/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

(function() {
"use strict";

if (!window.__firefox__) {
  Object.defineProperty(window, '__firefox__', {
    enumerable: false,
    configurable: false,
    writable: false,
    value: {}
  });
}

Object.defineProperty(window.__firefox__, 'NightMode', {
  enumerable: false,
  configurable: false,
  writable: false,
  value: { enabled: false }
});

const NIGHT_MODE_INVERT_FILTER_CSS = 'brightness(80%) invert(100%) hue-rotate(180deg)';

const NIGHT_MODE_STYLESHEET = `html {
  -webkit-filter: hue-rotate(180deg) invert(100%) !important;
}
img,video {
  -webkit-filter: ${NIGHT_MODE_INVERT_FILTER_CSS} !important;
}`;

var styleElement;

function getStyleElement() {
  if (styleElement) {
    return styleElement;
  }

  styleElement = document.createElement('style');
  styleElement.type = 'text/css';
  styleElement.appendChild(document.createTextNode(NIGHT_MODE_STYLESHEET));

  return styleElement;
}

function applyInvertFilterToChildBackgroundImageElements(parentNode) {
  parentNode.querySelectorAll('[style*="background"]').forEach(function(el) {
    if ((el.style.backgroundImage || '').startsWith('url')) {
      applyInvertFilterToElement(el);
    }
  });
}

function applyInvertFilterToElement(el) {
  invertedBackgroundImageElements.push(el);
  el.__firefox__NightMode_originalFilter = el.style.webkitFilter;
  el.style.webkitFilter = NIGHT_MODE_INVERT_FILTER_CSS;
}

function removeInvertFilterFromElement(el) {
  el.style.webkitFilter = el.__firefox__NightMode_originalFilter;
  delete el.__firefox__NightMode_originalFilter;
}

var invertedBackgroundImageElements = null;

// Create a `MutationObserver` that checks for new elements
// added that have a `background-image` in their `style`
// property/attribute.
var observer = new MutationObserver(function(mutations) {
  mutations.forEach(function(mutation) {
    mutation.addedNodes.forEach(function(node) {
      if (node.nodeType === Node.ELEMENT_NODE) {
        applyInvertFilterToChildBackgroundImageElements(node);
      }
    });
  });
});

Object.defineProperty(window.__firefox__.NightMode, 'setEnabled', {
  enumerable: false,
  configurable: false,
  writable: false,
  value: function(enabled) {
    if (enabled === window.__firefox__.NightMode.enabled) {
      return;
    }

    window.__firefox__.NightMode.enabled = enabled;

    var styleElement = getStyleElement();

    if (enabled) {
      invertedBackgroundImageElements = [];

      // Apply the NightMode CSS to the document.
      document.documentElement.appendChild(styleElement);

      // Add the "invert" CSS class name to all elements with a
      // `background-image` in their `style` property/attribute.
      applyInvertFilterToChildBackgroundImageElements(document);

      // Observe for future elements in the document containing
      // `background-image` in their `style` property/attribute
      // so that we can also apply the "invert" CSS class name
      // to them as they are added.
      observer.observe(document.documentElement, {
        childList: true,
        subtree: true
      });
      return;
    }

    // Stop observing for future elements in the document.
    observer.disconnect();

    // Remove the "invert" CSS class name from all elements
    // it was previously applied to.
    invertedBackgroundImageElements.forEach(removeInvertFilterFromElement);

    // Remove the NightMode CSS from the document.
    var styleElementParentNode = styleElement.parentNode;
    if (styleElementParentNode) {
      styleElementParentNode.removeChild(styleElement);
    }

    invertedBackgroundImageElements = null;
  }
});

})();