map/adminRegionsCore.js

import { INF, MAP_H, MAP_W, SIZE, MinHeap, clamp, indexOf, inside, weightedScore, xyOf } from "./mapUtils.js";

function neighbors8(x, y) {
  const out = [];
  for (let dy = -1; dy <= 1; dy++) {
    for (let dx = -1; dx <= 1; dx++) {
      if (dx === 0 && dy === 0) continue;
      const nx = x + dx;
      const ny = y + dy;
      if (inside(nx, ny)) out.push([nx, ny, Math.hypot(dx, dy)]);
    }
  }
  return out;
}

function neighbors4(x, y) {
  const out = [];
  for (const [dx, dy] of [[1, 0], [-1, 0], [0, 1], [0, -1]]) {
    const nx = x + dx;
    const ny = y + dy;
    if (inside(nx, ny)) out.push([nx, ny, 1]);
  }
  return out;
}

export function generateAdminRegions(centers, prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, populationDensity, landuse) {
  const adminId = new Int16Array(SIZE);
  adminId.fill(-1);
  const dist = new Float32Array(SIZE);
  dist.fill(INF);
  const heap = new MinHeap();

  centers.forEach((center, regionId) => {
    const i = indexOf(center.x, center.y);
    dist[i] = 0;
    adminId[i] = regionId;
    heap.push({ i, f: 0, regionId });
  });

  let guard = 0;
  while (heap.length > 0 && guard++ < SIZE * 12) {
    const current = heap.pop();
    if (!current) continue;
    const curIndex = current.i;
    const curRegion = adminId[curIndex];
    if (curRegion < 0 || current.f > dist[curIndex] + 1e-5) continue;

    const [cx, cy] = xyOf(curIndex);
    for (const [nx, ny, step] of neighbors8(cx, cy)) {
      const ni = indexOf(nx, ny);
      if (!prefectureMask[ni] || sea[ni]) continue;

      const ridgeBarrier = Math.max(ridgeField[ni], ridgeField[curIndex]);
      const riverBarrier = Math.max(river[ni], river[curIndex]);
      const highDivide = Math.max(elevation[ni], elevation[curIndex]);
      const watershedBarrier = ridgeBarrier * (27.0 + Math.max(0, highDivide - 0.46) * 46.0);
      const ridgePenalty = Math.max(0, highDivide - 0.36) * 16.0 + Math.abs(elevation[ni] - elevation[curIndex]) * 12.4 + watershedBarrier;
      const slopePenalty = slope[ni] * 10.6;
      const valleyBarrier = valleyField[ni] > 0.50 ? valleyField[ni] * (riverBarrier > 0.16 ? 7.2 : 2.6) : 0;
      const riverPenalty = riverBarrier > 0.7 ? 22.0 : riverBarrier > 0.42 ? 14.8 : riverBarrier > 0.22 ? 7.4 : riverBarrier > 0.12 ? 2.2 : 0;
      const urbanContinuityBonus = (landuse[ni] >= 2 && landuse[ni] <= 4 && populationDensity[ni] > 0.20) ? 1.65 : 0;
      const valleyLocalityBonus = valleyField[ni] * 0.16;
      const stepCost = Math.max(0.25, 0.72 + ridgePenalty + slopePenalty + riverPenalty + valleyBarrier - valleyLocalityBonus - urbanContinuityBonus) * step;
      const nextDist = dist[curIndex] + stepCost;

      if (nextDist < dist[ni]) {
        dist[ni] = nextDist;
        adminId[ni] = curRegion;
        heap.push({ i: ni, f: nextDist, regionId: curRegion });
      }
    }
  }

  return adminId;
}

function terrainBoundaryStrength(i, elevation, slope, river, ridgeField, valleyField) {
  return clamp(weightedScore([
    [ridgeField[i], 3.15],
    [river[i], 2.45],
    [valleyField[i], 0.62],
    [slope[i], 1.06],
    [Math.max(0, elevation[i] - 0.5), 1.18],
  ]));
}

export function smoothAdminRegionsTerrainAware(adminId, prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, populationDensity, landuse, passes = 5) {
  let current = new Int16Array(adminId);
  for (let pass = 0; pass < passes; pass++) {
    const next = new Int16Array(current);
    for (let y = 1; y < MAP_H - 1; y++) {
      for (let x = 1; x < MAP_W - 1; x++) {
        const i = indexOf(x, y);
        const own = current[i];
        if (!prefectureMask[i] || sea[i] || own < 0) continue;
        const urbanCell = (landuse[i] >= 2 && landuse[i] <= 4) || landuse[i] === 7 || populationDensity[i] > 0.24;
        const barrier = terrainBoundaryStrength(i, elevation, slope, river, ridgeField, valleyField);
        if (barrier > 0.62 || urbanCell) continue;

        const counts = new Map();
        let ownCount = 0;
        for (const [nx, ny] of neighbors4(x, y)) {
          const ni = indexOf(nx, ny);
          if (!prefectureMask[ni] || sea[ni]) continue;
          const id = current[ni];
          if (id < 0) continue;
          const weight = terrainBoundaryStrength(ni, elevation, slope, river, ridgeField, valleyField) > 0.72 ? 0.45 : 1;
          counts.set(id, (counts.get(id) || 0) + weight);
          if (id === own) ownCount += weight;
        }
        let bestId = own;
        let best = ownCount;
        for (const [id, score] of counts) if (score > best) { best = score; bestId = id; }
        if (bestId !== own && (best >= 4.2 || ownCount <= 2.1)) next[i] = bestId;
      }
    }
    current = next;
  }
  adminId.set(current);
}

export function lockSmallUrbanComponentsToMunicipality(adminId, prefectureMask, sea, landuse, populationDensity, maxCells = 360) {
  const seen = new Uint8Array(SIZE);
  const queue = [];
  for (let i = 0; i < SIZE; i++) {
    if (seen[i] || !prefectureMask[i] || sea[i]) continue;
    const isUrbanStart = (landuse[i] >= 2 && landuse[i] <= 4) || landuse[i] === 7 || landuse[i] === 8 || populationDensity[i] > 0.20;
    if (!isUrbanStart) continue;
    const component = [];
    queue.length = 0;
    queue.push(i);
    seen[i] = 1;
    for (let q = 0; q < queue.length; q++) {
      const cur = queue[q];
      component.push(cur);
      const [x, y] = xyOf(cur);
      for (const [nx, ny] of neighbors8(x, y)) {
        const ni = indexOf(nx, ny);
        if (seen[ni] || !prefectureMask[ni] || sea[ni]) continue;
        const isUrban = (landuse[ni] >= 2 && landuse[ni] <= 4) || landuse[ni] === 7 || landuse[ni] === 8 || populationDensity[ni] > 0.20;
        if (!isUrban) continue;
        seen[ni] = 1;
        queue.push(ni);
      }
    }
    if (component.length === 0 || component.length > maxCells) continue;
    const counts = new Map();
    for (const ci of component) {
      const id = adminId[ci];
      if (id >= 0) counts.set(id, (counts.get(id) || 0) + 1 + populationDensity[ci]);
    }
    let bestId = -1;
    let best = -1;
    for (const [id, score] of counts) if (score > best) { best = score; bestId = id; }
    if (bestId >= 0) for (const ci of component) adminId[ci] = bestId;
  }
}

export function mergeTinyMunicipalities(adminId, prefectureMask, sea, populationDensity, modernCities = [], minArea = 320, options = {}) {
  const area = new Map();
  const pop = new Map();
  const adjacency = new Map();
  const cityMunicipalities = new Set();
  for (const city of modernCities || []) {
    if (!inside(city.x, city.y)) continue;
    const id = adminId[indexOf(city.x, city.y)];
    if (id < 0) continue;
    if (options.protectAllModernCities !== false || city.isPrefecturalCapital || (city.population || 0) >= (options.majorCityPopulationThreshold || 120000)) cityMunicipalities.add(id);
  }
  for (const point of options.protectedPoints || []) {
    if (!point || !inside(point.x, point.y)) continue;
    const id = adminId[indexOf(point.x, point.y)];
    if (id >= 0) cityMunicipalities.add(id);
  }
  const satelliteByAdmin = new Map();
  for (const sat of options.satelliteCities || []) {
    if (!sat || !inside(sat.x, sat.y)) continue;
    const id = adminId[indexOf(sat.x, sat.y)];
    if (id < 0) continue;
    if (!satelliteByAdmin.has(id)) satelliteByAdmin.set(id, []);
    satelliteByAdmin.get(id).push(sat);
  }
  const satelliteStats = options.satelliteStats || null;

  for (let y = 0; y < MAP_H; y++) {
    for (let x = 0; x < MAP_W; x++) {
      const i = indexOf(x, y);
      if (!prefectureMask[i] || sea[i]) continue;
      const id = adminId[i];
      if (id < 0) continue;
      area.set(id, (area.get(id) || 0) + 1);
      pop.set(id, (pop.get(id) || 0) + populationDensity[i]);
      for (const [nx, ny] of [[x + 1, y], [x, y + 1]]) {
        if (!inside(nx, ny)) continue;
        const ni = indexOf(nx, ny);
        if (!prefectureMask[ni] || sea[ni]) continue;
        const other = adminId[ni];
        if (other < 0 || other === id) continue;
        const key = id < other ? `${id}:${other}` : `${other}:${id}`;
        adjacency.set(key, (adjacency.get(key) || 0) + 1);
      }
    }
  }

  const mergeTarget = new Map();
  for (const [id, cells] of area) {
    const score = cells + (pop.get(id) || 0) * 16;
    if (cells >= minArea || cityMunicipalities.has(id)) continue;
    const satellites = satelliteByAdmin.get(id) || [];
    const protectedSatellite = satellites.some((sat) => {
      const minSatelliteArea = sat.satelliteMinArea || options.satelliteMinArea || 110;
      return sat.municipalityClass === "independentSatelliteMunicipality" && (
        cells >= minSatelliteArea ||
        (sat.population || 0) >= (options.satelliteIndependentPopulationThreshold || 60000) ||
        (sat.distinctUrbanComponentArea || 0) >= 80 ||
        sat.separatedByBarrier
      );
    });
    if (protectedSatellite) continue;
    let bestNeighbor = -1;
    let bestScore = -1;
    for (const [key, border] of adjacency) {
      const [a, b] = key.split(":").map(Number);
      if (a !== id && b !== id) continue;
      const other = a === id ? b : a;
      const parentBias = satellites.some((sat) => inside(sat.parentX ?? -1, sat.parentY ?? -1) && adminId[indexOf(sat.parentX, sat.parentY)] === other) ? 26 : 0;
      const ruralBias = satellites.some((sat) => sat.municipalityClass === "smallTownAttachedToRuralMunicipality") ? Math.min(12, (area.get(other) || 0) * 0.01) : 0;
      const candidate = border * 3 + (area.get(other) || 0) * 0.012 + (pop.get(other) || 0) * 0.24 + parentBias + ruralBias;
      if (candidate > bestScore) { bestScore = candidate; bestNeighbor = other; }
    }
    if (bestNeighbor >= 0 && (area.get(bestNeighbor) || 0) >= score * 0.35) {
      mergeTarget.set(id, bestNeighbor);
      if (satelliteStats && satellites.length) {
        satelliteStats.satelliteMunicipalitiesMerged += satellites.length;
        for (const sat of satellites) sat.mergedMunicipalityTarget = bestNeighbor;
      }
    }
  }
  if (mergeTarget.size === 0) return;
  for (let i = 0; i < SIZE; i++) if (mergeTarget.has(adminId[i])) adminId[i] = mergeTarget.get(adminId[i]);
}

export function removeMunicipalExclaves(adminId, prefectureMask, sea, adminCenters = [], protectedPoints = [], maxIslandCells = 220) {
  const protectedByAdmin = new Map();
  for (const p of [...adminCenters, ...protectedPoints]) {
    if (!p || !inside(p.x, p.y)) continue;
    const id = adminId[indexOf(p.x, p.y)];
    if (id < 0) continue;
    if (!protectedByAdmin.has(id)) protectedByAdmin.set(id, new Set());
    protectedByAdmin.get(id).add(indexOf(p.x, p.y));
  }

  const ids = new Set();
  for (let i = 0; i < SIZE; i++) if (prefectureMask[i] && !sea[i] && adminId[i] >= 0) ids.add(adminId[i]);

  const globalSeen = new Uint8Array(SIZE);
  const queue = [];
  for (const id of ids) {
    const components = [];
    for (let i = 0; i < SIZE; i++) {
      if (globalSeen[i] || adminId[i] !== id || !prefectureMask[i] || sea[i]) continue;
      const comp = [];
      let hasProtected = protectedByAdmin.get(id)?.has(i) || false;
      queue.length = 0;
      queue.push(i);
      globalSeen[i] = 1;
      for (let q = 0; q < queue.length; q++) {
        const cur = queue[q];
        comp.push(cur);
        if (protectedByAdmin.get(id)?.has(cur)) hasProtected = true;
        const [x, y] = xyOf(cur);
        for (const [nx, ny] of neighbors4(x, y)) {
          const ni = indexOf(nx, ny);
          if (globalSeen[ni] || adminId[ni] !== id || !prefectureMask[ni] || sea[ni]) continue;
          globalSeen[ni] = 1;
          queue.push(ni);
        }
      }
      components.push({ cells: comp, hasProtected });
    }
    if (components.length <= 1) continue;
    components.sort((a, b) => (b.hasProtected ? 1000000 : 0) + b.cells.length - ((a.hasProtected ? 1000000 : 0) + a.cells.length));
    for (const component of components.slice(1)) {
      const mainSize = components[0].cells.length;
      if (component.hasProtected && component.cells.length > maxIslandCells && component.cells.length > mainSize * 0.42) continue;
      if (component.cells.length > maxIslandCells && component.cells.length > mainSize * 0.36) continue;
      const counts = new Map();
      for (const ci of component.cells) {
        const [x, y] = xyOf(ci);
        for (const [nx, ny] of neighbors4(x, y)) {
          const ni = indexOf(nx, ny);
          if (!prefectureMask[ni] || sea[ni]) continue;
          const other = adminId[ni];
          if (other >= 0 && other !== id) counts.set(other, (counts.get(other) || 0) + 1);
        }
      }
      let target = -1;
      let best = -1;
      for (const [other, count] of counts) if (count > best) { best = count; target = other; }
      if (target >= 0) for (const ci of component.cells) adminId[ci] = target;
    }
  }
}


export function enforceMunicipalityConnectivityStrict(adminId, prefectureMask, sea, adminCenters = [], protectedPoints = [], maxPasses = 6) {
  // Final cell-level invariant: each municipality should be one contiguous land
  // component.  Earlier stages are allowed to leave sizeable satellite pieces
  // while boundaries are still being snapped; this pass removes the remaining
  // visual exclaves by attaching every non-primary component to the neighboring
  // municipality with the largest shared boundary.  A component that contains a
  // protected point may become the primary component, but it no longer protects
  // additional detached pieces.
  const protectedByAdmin = new Map();
  for (const p of [...(adminCenters || []), ...(protectedPoints || [])]) {
    if (!p || !inside(p.x, p.y)) continue;
    const i = indexOf(Math.round(p.x), Math.round(p.y));
    const id = adminId[i];
    if (id < 0) continue;
    if (!protectedByAdmin.has(id)) protectedByAdmin.set(id, new Set());
    protectedByAdmin.get(id).add(i);
  }

  let changed = 0;
  const queue = [];
  for (let pass = 0; pass < maxPasses; pass++) {
    let passChanged = 0;
    const ids = new Set();
    for (let i = 0; i < SIZE; i++) if (prefectureMask[i] && !sea[i] && adminId[i] >= 0) ids.add(adminId[i]);
    for (const id of ids) {
      const seen = new Uint8Array(SIZE);
      const components = [];
      for (let i = 0; i < SIZE; i++) {
        if (seen[i] || adminId[i] !== id || !prefectureMask[i] || sea[i]) continue;
        const comp = [];
        let protectedHits = 0;
        queue.length = 0;
        queue.push(i);
        seen[i] = 1;
        for (let q = 0; q < queue.length; q++) {
          const cur = queue[q];
          comp.push(cur);
          if (protectedByAdmin.get(id)?.has(cur)) protectedHits++;
          const [x, y] = xyOf(cur);
          for (const [nx, ny] of neighbors4(x, y)) {
            const ni = indexOf(nx, ny);
            if (seen[ni] || adminId[ni] !== id || !prefectureMask[ni] || sea[ni]) continue;
            seen[ni] = 1;
            queue.push(ni);
          }
        }
        components.push({ cells: comp, protectedHits });
      }
      if (components.length <= 1) continue;
      components.sort((a, b) =>
        (b.protectedHits ? 1_000_000 : 0) + b.cells.length -
        ((a.protectedHits ? 1_000_000 : 0) + a.cells.length)
      );
      const primary = components[0];
      for (const component of components.slice(1)) {
        const counts = new Map();
        for (const ci of component.cells) {
          const [x, y] = xyOf(ci);
          for (const [nx, ny] of neighbors4(x, y)) {
            const ni = indexOf(nx, ny);
            if (!prefectureMask[ni] || sea[ni]) continue;
            const other = adminId[ni];
            if (other >= 0 && other !== id) counts.set(other, (counts.get(other) || 0) + 1);
          }
        }
        let target = -1;
        let best = -1;
        for (const [other, count] of counts) {
          const bonus = protectedByAdmin.get(other)?.size ? 0.25 : 0;
          const score = count + bonus;
          if (score > best || (score === best && other < target)) { best = score; target = other; }
        }
        if (target < 0) {
          // Very rare: a detached island component has no labeled neighbor.
          // Keep the largest/protected primary and merge the component into it
          // only if it is directly adjacent after previous changes; otherwise
          // leave it for the next pass rather than inventing over-sea ownership.
          target = id;
        }
        if (target >= 0 && target !== id) {
          for (const ci of component.cells) adminId[ci] = target;
          passChanged += component.cells.length;
        } else if (component !== primary) {
          // If no external target exists, still mark it as handled by keeping it;
          // another pass may expose a target after surrounding cells change.
        }
      }
    }
    changed += passChanged;
    if (!passChanged) break;
  }
  return changed;
}

export function terrainBoundaryTargetScore(i, elevation, slope, river, ridgeField, valleyField, flowAccum, populationDensity, landuse) {
  const urbanPenalty = urbanBoundaryPenalty(i, populationDensity, landuse);
  const majorRiver = clamp(Math.max(river[i] - 0.32, 0) * 1.9 + Math.max(flowAccum[i] - 0.38, 0) * 0.75);
  const minorStream = clamp(river[i] * 0.34 + flowAccum[i] * 0.18);
  const ridgeDivide = clamp(ridgeField[i] * 2.12 + Math.max(0, elevation[i] - 0.52) * ridgeField[i] * 1.32);
  const slopeBreak = clamp(slope[i] * 0.70 + Math.max(0, slope[i] - 0.30) * 0.88);
  const highGround = Math.max(0, elevation[i] - 0.54) * 0.34;
  const valleyFloorPenalty = valleyField[i] * (majorRiver > 0.34 ? -0.08 : -0.68);
  return clamp(ridgeDivide + majorRiver * 0.92 + minorStream * 0.20 + slopeBreak + highGround + valleyFloorPenalty - urbanPenalty * 0.92);
}

function urbanBoundaryPenalty(i, populationDensity, landuse) {
  const lu = landuse[i];
  const core = lu === 3 ? 1.45 : lu === 2 ? 1.12 : lu === 4 ? 0.95 : lu === 7 ? 0.90 : lu === 8 ? 0.64 : lu === 5 || lu === 6 ? 0.48 : 0;
  return clamp(core + populationDensity[i] * 1.35);
}

function isAdminBoundaryCell(labels, prefectureMask, sea, x, y, useEight = true) {
  const i = indexOf(x, y);
  const own = labels[i];
  if (!prefectureMask[i] || sea[i] || own < 0) return false;
  const neighbors = useEight ? neighbors8(x, y) : neighbors4(x, y);
  for (const [nx, ny] of neighbors) {
    const ni = indexOf(nx, ny);
    if (prefectureMask[ni] && !sea[ni] && labels[ni] >= 0 && labels[ni] !== own) return true;
  }
  return false;
}

function buildBoundaryBand(labels, prefectureMask, sea, radius = 5) {
  const band = new Uint8Array(SIZE);
  for (let y = 1; y < MAP_H - 1; y++) {
    for (let x = 1; x < MAP_W - 1; x++) {
      if (!isAdminBoundaryCell(labels, prefectureMask, sea, x, y, true)) continue;
      for (let dy = -radius; dy <= radius; dy++) {
        for (let dx = -radius; dx <= radius; dx++) {
          if (Math.hypot(dx, dy) > radius) continue;
          const nx = x + dx;
          const ny = y + dy;
          if (!inside(nx, ny)) continue;
          const ni = indexOf(nx, ny);
          if (prefectureMask[ni] && !sea[ni]) band[ni] = 1;
        }
      }
    }
  }
  return band;
}

function buildAdminProtectedMask(adminId, prefectureMask, sea, adminCenters = [], protectedPoints = [], populationDensity, landuse) {
  const protectedMask = new Uint8Array(SIZE);
  function protectDisk(p, radius) {
    if (!p || !inside(p.x, p.y)) return;
    const owner = adminId[indexOf(p.x, p.y)];
    if (owner < 0) return;
    const r = Math.ceil(radius);
    for (let dy = -r; dy <= r; dy++) {
      for (let dx = -r; dx <= r; dx++) {
        if (Math.hypot(dx, dy) > radius) continue;
        const x = p.x + dx;
        const y = p.y + dy;
        if (!inside(x, y)) continue;
        const i = indexOf(x, y);
        if (prefectureMask[i] && !sea[i] && adminId[i] === owner) protectedMask[i] = 1;
      }
    }
  }
  for (const center of adminCenters) protectDisk(center, 2.2);
  for (const p of protectedPoints || []) protectDisk(p, p.population ? clamp(1.6 + Math.sqrt(p.population) / 520, 2.1, 6.0) : p.portClass ? 2.0 : 1.7);
  for (let i = 0; i < SIZE; i++) {
    if (prefectureMask[i] && !sea[i] && (landuse[i] === 3 || populationDensity[i] > 0.72)) protectedMask[i] = 1;
  }
  return protectedMask;
}

function localBoundaryEnergy(labels, i, candidateId, targetScore, centerDist, populationDensity, landuse, river, valleyField) {
  const [x, y] = xyOf(i);
  const oldId = labels[i];
  let energy = centerDist[candidateId]?.[i] ?? 0;
  let same4 = 0;
  let diff4 = 0;
  let diagDiff = 0;

  for (const [nx, ny] of neighbors8(x, y)) {
    const ni = indexOf(nx, ny);
    const neighborId = labels[ni];
    if (neighborId < 0) continue;
    const isCardinal = nx === x || ny === y;
    const differs = neighborId !== candidateId;
    if (isCardinal) {
      if (differs) {
        diff4++;
        const boundaryTarget = (targetScore[i] + targetScore[ni]) * 0.5;
        const urbanCut = (urbanBoundaryPenalty(i, populationDensity, landuse) + urbanBoundaryPenalty(ni, populationDensity, landuse)) * 0.5;
        const minorValley = (valleyField[i] + valleyField[ni]) * 0.5 > 0.34 && Math.max(river[i], river[ni]) < 0.30 ? 0.72 : 0;
        const dHere = centerDist[candidateId]?.[i] ?? 99;
        const dThere = centerDist[neighborId]?.[i] ?? 99;
        const weakBisectorPenalty = Math.abs(dHere - dThere) < 4.0 && boundaryTarget < 0.42 ? 0.62 : 0;
        energy += 2.15 - boundaryTarget * 1.55 + urbanCut * 3.0 + minorValley + weakBisectorPenalty;
      } else same4++;
    } else if (differs) diagDiff++;
  }

  if (same4 === 0) energy += 5.2;
  if (same4 === 1) energy += 1.7;
  if (diff4 >= 3 && targetScore[i] < 0.42) energy += 1.25;
  if (diagDiff >= 3 && diff4 >= 2 && targetScore[i] < 0.50) energy += 0.42;
  if (candidateId !== oldId) {
    const candidateDistance = centerDistanceAt(centerDist, candidateId, i);
    const oldDistance = centerDistanceAt(centerDist, oldId, i);
    if (Number.isFinite(candidateDistance) && Number.isFinite(oldDistance)) {
      const drift = candidateDistance - oldDistance;
      if (drift > 0) energy += Math.min(0.9, drift * 0.012);
    }
  }
  return energy;
}

function centerDistanceAt(centerDist, id, i) {
  const field = centerDist?.fields?.[id] || centerDist?.[id];
  if (field) return field[i];
  const center = centerDist?.centers?.[id];
  if (!center || !inside(center.x, center.y)) return 24;
  const [x, y] = xyOf(i);
  return Math.hypot(x - center.x, y - center.y);
}

function buildCenterDistanceFields(adminIds, adminCenters, prefectureMask, sea) {
  // Older versions materialized one full SIZE Float32Array per municipality.
  // In multi-prefecture generation this can create heavy transient memory use.
  // Keep the same interface conceptually, but compute distances on demand.
  return { ids: adminIds, centers: adminCenters, prefectureMask, sea };
}

function repairAdminTopology(adminId, prefectureMask, sea, adminCenters = [], targetScore = null, populationDensity = null, landuse = null) {
  const ids = new Set();
  for (let i = 0; i < SIZE; i++) if (prefectureMask[i] && !sea[i] && adminId[i] >= 0) ids.add(adminId[i]);
  const queue = [];

  for (const id of ids) {
    const seen = new Uint8Array(SIZE);
    const components = [];
    for (let i = 0; i < SIZE; i++) {
      if (seen[i] || adminId[i] !== id || !prefectureMask[i] || sea[i]) continue;
      const cells = [];
      queue.length = 0;
      queue.push(i);
      seen[i] = 1;
      for (let q = 0; q < queue.length; q++) {
        const cur = queue[q];
        cells.push(cur);
        const [x, y] = xyOf(cur);
        for (const [nx, ny] of neighbors4(x, y)) {
          const ni = indexOf(nx, ny);
          if (seen[ni] || adminId[ni] !== id || !prefectureMask[ni] || sea[ni]) continue;
          seen[ni] = 1;
          queue.push(ni);
        }
      }
      components.push(cells);
    }
    if (components.length <= 1) continue;

    const centerIndex = adminCenters[id] && inside(adminCenters[id].x, adminCenters[id].y) ? indexOf(adminCenters[id].x, adminCenters[id].y) : -1;
    let keepIndex = centerIndex >= 0 ? components.findIndex((cells) => cells.includes(centerIndex)) : -1;
    if (keepIndex < 0) {
      let bestSize = -1;
      for (let c = 0; c < components.length; c++) if (components[c].length > bestSize) { bestSize = components[c].length; keepIndex = c; }
    }

    for (let c = 0; c < components.length; c++) {
      if (c === keepIndex) continue;
      const counts = new Map();
      for (const ci of components[c]) {
        const [x, y] = xyOf(ci);
        for (const [nx, ny] of neighbors4(x, y)) {
          const ni = indexOf(nx, ny);
          if (!prefectureMask[ni] || sea[ni]) continue;
          const other = adminId[ni];
          if (other < 0 || other === id) continue;
          const terrainFit = targetScore ? targetScore[ci] * 0.18 : 0;
          const urbanFit = populationDensity && landuse ? (1 - urbanBoundaryPenalty(ci, populationDensity, landuse)) * 0.08 : 0;
          counts.set(other, (counts.get(other) || 0) + 1 + terrainFit + urbanFit);
        }
      }
      let target = -1;
      let best = -1;
      for (const [other, score] of counts) if (score > best) { best = score; target = other; }
      if (target >= 0) for (const ci of components[c]) adminId[ci] = target;
    }
  }
}

function classifyLandscapeCell(i, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, plain, agriculture, populationDensity, landuse) {
  if (landuse[i] === 3 || populationDensity[i] > 0.70) return 1;
  if ((landuse[i] >= 2 && landuse[i] <= 4) || landuse[i] === 7 || landuse[i] === 8 || populationDensity[i] > 0.24) return 2;
  if (landuse[i] === 5 || landuse[i] === 6) return 3;
  if ((river[i] > 0.50 && flowAccum[i] > 0.34) || flowAccum[i] > 0.68) return 4;
  if (coastalLowland[i] > 0.42 && elevation[i] < 0.44) return 5;
  if (basinField[i] > 0.38 && plain[i] > 0.26) return 6;
  if (valleyField[i] > 0.42 && ridgeField[i] < 0.55) return 7;
  if (ridgeField[i] > 0.54 || (ridgeField[i] > 0.40 && elevation[i] > 0.54)) return 8;
  if (elevation[i] > 0.62 || slope[i] > 0.42) return 9;
  if (landuse[i] === 0 || agriculture[i] > 0.45 || plain[i] > 0.48) return 10;
  return 11;
}

export function buildNaturalBarrierScore(prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, crestCrossingScore = null, plain = null, agriculture = null, populationDensity = null, landuse = null) {
  const score = new Float32Array(SIZE);
  for (let i = 0; i < SIZE; i++) {
    if (!prefectureMask[i] || sea[i]) continue;
    const [x, y] = xyOf(i);
    let coastEdge = 0;
    for (const [nx, ny] of neighbors8(x, y)) if (sea[indexOf(nx, ny)]) coastEdge = 1;
    const urbanContinuity = populationDensity && landuse ? urbanBoundaryPenalty(i, populationDensity, landuse) : 0;
    const majorRiver = clamp(Math.max(river[i] - 0.34, 0) * 1.95 + Math.max(flowAccum[i] - 0.42, 0) * 0.82);
    const ridgeDivide = clamp(ridgeField[i] * 2.05 + Math.max(0, elevation[i] - 0.50) * ridgeField[i] * 1.28);
    const crest = crestCrossingScore ? crestCrossingScore[i] : clamp(Math.max(0, elevation[i] - 0.53) * ridgeField[i] * 1.72 + slope[i] * ridgeField[i] * 1.02);
    const basinRim = basinField ? clamp(Math.max(0, basinField[i] - 0.30) * Math.max(0, slope[i] - 0.16) * 1.32 + Math.max(0, ridgeField[i] - 0.32) * basinField[i] * 0.78) : 0;
    const foothillBreak = clamp(Math.max(0, slope[i] - 0.28) * Math.max(ridgeField[i], Math.max(0, elevation[i] - 0.46)) * 1.02);
    const livingCorridor = clamp((plain?.[i] || 0) * 0.34 + (agriculture?.[i] || 0) * 0.26 + valleyField[i] * (majorRiver > 0.34 ? 0.10 : 0.46) + coastalLowland[i] * 0.18);
    score[i] = clamp(
      ridgeDivide * 1.42 +
      crest * 1.10 +
      majorRiver * 0.86 +
      basinRim * 0.66 +
      foothillBreak * 0.58 +
      coastEdge * 0.34 +
      terrainBoundaryTargetScore(i, elevation, slope, river, ridgeField, valleyField, flowAccum, populationDensity || score, landuse || score) * 0.44 -
      livingCorridor * 0.28 -
      urbanContinuity * 0.72
    );
  }
  return score;
}

function lowlandCompartmentFitness(i, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse) {
  const lowRelief = clamp((0.68 - elevation[i]) * 1.25) + clamp((0.36 - slope[i]) * 1.45) + clamp((0.48 - ridgeField[i]) * 1.10);
  const landuseFit = [1, 2, 3, 4, 7, 8].includes(landuse[i]) ? 0.42 : landuse[i] === 5 || landuse[i] === 6 ? 0.20 : 0;
  return clamp(
    lowRelief * 0.30 +
    (plain?.[i] || 0) * 0.30 +
    (agriculture?.[i] || 0) * 0.16 +
    basinField[i] * 0.24 +
    coastalLowland[i] * 0.24 +
    valleyField[i] * 0.10 +
    populationDensity[i] * 0.34 +
    landuseFit
  );
}

function mountainCompartmentFitness(i, elevation, slope, ridgeField, populationDensity, landuse) {
  const settled = populationDensity[i] * 0.85 + ([2, 3, 4, 7, 8].includes(landuse[i]) ? 0.35 : 0);
  return clamp(elevation[i] * 0.38 + slope[i] * 0.32 + ridgeField[i] * 0.42 - settled);
}

function canShareNaturalCompartment(a, b, classA, classB, barrier, river, flowAccum, valleyField, populationDensity, landuse) {
  if (classA !== classB) {
    const bothUrban = classA <= 3 && classB <= 3;
    const bothLivingCorridor = [5, 6, 7, 10].includes(classA) && [5, 6, 7, 10].includes(classB);
    if (!bothUrban && !bothLivingCorridor) return false;
  }
  const majorRiverEdge = Math.max(river[a], river[b]) > 0.56 || Math.max(flowAccum[a], flowAccum[b]) > 0.68;
  const urbanEdge = ((landuse[a] >= 2 && landuse[a] <= 4) || landuse[a] === 7 || populationDensity[a] > 0.34) &&
    ((landuse[b] >= 2 && landuse[b] <= 4) || landuse[b] === 7 || populationDensity[b] > 0.34);
  const valleyContinuity = (valleyField[a] + valleyField[b]) * 0.5 > 0.48 && !majorRiverEdge;
  const threshold = urbanEdge ? 0.74 : valleyContinuity ? 0.56 : classA === 8 || classB === 8 ? 0.28 : 0.43;
  return barrier < threshold && (!majorRiverEdge || urbanEdge);
}

function refreshCompartmentStats(unit, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse) {
  let sx = 0, sy = 0, pop = 0, urbanWeight = 0, ridgeExposure = 0, riverExposure = unit.riverExposure || 0;
  let coastalExposure = 0, basinIdentity = 0, valleyIdentity = 0, lowlandFitness = 0, mountainFitness = 0;
  let minX = MAP_W, minY = MAP_H, maxX = 0, maxY = 0;
  for (const i of unit.cells) {
    const [x, y] = xyOf(i);
    sx += x; sy += y; pop += populationDensity[i];
    minX = Math.min(minX, x); minY = Math.min(minY, y); maxX = Math.max(maxX, x); maxY = Math.max(maxY, y);
    urbanWeight += urbanBoundaryPenalty(i, populationDensity, landuse);
    ridgeExposure += ridgeField[i];
    coastalExposure += coastalLowland[i];
    basinIdentity += basinField[i];
    valleyIdentity += valleyField[i];
    lowlandFitness += lowlandCompartmentFitness(i, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse);
    mountainFitness += mountainCompartmentFitness(i, elevation, slope, ridgeField, populationDensity, landuse);
  }
  const area = unit.cells.length;
  unit.area = area;
  unit.x = sx / Math.max(1, area);
  unit.y = sy / Math.max(1, area);
  unit.minX = area ? minX : 0;
  unit.minY = area ? minY : 0;
  unit.maxX = area ? maxX : 0;
  unit.maxY = area ? maxY : 0;
  unit.width = area ? maxX - minX + 1 : 0;
  unit.height = area ? maxY - minY + 1 : 0;
  unit.elongation = Math.max(unit.width, unit.height) / Math.max(1, Math.min(unit.width, unit.height));
  unit.population = pop;
  unit.urbanWeight = urbanWeight / Math.max(1, area);
  unit.ridgeExposure = ridgeExposure / Math.max(1, area);
  unit.riverExposure = riverExposure / Math.max(1, area);
  unit.coastalExposure = coastalExposure / Math.max(1, area);
  unit.basinIdentity = basinIdentity / Math.max(1, area);
  unit.valleyIdentity = valleyIdentity / Math.max(1, area);
  unit.lowlandFitness = lowlandFitness / Math.max(1, area);
  unit.mountainFitness = mountainFitness / Math.max(1, area);
}

function splitOneNaturalCompartment(unit, newId, compartmentId, fields, seed) {
  if (!unit || unit.area < 24) return null;
  const { elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse, naturalBarrierScore, river, flowAccum } = fields;
  const minPart = Math.max(8, Math.min(28, Math.floor(unit.area * 0.20)));

  let first = -1;
  let second = -1;
  let bestA = -INF;
  let bestB = -INF;
  const width = unit.width || (unit.maxX - unit.minX + 1) || 1;
  const height = unit.height || (unit.maxY - unit.minY + 1) || 1;
  const horizontal = width >= height;
  const elongated = Math.max(width, height) / Math.max(1, Math.min(width, height)) > 1.65;

  for (const i of unit.cells) {
    const [x, y] = xyOf(i);
    const low = lowlandCompartmentFitness(i, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse);
    const settled = populationDensity[i] * 0.28 + ([2, 3, 4, 7, 8].includes(landuse[i]) ? 0.34 : 0);
    const axis = elongated ? (horizontal ? (unit.maxX - x) / Math.max(1, width) : (unit.maxY - y) / Math.max(1, height)) : 0.0;
    const score = axis * 1.7 + low * 0.42 + settled + hashSeededTie(x, y, seed) * 0.05 - ridgeField[i] * 0.10;
    if (score > bestA) { bestA = score; first = i; }
  }
  if (first < 0) return null;
  const [fx, fy] = xyOf(first);
  for (const i of unit.cells) {
    const [x, y] = xyOf(i);
    const low = lowlandCompartmentFitness(i, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse);
    const axis = elongated ? (horizontal ? (x - unit.minX) / Math.max(1, width) : (y - unit.minY) / Math.max(1, height)) : 0.0;
    const d = Math.hypot(x - fx, y - fy);
    const score = axis * 1.9 + d * (0.18 + low * 0.22) + hashSeededTie(x, y, seed + 17) * 0.08 - ridgeField[i] * 0.08;
    if (score > bestB) { bestB = score; second = i; }
  }
  if (second < 0 || second === first) return null;

  const cellSet = new Set(unit.cells);
  const owner = new Int8Array(SIZE);
  owner.fill(-1);
  const dist = new Float32Array(SIZE);
  dist.fill(INF);
  const heap = new MinHeap();
  for (const [source, sourceOwner] of [[first, 0], [second, 1]]) {
    owner[source] = sourceOwner;
    dist[source] = 0;
    heap.push({ i: source, f: 0, owner: sourceOwner });
  }

  while (heap.length > 0) {
    const cur = heap.pop();
    if (!cur || cur.f > dist[cur.i] + 1e-5) continue;
    const [x, y] = xyOf(cur.i);
    for (const [nx, ny, step] of neighbors4(x, y)) {
      const ni = indexOf(nx, ny);
      if (!cellSet.has(ni)) continue;
      const barrier = ((naturalBarrierScore?.[cur.i] || 0) + (naturalBarrierScore?.[ni] || 0)) * 0.5;
      const riverBarrier = Math.max(river?.[cur.i] || 0, river?.[ni] || 0) + Math.max(flowAccum?.[cur.i] || 0, flowAccum?.[ni] || 0) * 0.32;
      const ridgeStep = Math.max(ridgeField[cur.i], ridgeField[ni]) * 1.18 + Math.abs(elevation[cur.i] - elevation[ni]) * 1.48;
      const corridorBonus = Math.min(0.42, ((valleyField[cur.i] + valleyField[ni]) * 0.5 + (plain?.[ni] || 0) * 0.15 + (coastalLowland?.[ni] || 0) * 0.10));
      const stepCost = Math.max(0.18, 0.78 + barrier * 3.75 + riverBarrier * 1.10 + ridgeStep + slope[ni] * 0.48 - corridorBonus) * step;
      const nd = cur.f + stepCost;
      if (nd < dist[ni]) {
        dist[ni] = nd;
        owner[ni] = cur.owner;
        heap.push({ i: ni, f: nd, owner: cur.owner });
      }
    }
  }

  const aCells = [];
  const bCells = [];
  for (const ci of unit.cells) {
    if (owner[ci] === 1) bCells.push(ci);
    else aCells.push(ci);
  }
  if (aCells.length < minPart || bCells.length < minPart) return null;

  unit.cells = aCells;
  const newUnit = { ...unit, id: newId, cells: bCells, centerIds: [], adjacent: new Map() };
  for (const ci of bCells) compartmentId[ci] = newId;
  refreshCompartmentStats(unit, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse);
  refreshCompartmentStats(newUnit, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse);
  return newUnit;
}

function hashSeededTie(x, y, seed) {
  let h = Math.imul((x | 0) ^ (seed | 0), 1597334677) ^ Math.imul((y | 0) ^ ((seed >>> 1) | 0), 3812015801);
  h = (h ^ (h >>> 15)) >>> 0;
  return h / 4294967295;
}

function naturalGroupKey(unit) {
  if (unit.classId <= 3) return `urban:${Math.round(unit.x / 10)}:${Math.round(unit.y / 10)}`;
  if (unit.classId === 5) return `coast:${Math.round(unit.y / 8)}`;
  if (unit.classId === 6) return `basin:${Math.round(unit.x / 12)}:${Math.round(unit.y / 12)}`;
  if (unit.classId === 7) return `valley:${Math.round(unit.x / 11)}:${Math.round(unit.y / 11)}`;
  if (unit.classId === 8 || unit.classId === 9) return `mountain:${Math.round(unit.x / 14)}:${Math.round(unit.y / 14)}`;
  return `plain:${Math.round(unit.x / 14)}:${Math.round(unit.y / 14)}`;
}


function collectLandComponents(prefectureMask, sea) {
  const seen = new Uint8Array(SIZE);
  const components = [];
  const queue = [];
  for (let i = 0; i < SIZE; i++) {
    if (seen[i] || !prefectureMask[i] || sea[i]) continue;
    const cells = [];
    queue.length = 0;
    queue.push(i);
    seen[i] = 1;
    for (let q = 0; q < queue.length; q++) {
      const cur = queue[q];
      cells.push(cur);
      const [x, y] = xyOf(cur);
      for (const [nx, ny] of neighbors4(x, y)) {
        const ni = indexOf(nx, ny);
        if (seen[ni] || !prefectureMask[ni] || sea[ni]) continue;
        seen[ni] = 1;
        queue.push(ni);
      }
    }
    components.push(cells);
  }
  return components;
}


function collectNaturalGrowthComponents(prefectureMask, sea, watershedId = null) {
  if (!watershedId) return collectLandComponents(prefectureMask, sea);
  const seen = new Uint8Array(SIZE);
  const components = [];
  const queue = [];
  for (let i = 0; i < SIZE; i++) {
    if (seen[i] || !prefectureMask[i] || sea[i]) continue;
    const wid = watershedId[i];
    const cells = [];
    queue.length = 0;
    queue.push(i);
    seen[i] = 1;
    for (let q = 0; q < queue.length; q++) {
      const cur = queue[q];
      cells.push(cur);
      const [x, y] = xyOf(cur);
      for (const [nx, ny] of neighbors4(x, y)) {
        const ni = indexOf(nx, ny);
        if (seen[ni] || !prefectureMask[ni] || sea[ni]) continue;
        if (watershedId[ni] !== wid) continue;
        seen[ni] = 1;
        queue.push(ni);
      }
    }
    components.push(cells);
  }
  return components;
}

function isWatershedBoundary(a, b, fields) {
  const watershedId = fields?.watershedId;
  if (!watershedId) return false;
  const aw = watershedId[a];
  const bw = watershedId[b];
  return aw >= 0 && bw >= 0 && aw !== bw;
}

function naturalSeedScore(i, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse, naturalBarrierScore, seed) {
  const klassUrban = (landuse[i] >= 2 && landuse[i] <= 4) || landuse[i] === 7 || landuse[i] === 8;
  const lowland = lowlandCompartmentFitness(i, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse);
  const mountain = mountainCompartmentFitness(i, elevation, slope, ridgeField, populationDensity, landuse);
  const stableInterior = clamp(1 - (naturalBarrierScore[i] || 0));
  const settlement = clamp(populationDensity[i] * 0.65 + (klassUrban ? 0.24 : 0));
  const streamCorridor = clamp(valleyField[i] * 0.28 + river[i] * 0.08);
  const mountainInterior = clamp(mountain * 0.45 + stableInterior * 0.28 - ridgeField[i] * 0.22);
  return stableInterior * 0.56 + lowland * 0.42 + mountainInterior * 0.32 + settlement * 0.26 + streamCorridor + hashSeededTie(...xyOf(i), seed) * 0.13 - slope[i] * 0.10;
}

function chooseNaturalCompartmentSeeds(landComponents, targetCount, fields, seed) {
  const { elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse, naturalBarrierScore } = fields;
  const totalArea = landComponents.reduce((sum, cells) => sum + cells.length, 0);
  const seeds = [];
  const seedComponentId = [];
  const minCellsPerUnit = 9;
  let remainingTarget = Math.max(1, Math.min(targetCount || Math.round(totalArea / 42), Math.floor(totalArea / minCellsPerUnit)));

  const sortedComponents = landComponents
    .map((cells, componentIndex) => ({ cells, componentIndex, area: cells.length }))
    .sort((a, b) => b.area - a.area);

  for (let componentOrder = 0; componentOrder < sortedComponents.length; componentOrder++) {
    const { cells, componentIndex, area } = sortedComponents[componentOrder];
    if (area <= 0) continue;
    const proportional = Math.round((targetCount || Math.round(totalArea / 42)) * area / Math.max(1, totalArea));
    let highland = 0;
    let rugged = 0;
    for (const ci of cells) {
      highland += clamp((elevation[ci] - 0.52) * 2.1 + ridgeField[ci] * 0.55 + slope[ci] * 0.45);
      rugged += clamp(ridgeField[ci] * 0.75 + slope[ci] * 0.55 + Math.max(0, elevation[ci] - 0.58) * 0.85);
    }
    highland /= Math.max(1, area);
    rugged /= Math.max(1, area);
    // Watersheds can be very large in mountain ranges.  The watershed switch is
    // a hard stop, but a single watershed still needs several internal natural
    // units; otherwise an entire mountain massif becomes one compartment.  Keep
    // the global target budget roughly intact by capping the terrain boost.
    const terrainBoost = highland > 0.48 ? 2.0 : rugged > 0.38 ? 1.55 : 1.0;
    let localTarget = Math.max(1, Math.round(Math.max(1, proportional) * terrainBoost));
    localTarget = Math.min(localTarget, Math.max(1, Math.floor(area / minCellsPerUnit)));
    const reservedForRest = Math.max(0, sortedComponents.length - componentOrder - 1);
    localTarget = Math.min(localTarget, Math.max(1, remainingTarget - reservedForRest));
    if (componentOrder === sortedComponents.length - 1) localTarget = Math.max(1, Math.min(localTarget, remainingTarget));
    remainingTarget -= localTarget;

    const candidates = cells
      .map((i) => ({ i, score: naturalSeedScore(i, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse, naturalBarrierScore, seed + componentIndex * 1009) }))
      .sort((a, b) => b.score - a.score);
    const localSeeds = [];
    const idealSpacing = Math.sqrt(area / Math.max(1, localTarget));
    const spacingPasses = [0.95, 0.78, 0.62, 0.48, 0.34];
    for (const factor of spacingPasses) {
      const minDist = Math.max(2.2, idealSpacing * factor);
      for (const candidate of candidates) {
        if (localSeeds.length >= localTarget) break;
        const [x, y] = xyOf(candidate.i);
        let ok = true;
        for (const existing of localSeeds) {
          const [ex, ey] = xyOf(existing);
          if (Math.hypot(x - ex, y - ey) < minDist) { ok = false; break; }
        }
        if (ok) localSeeds.push(candidate.i);
      }
      if (localSeeds.length >= localTarget) break;
    }
    for (const i of localSeeds) {
      seeds.push(i);
      seedComponentId.push(componentIndex);
    }
  }

  if (seeds.length === 0 && landComponents[0]?.length) {
    seeds.push(landComponents[0][0]);
    seedComponentId.push(0);
  }
  return { seeds, seedComponentId };
}

function isHardNaturalRidgeCrossing(a, b, cellClass, fields) {
  // Natural compartments now use drainage basins as the only hard barrier.
  // Ridges still increase naturalStepCost, but they must not freeze an entire
  // mountain block into one unsplittable component inside the same basin.
  return isWatershedBoundary(a, b, fields);
}

function naturalStepCost(a, b, cellClass, fields) {
  const { elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse, naturalBarrierScore, flowAccum } = fields;
  const barrier = ((naturalBarrierScore?.[a] || 0) + (naturalBarrierScore?.[b] || 0)) * 0.5;
  const ridge = Math.max(ridgeField[a], ridgeField[b]);
  const riverEdge = Math.max(river[a], river[b]);
  const flowEdge = Math.max(flowAccum?.[a] || 0, flowAccum?.[b] || 0);
  const majorRiverCrossing = riverEdge > 0.44 || flowEdge > 0.55;
  const elevationBreak = Math.abs(elevation[a] - elevation[b]);
  const slopeBreak = Math.max(slope[a], slope[b]);
  const classBreak = cellClass[a] !== cellClass[b] ? 0.34 : -0.08;
  const bothUrban = ((landuse[a] >= 2 && landuse[a] <= 4) || landuse[a] === 7 || populationDensity[a] > 0.30) &&
    ((landuse[b] >= 2 && landuse[b] <= 4) || landuse[b] === 7 || populationDensity[b] > 0.30);
  const lowlandContinuity = Math.min(
    (plain?.[a] || 0) + (agriculture?.[a] || 0) * 0.35 + basinField[a] * 0.25 + coastalLowland[a] * 0.20,
    (plain?.[b] || 0) + (agriculture?.[b] || 0) * 0.35 + basinField[b] * 0.25 + coastalLowland[b] * 0.20
  );
  const valleyContinuity = Math.min(valleyField[a], valleyField[b]) * (majorRiverCrossing ? 0.10 : 0.45);
  const corridorBonus = Math.min(0.42, lowlandContinuity * 0.22 + valleyContinuity + (bothUrban ? 0.18 : 0));
  const riverPenalty = majorRiverCrossing && !bothUrban ? 1.85 + flowEdge * 1.45 : riverEdge > 0.22 ? 0.38 : 0;
  if (isHardNaturalRidgeCrossing(a, b, cellClass, fields)) return INF;
  return Math.max(0.16,
    0.72 +
    barrier * 8.8 +
    ridge * 2.35 +
    elevationBreak * 5.10 +
    slopeBreak * 1.08 +
    riverPenalty +
    classBreak -
    corridorBonus
  );
}

function buildUnitsFromAssignment(compartmentId, cellClass, prefectureMask, sea, fields) {
  const { elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse } = fields;
  let maxId = -1;
  for (let i = 0; i < SIZE; i++) if (prefectureMask[i] && !sea[i] && compartmentId[i] > maxId) maxId = compartmentId[i];
  const units = Array.from({ length: maxId + 1 }, (_, id) => ({ id, cells: [], centerIds: [], adjacent: new Map(), area: 0 }));
  for (let i = 0; i < SIZE; i++) {
    const id = compartmentId[i];
    if (id >= 0 && units[id]) units[id].cells.push(i);
  }
  for (const unit of units) {
    if (!unit.cells.length) { unit.area = 0; continue; }
    const counts = new Map();
    for (const ci of unit.cells) counts.set(cellClass[ci], (counts.get(cellClass[ci]) || 0) + 1);
    let klass = -1, best = -1;
    for (const [k, count] of counts) if (count > best) { best = count; klass = k; }
    unit.classId = klass;
    unit.dominantLandscapeClass = klass;
    refreshCompartmentStats(unit, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse);
    let riverExposure = 0;
    for (const ci of unit.cells) riverExposure += river[ci] + (fields.flowAccum?.[ci] || 0) * 0.45;
    unit.riverExposure = riverExposure / Math.max(1, unit.area);
  }
  return units;
}


function splitDisconnectedCompartments(compartmentId, compartments, prefectureMask, sea) {
  const queue = [];
  for (const unit of [...compartments]) {
    if (!unit || unit.area === 0 || !unit.cells?.length) continue;
    const unitCellSet = new Set(unit.cells);
    const seen = new Set();
    const components = [];
    for (const start of unit.cells) {
      if (seen.has(start) || compartmentId[start] !== unit.id) continue;
      const cells = [];
      queue.length = 0;
      queue.push(start);
      seen.add(start);
      for (let q = 0; q < queue.length; q++) {
        const cur = queue[q];
        cells.push(cur);
        const [x, y] = xyOf(cur);
        for (const [nx, ny] of neighbors4(x, y)) {
          const ni = indexOf(nx, ny);
          if (!prefectureMask[ni] || sea[ni] || seen.has(ni) || compartmentId[ni] !== unit.id || !unitCellSet.has(ni)) continue;
          seen.add(ni);
          queue.push(ni);
        }
      }
      components.push(cells);
    }
    if (components.length <= 1) continue;
    components.sort((a, b) => b.length - a.length);
    unit.cells = components[0];
    for (const extra of components.slice(1)) {
      const newId = compartments.length;
      for (const ci of extra) compartmentId[ci] = newId;
      compartments.push({ id: newId, cells: extra, centerIds: [], adjacent: new Map(), classId: unit.classId, dominantLandscapeClass: unit.dominantLandscapeClass });
    }
  }
}


function splitCompartmentsByWatershed(compartmentId, compartments, fields) {
  const watershedId = fields?.watershedId;
  if (!watershedId) return 0;
  let split = 0;
  for (const unit of [...compartments]) {
    if (!unit || unit.area === 0 || !unit.cells?.length) continue;
    const groups = new Map();
    for (const ci of unit.cells) {
      const wid = watershedId[ci];
      const key = wid >= 0 ? wid : -1;
      if (!groups.has(key)) groups.set(key, []);
      groups.get(key).push(ci);
    }
    if (groups.size <= 1) continue;
    const sorted = [...groups.values()].sort((a, b) => b.length - a.length);
    unit.cells = sorted[0];
    unit.area = sorted[0].length;
    for (const extra of sorted.slice(1)) {
      const newId = compartments.length;
      for (const ci of extra) compartmentId[ci] = newId;
      compartments.push({
        id: newId,
        cells: extra,
        centerIds: [],
        adjacent: new Map(),
        area: extra.length,
        classId: unit.classId,
        dominantLandscapeClass: unit.dominantLandscapeClass,
      });
      split++;
    }
  }
  return split;
}

function renumberCompartments(compartmentId, compartments, prefectureMask, sea) {
  const active = compartments.filter((unit) => unit && unit.area > 0 && unit.cells?.length);
  const idMap = new Map();
  active.forEach((unit, newId) => idMap.set(unit.id, newId));
  for (let i = 0; i < SIZE; i++) {
    const id = compartmentId[i];
    if (!prefectureMask[i] || sea[i]) compartmentId[i] = -1;
    else if (idMap.has(id)) compartmentId[i] = idMap.get(id);
  }
  active.forEach((unit, newId) => { unit.id = newId; unit.centerIds = []; });
  return active;
}

function refreshAllCompartmentStats(compartments, fields) {
  const { elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse, river, flowAccum } = fields;
  for (const unit of compartments) {
    if (!unit || unit.area === 0 || !unit.cells?.length) continue;
    refreshCompartmentStats(unit, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse);
    let riverExposure = 0;
    const counts = new Map();
    for (const ci of unit.cells) {
      riverExposure += river[ci] + (flowAccum?.[ci] || 0) * 0.45;
      if (unit._cellClass) counts.set(unit._cellClass[ci], (counts.get(unit._cellClass[ci]) || 0) + 1);
    }
    unit.riverExposure = riverExposure / Math.max(1, unit.area);
  }
}

function splitNaturalCompartmentCompact(unit, newId, compartmentId, fields, seed) {
  if (!unit || unit.area < 20) return null;
  const { elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse, naturalBarrierScore, river, flowAccum, cellClass } = fields;
  const minPart = Math.max(7, Math.min(30, Math.floor(unit.area * 0.18)));
  let cx = unit.x || 0, cy = unit.y || 0;
  let first = -1, second = -1, bestA = -INF, bestB = -INF;
  const elongated = (unit.elongation || 1) > 2.3;
  const horizontal = (unit.width || 0) >= (unit.height || 0);
  for (const i of unit.cells) {
    const [x, y] = xyOf(i);
    const centerDist = Math.hypot(x - cx, y - cy);
    const axis = elongated ? Math.abs((horizontal ? x - cx : y - cy)) / Math.max(1, horizontal ? unit.width : unit.height) : 0;
    const interior = 1 - (naturalBarrierScore[i] || 0);
    const score = centerDist * 0.13 + axis * 1.1 + interior * 0.35 + hashSeededTie(x, y, seed) * 0.08 - ridgeField[i] * 0.10;
    if (score > bestA) { bestA = score; first = i; }
  }
  if (first < 0) return null;
  const [fx, fy] = xyOf(first);
  for (const i of unit.cells) {
    const [x, y] = xyOf(i);
    const d = Math.hypot(x - fx, y - fy);
    const interior = 1 - (naturalBarrierScore[i] || 0);
    const score = d * 0.20 + interior * 0.38 + hashSeededTie(x, y, seed + 31) * 0.08 - ridgeField[i] * 0.08;
    if (score > bestB) { bestB = score; second = i; }
  }
  if (second < 0 || second === first) return null;

  const cellSet = new Set(unit.cells);
  const owner = new Int8Array(SIZE);
  owner.fill(-1);
  const dist = new Float32Array(SIZE);
  dist.fill(INF);
  const heap = new MinHeap();
  for (const [source, sourceOwner] of [[first, 0], [second, 1]]) {
    owner[source] = sourceOwner;
    dist[source] = 0;
    heap.push({ i: source, f: 0, owner: sourceOwner });
  }
  while (heap.length > 0) {
    const cur = heap.pop();
    if (!cur || cur.f > dist[cur.i] + 1e-5) continue;
    const [x, y] = xyOf(cur.i);
    for (const [nx, ny, step] of neighbors4(x, y)) {
      const ni = indexOf(nx, ny);
      if (!cellSet.has(ni)) continue;
      const stepCost = naturalStepCost(cur.i, ni, cellClass, fields);
      if (stepCost >= INF) continue;
      const nd = cur.f + stepCost * step;
      if (nd < dist[ni]) {
        dist[ni] = nd;
        owner[ni] = cur.owner;
        heap.push({ i: ni, f: nd, owner: cur.owner });
      }
    }
  }
  const aCells = [], bCells = [];
  for (const ci of unit.cells) (owner[ci] === 1 ? bCells : aCells).push(ci);
  if (aCells.length < minPart || bCells.length < minPart) return null;
  unit.cells = aCells;
  for (const ci of bCells) compartmentId[ci] = newId;
  const newUnit = { id: newId, cells: bCells, centerIds: [], adjacent: new Map(), classId: unit.classId, dominantLandscapeClass: unit.dominantLandscapeClass };
  refreshCompartmentStats(unit, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse);
  refreshCompartmentStats(newUnit, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse);
  return newUnit;
}

function splitNaturalCompartmentByAxis(unit, newId, compartmentId, fields, seed) {
  if (!unit || unit.area < 20 || !unit.cells?.length) return null;
  const { elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse } = fields;
  const minPart = Math.max(7, Math.min(34, Math.floor(unit.area * 0.20)));
  const horizontal = (unit.width || 0) >= (unit.height || 0);
  const cx = unit.x || 0;
  const cy = unit.y || 0;
  const sorted = [...unit.cells].sort((a, b) => {
    const [ax, ay] = xyOf(a);
    const [bx, by] = xyOf(b);
    const av = (horizontal ? ax : ay) + hashSeededTie(ax, ay, seed) * 0.35 + Math.abs((horizontal ? ay - cy : ax - cx)) * 0.015;
    const bv = (horizontal ? bx : by) + hashSeededTie(bx, by, seed) * 0.35 + Math.abs((horizontal ? by - cy : bx - cx)) * 0.015;
    return av - bv;
  });
  const cut = Math.max(minPart, Math.min(sorted.length - minPart, Math.floor(sorted.length * 0.50)));
  if (cut <= 0 || sorted.length - cut < minPart) return null;
  const aCells = sorted.slice(0, cut);
  const bCells = sorted.slice(cut);
  unit.cells = aCells;
  unit.area = aCells.length;
  for (const ci of aCells) compartmentId[ci] = unit.id;
  for (const ci of bCells) compartmentId[ci] = newId;
  const newUnit = { id: newId, cells: bCells, area: bCells.length, centerIds: [], adjacent: new Map(), classId: unit.classId, dominantLandscapeClass: unit.dominantLandscapeClass };
  refreshCompartmentStats(unit, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse);
  refreshCompartmentStats(newUnit, elevation, slope, ridgeField, valleyField, basinField, coastalLowland, plain, agriculture, populationDensity, landuse);
  return newUnit;
}

function buildSeededNaturalCompartments(prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, crestCrossingScore = null, plain = null, agriculture = null, populationDensity = null, landuse = null, options = {}) {
  const progress = typeof options.progress === "function" ? options.progress : null;
  const naturalBarrierScore = buildNaturalBarrierScore(prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, crestCrossingScore, plain, agriculture, populationDensity, landuse);
  const cellClass = new Int16Array(SIZE);
  cellClass.fill(-1);
  for (let i = 0; i < SIZE; i++) if (prefectureMask[i] && !sea[i]) cellClass[i] = classifyLandscapeCell(i, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, plain, agriculture, populationDensity, landuse);
  const watershedId = options.watershedId || null;
  const fields = { elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, plain, agriculture, populationDensity, landuse, naturalBarrierScore, cellClass, watershedId };
  const landComponents = collectNaturalGrowthComponents(prefectureMask, sea, watershedId);
  const landArea = landComponents.reduce((sum, cells) => sum + cells.length, 0);
  const requestedTarget = options.targetCompartmentCount || clamp(Math.round(landArea / 34), 40, 360);
  const targetCount = clamp(Math.round(requestedTarget), Math.min(1, landArea), Math.max(1, Math.floor(landArea / 8)));
  const { seeds } = chooseNaturalCompartmentSeeds(landComponents, targetCount, fields, options.seed || 0);
  progress?.(`natural seeds chosen: ${seeds.length}/${targetCount}`);
  const compartmentId = new Int32Array(SIZE);
  compartmentId.fill(-1);
  const dist = new Float32Array(SIZE);
  dist.fill(INF);
  const heap = new MinHeap();
  seeds.forEach((i, id) => {
    compartmentId[i] = id;
    dist[i] = 0;
    heap.push({ i, f: 0, id });
  });
  while (heap.length > 0) {
    const cur = heap.pop();
    if (!cur || cur.f > dist[cur.i] + 1e-5) continue;
    const [x, y] = xyOf(cur.i);
    for (const [nx, ny, step] of neighbors4(x, y)) {
      const ni = indexOf(nx, ny);
      if (!prefectureMask[ni] || sea[ni]) continue;
      const stepCost = naturalStepCost(cur.i, ni, cellClass, fields);
      if (stepCost >= INF) continue;
      const nd = cur.f + stepCost * step;
      if (nd < dist[ni]) {
        dist[ni] = nd;
        compartmentId[ni] = cur.id;
        heap.push({ i: ni, f: nd, id: cur.id });
      }
    }
  }
  for (let i = 0; i < SIZE; i++) if (prefectureMask[i] && !sea[i] && compartmentId[i] < 0) compartmentId[i] = 0;
  progress?.("natural seeded growth complete");

  let compartments = buildUnitsFromAssignment(compartmentId, cellClass, prefectureMask, sea, fields);
  rebuildLandscapeUnitAdjacency(compartmentId, compartments, naturalBarrierScore, prefectureMask, sea);
  mergeTinyLandscapeUnits(compartmentId, compartments, 9);
  mergeWeakArtificialLandscapeUnits(compartmentId, compartments, fields, prefectureMask, sea, Math.max(42, Math.round(landArea / Math.max(1, targetCount) * 2.15)), 5);
  progress?.(`natural post-split units: ${compartments.filter((unit) => unit && unit.area > 0).length}`);
  splitDisconnectedCompartments(compartmentId, compartments, prefectureMask, sea);
  splitCompartmentsByWatershed(compartmentId, compartments, fields);
  compartments = renumberCompartments(compartmentId, compartments, prefectureMask, sea);
  refreshAllCompartmentStats(compartments, fields);
  rebuildLandscapeUnitAdjacency(compartmentId, compartments, naturalBarrierScore, prefectureMask, sea);
  progress?.(`natural pre-split units: ${compartments.filter((unit) => unit && unit.area > 0).length}`);

  const maxNaturalCompartmentArea = options.maxNaturalCompartmentArea || Math.max(28, Math.round(landArea / Math.max(1, targetCount) * 1.55));
  let guard = Math.max(60, targetCount * 2);
  while (guard-- > 0) {
    let active = compartments.filter((unit) => unit && unit.area > 0);
    const needMore = active.length < targetCount;
    const worst = active
      .filter((unit) => unit.area >= 20 && (unit._splitRejected || 0) < 3 && (needMore || unit.area > maxNaturalCompartmentArea * 1.18 || (unit.elongation || 1) > 4.2))
      .sort((a, b) => {
        const sa = (a.area / maxNaturalCompartmentArea) * 1.8 + Math.max(0, (a.elongation || 1) - 3.0) * 1.2;
        const sb = (b.area / maxNaturalCompartmentArea) * 1.8 + Math.max(0, (b.elongation || 1) - 3.0) * 1.2;
        return sb - sa;
      })[0];
    if (!worst) break;
    const newUnit = splitNaturalCompartmentCompact(worst, compartments.length, compartmentId, fields, (options.seed || 0) + guard * 97)
      || splitNaturalCompartmentByAxis(worst, compartments.length, compartmentId, fields, (options.seed || 0) + guard * 131);
    if (!newUnit) {
      worst._splitRejected = (worst._splitRejected || 0) + 1;
      if (worst._splitRejected > 2) worst.elongation = Math.min(worst.elongation || 1, 3.1);
      if (!needMore) break;
      continue;
    }
    compartments.push(newUnit);
    if (compartments.filter((unit) => unit && unit.area > 0).length >= targetCount && newUnit.area <= maxNaturalCompartmentArea) {
      const stillBad = compartments.some((unit) => unit && unit.area >= 20 && (
        unit.area > maxNaturalCompartmentArea * 1.35 ||
        ((unit.elongation || 1) > 4.2 && unit.area > 28)
      ));
      if (!stillBad) break;
    }
  }

  mergeWeakArtificialLandscapeUnits(compartmentId, compartments, fields, prefectureMask, sea, Math.max(48, Math.round(landArea / Math.max(1, targetCount) * 2.05)), 4);
  splitDisconnectedCompartments(compartmentId, compartments, prefectureMask, sea);
  splitCompartmentsByWatershed(compartmentId, compartments, fields);
  compartments = renumberCompartments(compartmentId, compartments, prefectureMask, sea);
  refreshAllCompartmentStats(compartments, fields);
  rebuildLandscapeUnitAdjacency(compartmentId, compartments, naturalBarrierScore, prefectureMask, sea);
  return { compartmentId, compartments, naturalBarrierScore };
}

export function buildNaturalCompartments(prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, crestCrossingScore = null, plain = null, agriculture = null, populationDensity = null, landuse = null, options = {}) {
  return buildSeededNaturalCompartments(prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, crestCrossingScore, plain, agriculture, populationDensity, landuse, options);
}

function buildLandscapeUnits(prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, plain, agriculture, populationDensity, landuse) {
  const unitId = new Int32Array(SIZE);
  unitId.fill(-1);
  const cellClass = new Int16Array(SIZE);
  cellClass.fill(-1);
  for (let i = 0; i < SIZE; i++) if (prefectureMask[i] && !sea[i]) cellClass[i] = classifyLandscapeCell(i, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, plain, agriculture, populationDensity, landuse);

  const units = [];
  const queue = [];
  for (let i = 0; i < SIZE; i++) {
    if (cellClass[i] < 0 || unitId[i] >= 0) continue;
    const id = units.length;
    const klass = cellClass[i];
    const cells = [];
    let sx = 0, sy = 0, pop = 0, urbanWeight = 0, ridgeExposure = 0, riverExposure = 0;
    queue.length = 0;
    queue.push(i);
    unitId[i] = id;
    for (let q = 0; q < queue.length; q++) {
      const cur = queue[q];
      const [x, y] = xyOf(cur);
      cells.push(cur);
      sx += x; sy += y; pop += populationDensity[cur];
      urbanWeight += urbanBoundaryPenalty(cur, populationDensity, landuse);
      ridgeExposure += ridgeField[cur];
      riverExposure += river[cur] + flowAccum[cur] * 0.45;
      for (const [nx, ny] of neighbors4(x, y)) {
        const ni = indexOf(nx, ny);
        if (unitId[ni] >= 0 || cellClass[ni] !== klass) continue;
        unitId[ni] = id;
        queue.push(ni);
      }
    }
    units.push({ id, classId: klass, cells, area: cells.length, x: sx / cells.length, y: sy / cells.length, population: pop, urbanWeight: urbanWeight / cells.length, ridgeExposure: ridgeExposure / cells.length, riverExposure: riverExposure / cells.length, adjacent: new Map(), centerIds: [], owner: -1 });
  }

  const targetScore = new Float32Array(SIZE);
  for (let i = 0; i < SIZE; i++) if (cellClass[i] >= 0) targetScore[i] = terrainBoundaryTargetScore(i, elevation, slope, river, ridgeField, valleyField, flowAccum, populationDensity, landuse);
  rebuildLandscapeUnitAdjacency(unitId, units, targetScore, prefectureMask, sea);
  mergeTinyLandscapeUnits(unitId, units, 10);
  rebuildLandscapeUnitAdjacency(unitId, units, targetScore, prefectureMask, sea);
  return { unitId, units, targetScore };
}

function rebuildLandscapeUnitAdjacency(unitId, units, targetScore, prefectureMask, sea) {
  for (const unit of units) unit.adjacent = new Map();
  for (let y = 0; y < MAP_H; y++) {
    for (let x = 0; x < MAP_W; x++) {
      const i = indexOf(x, y);
      if (!prefectureMask[i] || sea[i]) continue;
      const a = unitId[i];
      if (a < 0 || !units[a] || units[a].area === 0) continue;
      for (const [nx, ny] of [[x + 1, y], [x, y + 1]]) {
        if (!inside(nx, ny)) continue;
        const ni = indexOf(nx, ny);
        if (!prefectureMask[ni] || sea[ni]) continue;
        const b = unitId[ni];
        if (b < 0 || b === a || !units[b] || units[b].area === 0) continue;
        const v = (targetScore[i] + targetScore[ni]) * 0.5;
        const vertical = nx !== x ? 1 : 0;
        const keyA = units[a].adjacent.get(b) || { count: 0, target: 0, vertical: 0, horizontal: 0 };
        keyA.count++; keyA.target += v; if (vertical) keyA.vertical++; else keyA.horizontal++; units[a].adjacent.set(b, keyA);
        const keyB = units[b].adjacent.get(a) || { count: 0, target: 0, vertical: 0, horizontal: 0 };
        keyB.count++; keyB.target += v; if (vertical) keyB.vertical++; else keyB.horizontal++; units[b].adjacent.set(a, keyB);
      }
    }
  }
}

function mergeTinyLandscapeUnits(unitId, units, minArea = 10) {
  for (const unit of units) {
    if (unit.area === 0 || unit.area >= minArea) continue;
    let bestId = -1, bestScore = -INF;
    for (const [otherId, edge] of unit.adjacent) {
      const other = units[otherId];
      if (!other || other.area === 0) continue;
      const score = edge.count * 3 + (other.classId === unit.classId ? 8 : 0) + other.area * 0.01 - edge.target / Math.max(1, edge.count);
      if (score > bestScore) { bestScore = score; bestId = otherId; }
    }
    if (bestId < 0) continue;
    const target = units[bestId];
    for (const i of unit.cells) { unitId[i] = bestId; target.cells.push(i); }
    const totalArea = target.area + unit.area;
    target.x = (target.x * target.area + unit.x * unit.area) / totalArea;
    target.y = (target.y * target.area + unit.y * unit.area) / totalArea;
    target.population += unit.population;
    target.urbanWeight = (target.urbanWeight * target.area + unit.urbanWeight * unit.area) / totalArea;
    target.ridgeExposure = (target.ridgeExposure * target.area + unit.ridgeExposure * unit.area) / totalArea;
    target.riverExposure = (target.riverExposure * target.area + unit.riverExposure * unit.area) / totalArea;
    target.area = totalArea;
    unit.area = 0;
    unit.cells = [];
  }
}


function mergeUnitInto(unitId, units, fromId, toId, fields) {
  const from = units[fromId];
  const to = units[toId];
  if (!from || !to || from.area === 0 || to.area === 0 || fromId === toId) return false;
  for (const ci of from.cells) { unitId[ci] = toId; to.cells.push(ci); }
  from.area = 0;
  from.cells = [];
  refreshCompartmentStats(to, fields.elevation, fields.slope, fields.ridgeField, fields.valleyField, fields.basinField, fields.coastalLowland, fields.plain, fields.agriculture, fields.populationDensity, fields.landuse);
  return true;
}

function mergeWeakArtificialLandscapeUnits(unitId, units, fields, prefectureMask, sea, maxMergedArea = 84, passes = 5) {
  // Seeded graph growth can create diagonal stair-step borders in uniform plains
  // and gentle hills.  If the shared edge is weak, balanced H/V, and the two
  // sides are the same natural group, merge it instead of preserving an
  // artificial Voronoi-like cut.
  for (let pass = 0; pass < passes; pass++) {
    rebuildLandscapeUnitAdjacency(unitId, units, fields.naturalBarrierScore, prefectureMask, sea);
    let best = null;
    let bestScore = 0.0;
    for (const unit of units) {
      if (!unit || unit.area === 0) continue;
      for (const [otherId, edge] of unit.adjacent) {
        if (otherId <= unit.id) continue;
        const other = units[otherId];
        if (!other || other.area === 0) continue;
        const avgBarrier = edge.target / Math.max(1, edge.count);
        const sameClass = unit.classId === other.classId;
        const sameGroup = naturalGroupKey(unit) === naturalGroupKey(other);
        const combinedArea = unit.area + other.area;
        if (!sameClass && !sameGroup) continue;
        if (combinedArea > maxMergedArea && unit.area > 18 && other.area > 18) continue;
        const h = edge.horizontal || 0;
        const v = edge.vertical || 0;
        const balancedStair = Math.min(h, v) / Math.max(1, h + v);
        const lowlandContinuity = Math.min(unit.lowlandFitness || 0, other.lowlandFitness || 0);
        const mountainContinuity = Math.min(unit.mountainFitness || 0, other.mountainFitness || 0);
        const urbanGuard = Math.max(unit.urbanWeight || 0, other.urbanWeight || 0);
        const ridgeDivider = avgBarrier > 0.58 || Math.max(unit.ridgeExposure || 0, other.ridgeExposure || 0) > 0.62;
        const weakDivider = avgBarrier < (sameClass ? 0.40 : 0.30);
        const bothMountain = (unit.mountainFitness || 0) > 0.56 && (other.mountainFitness || 0) > 0.56;
        // Large highland units are visually important.  Do not erase their
        // internal subdivision just because two neighbouring cells share a class
        // inside the same watershed.
        if (bothMountain && combinedArea > Math.max(32, maxMergedArea * 0.72)) continue;
        if (!weakDivider || ridgeDivider) continue;
        const score =
          (sameClass ? 1.7 : 0) +
          (sameGroup ? 1.2 : 0) +
          balancedStair * 1.4 +
          edge.count * 0.018 +
          lowlandContinuity * 0.85 +
          mountainContinuity * 0.35 -
          avgBarrier * 4.8 -
          Math.max(0, combinedArea - maxMergedArea) * 0.020 -
          urbanGuard * 0.20;
        if (score > bestScore) best = { fromId: unit.area <= other.area ? unit.id : otherId, toId: unit.area <= other.area ? otherId : unit.id }, bestScore = score;
      }
    }
    if (!best) break;
    mergeUnitInto(unitId, units, best.fromId, best.toId, fields);
  }
}

function naturalOwnershipAffinity(unit, neighbor, edge) {
  const boundaryTarget = edge.target / Math.max(1, edge.count);
  const sameClass = unit.classId === neighbor.classId ? 1.0 : 0;
  const sameGroup = naturalGroupKey(unit) === naturalGroupKey(neighbor) ? 1.1 : 0;
  const bothUrban = unit.classId <= 3 && neighbor.classId <= 3;
  const bothCorridor = [5, 6, 7, 10].includes(unit.classId) && [5, 6, 7, 10].includes(neighbor.classId);
  const urbanContinuity = bothUrban ? 1.35 : (unit.urbanWeight + neighbor.urbanWeight) > 0.75 && Math.abs(unit.urbanWeight - neighbor.urbanWeight) < 0.35 ? 0.58 : 0;
  const ridgeExposure = Math.max(unit.ridgeExposure || 0, neighbor.ridgeExposure || 0);
  const hardRidgePenalty = boundaryTarget > 0.62 || ridgeExposure > 0.62 ? 2.4 : 0;
  const strongDividerPenalty = boundaryTarget * (edge.count > 2 ? 5.2 : 3.8) + ridgeExposure * 1.35 + hardRidgePenalty;
  return edge.count * 0.50 + sameClass + sameGroup + urbanContinuity + (bothCorridor ? 0.58 : 0) - strongDividerPenalty;
}

function compartmentCrossingCost(unit, neighbor, edge) {
  const boundaryScore = edge.target / Math.max(1, edge.count);
  const sameClass = unit.classId === neighbor.classId ? 1 : 0;
  const sameGroup = naturalGroupKey(unit) === naturalGroupKey(neighbor) ? 1 : 0;
  const lowlandContinuity = Math.min(unit.lowlandFitness || 0, neighbor.lowlandFitness || 0);
  const urbanContinuity = Math.min(unit.urbanWeight || 0, neighbor.urbanWeight || 0);
  const mountainPenalty = Math.max(unit.mountainFitness || 0, neighbor.mountainFitness || 0);
  const ridgePenalty = Math.max(unit.ridgeExposure || 0, neighbor.ridgeExposure || 0);
  return Math.max(0.18,
    1.0 +
    boundaryScore * 9.2 +
    mountainPenalty * 2.65 +
    ridgePenalty * 2.75 +
    (boundaryScore > 0.64 || ridgePenalty > 0.64 ? 5.5 : 0) -
    sameClass * 0.45 -
    sameGroup * 0.35 -
    lowlandContinuity * 0.98 -
    urbanContinuity * 0.64 -
    Math.min(1.0, edge.count / 12) * 0.25
  );
}

function enrichCompartmentsWithUnifiedGeography(compartments, options = {}) {
  const geo = options.geography || options || {};
  const fields = [
    ["habitability", geo.habitability],
    ["accessibility", geo.accessibility],
    ["centrality", geo.centrality],
    ["boundaryAvoidance", geo.boundaryAvoidance],
    ["adminBoundaryPreference", geo.adminBoundaryPreference],
    ["geographicBarrier", geo.geographicBarrier],
  ];
  if (!fields.some(([, field]) => field)) return false;
  for (const unit of compartments || []) {
    if (!unit || !unit.cells?.length) continue;
    for (const [name, field] of fields) {
      if (!field) continue;
      let sum = 0;
      for (const i of unit.cells) sum += field[i] || 0;
      unit[name] = sum / Math.max(1, unit.cells.length);
    }
  }
  return true;
}

function graphVoronoiCompartmentOwners(compartments, compartmentId, adminCenters, options = {}) {
  const owner = new Int16Array(compartments.length);
  const dist = new Float32Array(compartments.length);
  owner.fill(-1);
  dist.fill(INF);
  const heap = new MinHeap();
  for (let id = 0; id < adminCenters.length; id++) {
    const center = adminCenters[id];
    if (!center || !inside(center.x, center.y)) continue;
    const compIndex = compartmentId[indexOf(center.x, center.y)];
    const unit = compartments[compIndex];
    if (compIndex < 0 || !unit || unit.area === 0) continue;
    unit.centerIds.push(id);
    if (dist[compIndex] > 0) {
      dist[compIndex] = 0;
      owner[compIndex] = id;
      heap.push({ i: compIndex, f: 0, owner: id });
    }
  }
  while (heap.length > 0) {
    const cur = heap.pop();
    if (!cur || cur.f > dist[cur.i] + 1e-5) continue;
    const unit = compartments[cur.i];
    if (!unit || unit.area === 0) continue;
    const center = adminCenters[cur.owner];
    for (const [neighborId, edge] of unit.adjacent) {
      const neighbor = compartments[neighborId];
      if (!neighbor || neighbor.area === 0) continue;
      const crossing = compartmentCrossingCost(unit, neighbor, edge);
      const euclideanTie = center ? Math.hypot(neighbor.x - center.x, neighbor.y - center.y) * 0.006 : 0;
      const hinterlandDrag = (neighbor.mountainFitness || 0) > 0.64 && (neighbor.population || 0) < 6 ? 0.35 : 0;
      const ridgeExpansionDrag = Math.max(0, (neighbor.ridgeExposure || 0) - (unit.ridgeExposure || 0)) * 1.75 + (crossing > 9.0 ? 2.8 : 0);
      const next = cur.f + crossing + euclideanTie + hinterlandDrag + ridgeExpansionDrag;
      if (next + 1e-5 < dist[neighborId]) {
        dist[neighborId] = next;
        owner[neighborId] = cur.owner;
        heap.push({ i: neighborId, f: next, owner: cur.owner });
      } else if (Math.abs(next - dist[neighborId]) < 0.08 && owner[neighborId] >= 0) {
        const oldCenter = adminCenters[owner[neighborId]];
        const oldD = oldCenter ? Math.hypot(neighbor.x - oldCenter.x, neighbor.y - oldCenter.y) : INF;
        const newD = center ? Math.hypot(neighbor.x - center.x, neighbor.y - center.y) : INF;
        if (newD < oldD - 1.5 || (newD < oldD + 1.5 && cur.owner < owner[neighborId])) owner[neighborId] = cur.owner;
      }
    }
  }
  for (const unit of compartments) {
    if (!unit || unit.area === 0 || owner[unit.id] >= 0) continue;
    let bestOwner = -1, bestScore = INF;
    for (const [neighborId, edge] of unit.adjacent) {
      if (owner[neighborId] < 0) continue;
      const neighbor = compartments[neighborId];
      const score = compartmentCrossingCost(unit, neighbor, edge) + (neighbor?.area || 0) * -0.001;
      if (score < bestScore) { bestScore = score; bestOwner = owner[neighborId]; }
    }
    owner[unit.id] = bestOwner >= 0 ? bestOwner : 0;
  }
  return owner;
}

function compartmentMunicipalityMetrics(compartments, owner, targetMunicipalityCount = 0, targetCompartmentCount = 0) {
  const counts = new Map();
  let active = 0;
  for (const unit of compartments) {
    if (!unit || unit.area === 0) continue;
    active++;
    const id = owner[unit.id];
    if (id >= 0) counts.set(id, (counts.get(id) || 0) + 1);
  }
  const actual = counts.size;
  const singles = [...counts.values()].filter((value) => value === 1).length;
  return {
    targetMunicipalityCount,
    actualMunicipalityCount: actual,
    targetNaturalCompartmentCount: targetCompartmentCount,
    naturalCompartmentCount: active,
    compartmentCount: active,
    averageCompartmentsPerMunicipality: actual ? active / actual : 0,
    singleCompartmentMunicipalityRatio: actual ? singles / actual : 0,
  };
}

function averageFinalBorderBarrier(adminId, prefectureMask, sea, naturalBarrierScore) {
  let sum = 0;
  let count = 0;
  for (let y = 1; y < MAP_H - 1; y++) {
    for (let x = 1; x < MAP_W - 1; x++) {
      const i = indexOf(x, y);
      if (!prefectureMask[i] || sea[i] || adminId[i] < 0) continue;
      for (const [nx, ny] of [[x + 1, y], [x, y + 1]]) {
        const ni = indexOf(nx, ny);
        if (!prefectureMask[ni] || sea[ni] || adminId[ni] < 0 || adminId[ni] === adminId[i]) continue;
        sum += (naturalBarrierScore[i] + naturalBarrierScore[ni]) * 0.5;
        count++;
      }
    }
  }
  return count ? sum / count : 0;
}

function averageFinalBorderField(adminId, prefectureMask, sea, field) {
  if (!field) return 0;
  let sum = 0;
  let count = 0;
  for (let y = 1; y < MAP_H - 1; y++) {
    for (let x = 1; x < MAP_W - 1; x++) {
      const i = indexOf(x, y);
      if (!prefectureMask[i] || sea[i] || adminId[i] < 0) continue;
      for (const [nx, ny] of [[x + 1, y], [x, y + 1]]) {
        const ni = indexOf(nx, ny);
        if (!prefectureMask[ni] || sea[ni] || adminId[ni] < 0 || adminId[ni] === adminId[i]) continue;
        sum += ((field[i] || 0) + (field[ni] || 0)) * 0.5;
        count++;
      }
    }
  }
  return count ? sum / count : 0;
}

function assignCompartmentsToAdminOwners(compartments, compartmentId, adminCenters, naturalBarrierScore, prefectureMask, sea) {
  const owner = new Int16Array(compartments.length);
  owner.fill(-1);
  for (let id = 0; id < adminCenters.length; id++) {
    const center = adminCenters[id];
    if (!center || !inside(center.x, center.y)) continue;
    const compIndex = compartmentId[indexOf(center.x, center.y)];
    if (compIndex >= 0 && compartments[compIndex]?.area > 0) {
      const unit = compartments[compIndex];
      unit.centerIds.push(id);
      owner[compIndex] = id;
    }
  }

  for (let pass = 0; pass < compartments.length + 8; pass++) {
    let changed = 0;
    for (const unit of compartments) {
      if (!unit || unit.area === 0 || owner[unit.id] >= 0) continue;
      let bestOwner = -1;
      let bestScore = -INF;
      for (const [neighborId, edge] of unit.adjacent) {
        const neighborOwner = owner[neighborId];
        if (neighborOwner < 0) continue;
        const neighbor = compartments[neighborId];
        if (!neighbor || neighbor.area === 0) continue;
        const center = adminCenters[neighborOwner];
        const d = center ? Math.hypot(unit.x - center.x, unit.y - center.y) : 0;
        const score = naturalOwnershipAffinity(unit, neighbor, edge) - d * 0.006 + Math.min(0.9, Math.sqrt(Math.max(1, neighbor.area)) * 0.020);
        if (score > bestScore) { bestScore = score; bestOwner = neighborOwner; }
      }
      const accept = unit.classId <= 3 ? bestScore > -0.35 : unit.classId === 8 || unit.classId === 9 ? bestScore > -1.05 : bestScore > -0.70;
      if (bestOwner >= 0 && accept) {
        owner[unit.id] = bestOwner;
        changed++;
      }
    }
    if (changed === 0) break;
  }

  for (const unit of compartments) {
    if (!unit || unit.area === 0 || owner[unit.id] >= 0) continue;
    let bestId = -1;
    let bestScore = -INF;
    for (let id = 0; id < adminCenters.length; id++) {
      const center = adminCenters[id];
      if (!center || !inside(center.x, center.y)) continue;
      const centerComp = compartments[compartmentId[indexOf(center.x, center.y)]];
      const sameGroup = centerComp && naturalGroupKey(centerComp) === naturalGroupKey(unit) ? 2.3 : 0;
      const sameClass = centerComp && centerComp.classId === unit.classId ? 0.8 : 0;
      const urbanFit = unit.urbanWeight > 0.55 && centerComp?.urbanWeight > 0.55 ? 1.3 : 0;
      const d = Math.hypot(unit.x - center.x, unit.y - center.y);
      const score = sameGroup + sameClass + urbanFit - d * 0.020 - unit.ridgeExposure * 0.16;
      if (score > bestScore) { bestScore = score; bestId = id; }
    }
    owner[unit.id] = bestId >= 0 ? bestId : 0;
  }
  return owner;
}

export function extractCompartmentBorders(compartmentId, prefectureMask, sea) {
  const segments = [];
  for (let y = 0; y < MAP_H; y++) {
    for (let x = 0; x < MAP_W; x++) {
      const i = indexOf(x, y);
      if (!prefectureMask[i] || sea[i] || compartmentId[i] < 0) continue;
      const a = compartmentId[i];
      if (x + 1 < MAP_W) {
        const ni = indexOf(x + 1, y);
        if (prefectureMask[ni] && !sea[ni] && compartmentId[ni] >= 0 && compartmentId[ni] !== a) segments.push([[x + 1, y], [x + 1, y + 1]]);
      }
      if (y + 1 < MAP_H) {
        const ni = indexOf(x, y + 1);
        if (prefectureMask[ni] && !sea[ni] && compartmentId[ni] >= 0 && compartmentId[ni] !== a) segments.push([[x, y + 1], [x + 1, y + 1]]);
      }
    }
  }
  return segments;
}

export function assignAdminRegionsFromNaturalCompartments(prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, plain, agriculture, populationDensity, landuse, adminCenters = [], options = {}) {
  const progress = typeof options.progress === "function" ? options.progress : null;
  const sharedCompartments = options.naturalCompartmentId && options.naturalCompartments
    ? { compartmentId: options.naturalCompartmentId, compartments: options.naturalCompartments, naturalBarrierScore: options.naturalBarrierScore || ridgeField }
    : null;
  const { compartmentId, compartments, naturalBarrierScore } = sharedCompartments ||
    buildNaturalCompartments(prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, null, plain, agriculture, populationDensity, landuse, options);
  progress?.("natural compartments built");
  const adminId = new Int16Array(SIZE);
  adminId.fill(-1);
  const unifiedGeographyApplied = enrichCompartmentsWithUnifiedGeography(compartments, options);
  const owner = graphVoronoiCompartmentOwners(compartments, compartmentId, adminCenters, options);
  progress?.("natural compartments assigned");
  for (const unit of compartments) {
    const assigned = owner[unit.id];
    if (assigned < 0) continue;
    for (const i of unit.cells) adminId[i] = assigned;
  }
  for (let i = 0; i < SIZE; i++) {
    if (!prefectureMask[i] || sea[i] || adminId[i] >= 0) continue;
    const comp = compartments[compartmentId[i]];
    adminId[i] = comp && owner[comp.id] >= 0 ? owner[comp.id] : 0;
  }
  repairAdminTopology(adminId, prefectureMask, sea, adminCenters, naturalBarrierScore, populationDensity, landuse);
  progress?.("natural topology repaired");
  const activeCompartments = compartments.filter((unit) => unit.area > 0);
  const relationMetrics = compartmentMunicipalityMetrics(compartments, owner, options.targetMunicipalityCount || adminCenters.length, options.targetCompartmentCount || 0);
  return {
    adminId,
    compartmentId,
    compartments,
    naturalBarrierScore,
    debug: {
      ...relationMetrics,
      compartmentBorders: extractCompartmentBorders(compartmentId, prefectureMask, sea),
      averageCompartmentArea: activeCompartments.length ? activeCompartments.reduce((sum, unit) => sum + unit.area, 0) / activeCompartments.length : 0,
      maxCompartmentArea: activeCompartments.length ? Math.max(...activeCompartments.map((unit) => unit.area || 0)) : 0,
      maxCompartmentElongation: activeCompartments.length ? Math.max(...activeCompartments.map((unit) => unit.elongation || 1)) : 1,
      worstNaturalCompartments: activeCompartments
        .map((unit) => ({ id: unit.id, area: unit.area || 0, width: unit.width || 0, height: unit.height || 0, elongation: unit.elongation || 1, classId: unit.classId, x: Math.round(unit.x || 0), y: Math.round(unit.y || 0) }))
        .sort((a, b) => (b.elongation * Math.sqrt(Math.max(1, b.area))) - (a.elongation * Math.sqrt(Math.max(1, a.area))))
        .slice(0, 8),
      finalBorderNaturalBarrierAverage: averageFinalBorderBarrier(adminId, prefectureMask, sea, naturalBarrierScore),
      unifiedGeographyAppliedToAdminCompartments: unifiedGeographyApplied,
      finalBorderUnifiedBoundaryPreferenceAverage: averageFinalBorderField(adminId, prefectureMask, sea, options.adminBoundaryPreference || options.geography?.adminBoundaryPreference),
      finalBorderUnifiedBoundaryAvoidanceAverage: averageFinalBorderField(adminId, prefectureMask, sea, options.boundaryAvoidance || options.geography?.boundaryAvoidance),
      finalBorderUnifiedCentralityAverage: averageFinalBorderField(adminId, prefectureMask, sea, options.centrality || options.geography?.centrality),
      voronoiLikeRateBefore: 0,
      voronoiLikeRateAfter: weakVoronoiLikeRate(adminId, adminCenters, prefectureMask, sea, naturalBarrierScore),
    },
  };
}

function weakVoronoiLikeRate(adminId, adminCenters, prefectureMask, sea, naturalBarrierScore) {
  let weak = 0;
  let total = 0;
  for (let y = 1; y < MAP_H - 1; y++) {
    for (let x = 1; x < MAP_W - 1; x++) {
      const i = indexOf(x, y);
      if (!prefectureMask[i] || sea[i] || adminId[i] < 0) continue;
      for (const [nx, ny] of [[x + 1, y], [x, y + 1]]) {
        const ni = indexOf(nx, ny);
        const a = adminId[i], b = adminId[ni];
        if (!prefectureMask[ni] || sea[ni] || a < 0 || b < 0 || a === b) continue;
        total++;
        const ca = adminCenters[a], cb = adminCenters[b];
        if (!ca || !cb) continue;
        const mx = (x + nx) * 0.5, my = (y + ny) * 0.5;
        const nearBisector = Math.abs(Math.hypot(mx - ca.x, my - ca.y) - Math.hypot(mx - cb.x, my - cb.y)) < 4.0;
        if (nearBisector && (naturalBarrierScore[i] + naturalBarrierScore[ni]) * 0.5 < 0.38) weak++;
      }
    }
  }
  return total ? weak / total : 0;
}

export function applyLandscapeUnitAdminPartition(adminId, prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, plain, agriculture, populationDensity, landuse, adminCenters = []) {
  const before = new Int16Array(adminId);
  const initialNaturalBarrierScore = buildNaturalBarrierScore(prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, null, plain, agriculture, populationDensity, landuse);
  const beforeVoronoiLikeRate = weakVoronoiLikeRate(adminId, adminCenters, prefectureMask, sea, initialNaturalBarrierScore);
  const { compartmentId, compartments, naturalBarrierScore } = buildNaturalCompartments(prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, null, plain, agriculture, populationDensity, landuse);
  if (compartments.length === 0) return;
  for (let id = 0; id < adminCenters.length; id++) {
    const center = adminCenters[id];
    if (!center || !inside(center.x, center.y)) continue;
    const unit = compartments[compartmentId[indexOf(center.x, center.y)]];
    if (unit) unit.centerIds.push(id);
  }

  const owner = new Int16Array(compartments.length);
  owner.fill(-1);
  for (const unit of compartments) {
    if (unit.area === 0 || unit.centerIds.length === 0) continue;
    owner[unit.id] = unit.centerIds[0];
  }

  for (let pass = 0; pass < compartments.length + 4; pass++) {
    let changed = 0;
    for (const unit of compartments) {
      if (!unit || unit.area === 0 || owner[unit.id] >= 0) continue;
      let bestOwner = -1;
      let bestScore = -INF;
      for (const [neighborId, edge] of unit.adjacent) {
        const neighborOwner = owner[neighborId];
        if (neighborOwner < 0) continue;
        const neighbor = compartments[neighborId];
        if (!neighbor || neighbor.area === 0) continue;
        const score = naturalOwnershipAffinity(unit, neighbor, edge) + Math.min(0.8, Math.sqrt(Math.max(1, neighbor.area)) * 0.018);
        if (score > bestScore) { bestScore = score; bestOwner = neighborOwner; }
      }
      const accept = unit.classId <= 3 ? bestScore > -0.15 : unit.classId === 8 || unit.classId === 9 ? bestScore > -0.80 : bestScore > -0.45;
      if (bestOwner >= 0 && accept) { owner[unit.id] = bestOwner; changed++; }
    }
    if (changed === 0) break;
  }

  for (const unit of compartments) {
    if (!unit || unit.area === 0 || owner[unit.id] >= 0) continue;
    let bestId = -1, bestScore = -INF;
    for (let id = 0; id < adminCenters.length; id++) {
      const center = adminCenters[id];
      if (!center || !inside(center.x, center.y)) continue;
      const centerComp = compartments[compartmentId[indexOf(center.x, center.y)]];
      const sameGroup = centerComp && naturalGroupKey(centerComp) === naturalGroupKey(unit) ? 2.4 : 0;
      const sameClass = centerComp && centerComp.classId === unit.classId ? 0.9 : 0;
      const urbanFit = unit.urbanWeight > 0.55 && centerComp?.urbanWeight > 0.55 ? 1.2 : 0;
      const d = Math.hypot(unit.x - center.x, unit.y - center.y);
      const score = sameGroup + sameClass + urbanFit - d * 0.018 - unit.ridgeExposure * 0.18;
      if (score > bestScore) { bestScore = score; bestId = id; }
    }
    owner[unit.id] = bestId >= 0 ? bestId : 0;
  }

  for (const unit of compartments) {
    const assigned = owner[unit.id];
    if (assigned >= 0) for (const i of unit.cells) adminId[i] = assigned;
  }
  for (let i = 0; i < SIZE; i++) {
    if (!prefectureMask[i] || sea[i] || adminId[i] >= 0) continue;
    const comp = compartments[compartmentId[i]];
    adminId[i] = comp && owner[comp.id] >= 0 ? owner[comp.id] : 0;
  }
  for (let id = 0; id < adminCenters.length; id++) {
    const center = adminCenters[id];
    if (!center || !inside(center.x, center.y)) continue;
    for (let dy = -2; dy <= 2; dy++) for (let dx = -2; dx <= 2; dx++) {
      if (Math.hypot(dx, dy) > 2) continue;
      const x = center.x + dx, y = center.y + dy;
      if (!inside(x, y)) continue;
      const i = indexOf(x, y);
      if (prefectureMask[i] && !sea[i]) adminId[i] = id;
    }
  }
  repairAdminTopology(adminId, prefectureMask, sea, adminCenters, naturalBarrierScore, populationDensity, landuse);
  let changedCells = 0;
  for (let i = 0; i < SIZE; i++) if (prefectureMask[i] && !sea[i] && before[i] !== adminId[i]) changedCells++;
  const activeCompartments = compartments.filter((unit) => unit.area > 0);
  applyLandscapeUnitAdminPartition.lastDebug = {
    compartmentCount: activeCompartments.length,
    averageCompartmentArea: activeCompartments.length ? activeCompartments.reduce((sum, unit) => sum + unit.area, 0) / activeCompartments.length : 0,
    changedAfterNaturalCompartmentPartition: changedCells,
    finalBorderNaturalBarrierAverage: averageFinalBorderBarrier(adminId, prefectureMask, sea, naturalBarrierScore),
    voronoiLikeRateBefore: beforeVoronoiLikeRate,
    voronoiLikeRateAfter: weakVoronoiLikeRate(adminId, adminCenters, prefectureMask, sea, naturalBarrierScore),
  };
}

export function snapAdminBoundariesToTerrain(adminId, prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, flowAccum, populationDensity, landuse, adminCenters = [], protectedPoints = [], passes = 6) {
  const targetScore = new Float32Array(SIZE);
  for (let i = 0; i < SIZE; i++) if (prefectureMask[i] && !sea[i]) targetScore[i] = terrainBoundaryTargetScore(i, elevation, slope, river, ridgeField, valleyField, flowAccum, populationDensity, landuse);
  const protectedMask = buildAdminProtectedMask(adminId, prefectureMask, sea, adminCenters, protectedPoints, populationDensity, landuse);
  const band = buildBoundaryBand(adminId, prefectureMask, sea, 5);
  const adminIds = [...new Set([...adminId].filter((id) => id >= 0))];
  const centerDist = buildCenterDistanceFields(adminIds, adminCenters, prefectureMask, sea);
  let current = new Int16Array(adminId);

  for (let pass = 0; pass < passes; pass++) {
    const next = new Int16Array(current);
    let changed = 0;
    for (let y = 1; y < MAP_H - 1; y++) {
      for (let x = 1; x < MAP_W - 1; x++) {
        const i = indexOf(x, y);
        const own = current[i];
        if (!band[i] || protectedMask[i] || !prefectureMask[i] || sea[i] || own < 0) continue;
        if (!isAdminBoundaryCell(current, prefectureMask, sea, x, y, true)) continue;
        const candidates = new Set();
        for (const [nx, ny] of neighbors8(x, y)) {
          const ni = indexOf(nx, ny);
          if (prefectureMask[ni] && !sea[ni] && current[ni] >= 0 && current[ni] !== own) candidates.add(current[ni]);
        }
        if (candidates.size === 0) continue;
        const currentEnergy = localBoundaryEnergy(current, i, own, targetScore, centerDist, populationDensity, landuse, river, valleyField);
        let bestId = own, bestEnergy = currentEnergy;
        for (const candidate of candidates) {
          const candidateEnergy = localBoundaryEnergy(current, i, candidate, targetScore, centerDist, populationDensity, landuse, river, valleyField);
          const threshold = 0.18 + (targetScore[i] < 0.36 ? 0.16 : 0) + urbanBoundaryPenalty(i, populationDensity, landuse) * 0.25;
          if (bestEnergy - candidateEnergy > threshold) { bestEnergy = candidateEnergy; bestId = candidate; }
        }
        if (bestId !== own) { next[i] = bestId; changed++; }
      }
    }
    current = next;
    if (changed === 0) break;
  }
  adminId.set(current);
  repairAdminTopology(adminId, prefectureMask, sea, adminCenters, targetScore, populationDensity, landuse);
}

export function splitOversizedRuralMunicipalities(adminId, prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, plain, agriculture, populationDensity, landuse, adminCenters = [], settlements = []) {
  const before = new Int16Array(adminId);
  const area = new Map();
  const lowland = new Map();
  const mountain = new Map();
  for (let i = 0; i < SIZE; i++) {
    if (!prefectureMask[i] || sea[i] || adminId[i] < 0) continue;
    const id = adminId[i];
    area.set(id, (area.get(id) || 0) + 1);
    const living = (plain[i] || 0) * 0.42 + (agriculture[i] || 0) * 0.28 + basinField[i] * 0.20 + coastalLowland[i] * 0.20 + valleyField[i] * 0.12;
    const rough = ridgeField[i] * 0.54 + slope[i] * 0.36 + Math.max(0, elevation[i] - 0.58) * 0.38;
    lowland.set(id, (lowland.get(id) || 0) + living);
    mountain.set(id, (mountain.get(id) || 0) + rough);
  }
  const areas = [...area.values()].sort((a, b) => a - b);
  const median = areas.length ? areas[Math.floor(areas.length / 2)] : 0;
  if (!median) return { changedCells: 0, splitMunicipalities: 0 };

  const { compartmentId, compartments, naturalBarrierScore } = buildNaturalCompartments(prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, null, plain, agriculture, populationDensity, landuse);
  const unitOwner = new Int16Array(compartments.length);
  unitOwner.fill(-1);
  for (const unit of compartments) {
    if (!unit || unit.area === 0) continue;
    const counts = new Map();
    for (const i of unit.cells) {
      const id = adminId[i];
      if (id >= 0) counts.set(id, (counts.get(id) || 0) + 1);
    }
    let bestId = -1, best = -1;
    for (const [id, count] of counts) if (count > best) { best = count; bestId = id; }
    unitOwner[unit.id] = bestId;
  }

  const adminCenterIndex = new Map();
  for (let id = 0; id < adminCenters.length; id++) {
    const c = adminCenters[id];
    if (c && inside(c.x, c.y)) adminCenterIndex.set(id, indexOf(c.x, c.y));
  }

  let splitMunicipalities = 0;
  let rejectedMunicipalities = 0;
  for (const [id, cells] of area) {
    const averageLowland = (lowland.get(id) || 0) / cells;
    const averageMountain = (mountain.get(id) || 0) / cells;
    if (cells < median * 1.85 || averageLowland < 0.24 || averageMountain > 0.48) {
      if (cells >= median * 1.85) rejectedMunicipalities++;
      continue;
    }
    const localSettlements = settlements.filter((p) => p && inside(p.x, p.y) && adminId[indexOf(p.x, p.y)] === id);
    const meaningfulNodes = localSettlements.filter((p) => p.kind === "Satellite City" || p.kind === "New Town" || p.kind === "Market Town" || (p.population || 0) >= 30000);
    if (meaningfulNodes.length < 2) {
      rejectedMunicipalities++;
      continue;
    }
    let changedHere = 0;
    for (const unit of compartments) {
      if (!unit || unit.area === 0 || unitOwner[unit.id] !== id) continue;
      const centerIndex = adminCenterIndex.get(id);
      if (centerIndex >= 0 && unit.cells.includes(centerIndex)) continue;
      if (unit.classId === 8 || unit.classId === 9) continue;
      let bestNeighbor = -1;
      let bestScore = -INF;
      for (const [neighborId, edge] of unit.adjacent) {
        const neighborOwner = unitOwner[neighborId];
        if (neighborOwner < 0 || neighborOwner === id) continue;
        const boundaryTarget = edge.target / Math.max(1, edge.count);
        const neighbor = compartments[neighborId];
        const nodePull = meaningfulNodes.reduce((best, p) => Math.max(best, 1 / (1 + Math.hypot(p.x - unit.x, p.y - unit.y) / 6)), 0);
        const score = edge.count * 0.7 + boundaryTarget * 1.4 + nodePull * 1.2 - Math.max(0, (neighbor?.ridgeExposure || 0) - unit.ridgeExposure) * 0.35;
        if (score > bestScore) { bestScore = score; bestNeighbor = neighborOwner; }
      }
      if (bestNeighbor < 0 || bestScore < 2.2) continue;
      for (const ci of unit.cells) {
        if (adminId[ci] === id) {
          adminId[ci] = bestNeighbor;
          changedHere++;
        }
      }
    }
    if (changedHere > Math.max(28, cells * 0.035)) splitMunicipalities++;
  }
  repairAdminTopology(adminId, prefectureMask, sea, adminCenters, naturalBarrierScore, populationDensity, landuse);
  let changedCells = 0;
  for (let i = 0; i < SIZE; i++) if (prefectureMask[i] && !sea[i] && before[i] !== adminId[i]) changedCells++;
  return { changedCells, splitMunicipalities, rejectedMunicipalities };
}

export function splitOversizedLowlandMunicipalities(adminId, prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, plain, agriculture, populationDensity, landuse, adminCenters = [], settlements = []) {
  return splitOversizedRuralMunicipalities(adminId, prefectureMask, sea, elevation, slope, river, ridgeField, valleyField, basinField, coastalLowland, flowAccum, plain, agriculture, populationDensity, landuse, adminCenters, settlements);
}