/* 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/. */

const lazy = {};

ChromeUtils.defineESModuleGetters(lazy, {
  setTimeout: "resource://gre/modules/Timer.sys.mjs",
  clearTimeout: "resource://gre/modules/Timer.sys.mjs",
});

ChromeUtils.defineLazyGetter(lazy, "console", () => {
  return console.createInstance({
    maxLogLevelPref: "browser.translations.logLevel",
    prefix: "Translations",
  });
});

/**
 * Map the NodeFilter enums that are used by the TreeWalker into enums that make
 * sense for determining the status of the nodes for the TranslationsDocument process.
 * This aligns the meanings of the filtering for the translations process.
 */
const NodeStatus = {
  // This node is ready to translate as is.
  READY_TO_TRANSLATE: NodeFilter.FILTER_ACCEPT,

  // This node is a shadow host and needs to be subdivided further.
  SHADOW_HOST: NodeFilter.FILTER_ACCEPT,

  // This node contains too many block elements and needs to be subdivided further.
  SUBDIVIDE_FURTHER: NodeFilter.FILTER_SKIP,

  // This node should not be considered for translation.
  NOT_TRANSLATABLE: NodeFilter.FILTER_REJECT,
};

/**
 * @typedef {import("../translations").NodeVisibility} NodeVisibility
 * @typedef {(message: string) => Promise<string>} TranslationFunction
 */

/**
 * Create a translation cache with a limit. It implements a "least recently used" strategy
 * to remove old translations. After `#cacheExpirationMS` the cache will be emptied.
 * This cache is owned statically by the TranslationsChild. This means that it will be
 * re-used on page reloads if the origin of the site does not change.
 */
export class LRUCache {
  /** @type {Map<string, string>} */
  #htmlCache = new Map();
  /** @type {Map<string, string>} */
  #textCache = new Map();
  /** @type {string} */
  #fromLanguage;
  /** @type {string} */
  #toLanguage;

  /**
   * This limit is used twice, once for Text translations, and once for HTML translations.
   */
  #cacheLimit = 5_000;

  /**
   * This cache will self-destruct after 10 minutes.
   */
  #cacheExpirationMS = 10 * 60_000;

  /**
   * @param {string} fromLanguage
   * @param {string} toLanguage
   */
  constructor(fromLanguage, toLanguage) {
    this.#fromLanguage = fromLanguage;
    this.#toLanguage = toLanguage;
  }

  /**
   * @param {boolean} isHTML
   * @returns {boolean}
   */
  #getCache(isHTML) {
    return isHTML ? this.#htmlCache : this.#textCache;
  }

  /**
   * Get a translation if it exists from the cache, and move it to the end of the cache
   * to keep it alive longer.
   *
   * @param {string} sourceString
   * @param {boolean} isHTML
   * @returns {string}
   */
  get(sourceString, isHTML) {
    const cache = this.#getCache(isHTML);
    const targetString = cache.get(sourceString);

    if (targetString === undefined) {
      return undefined;
    }

    // Maps are ordered, move this item to the end of the list so it will stay
    // alive longer.
    cache.delete(sourceString);
    cache.set(sourceString, targetString);

    this.keepAlive();

    return targetString;
  }

  /**
   * @param {string} sourceString
   * @param {string} targetString
   * @param {boolean} isHTML
   */
  set(sourceString, targetString, isHTML) {
    const cache = this.#getCache(isHTML);
    if (cache.size === this.#cacheLimit) {
      // If the cache is at the limit, get the least recently used translation and
      // remove it. This works since Maps have keys ordered by insertion order.
      const key = cache.keys().next().value;
      cache.delete(key);
    }
    cache.set(sourceString, targetString);
    this.keepAlive();
  }

  /**
   * @param {string} fromLanguage
   * @param {string} toLanguage
   */
  matches(fromLanguage, toLanguage) {
    return (
      this.#fromLanguage === fromLanguage && this.#toLanguage === toLanguage
    );
  }

  /**
   * @type {number}
   */
  #timeoutId = 0;

  #pendingKeepAlive = false;

  /**
   * Clear out the cache on a timer.
   */
  keepAlive() {
    if (this.#timeoutId) {
      lazy.clearTimeout(this.#timeoutId);
    }
    if (!this.#pendingKeepAlive) {
      // Rather than continuously creating new functions in a tight loop, only schedule
      // one keepAlive timeout on the next tick.
      this.#pendingKeepAlive = true;

      lazy.setTimeout(() => {
        this.#pendingKeepAlive = false;
        this.#timeoutId = lazy.setTimeout(() => {
          this.#htmlCache = new Map();
          this.#textCache = new Map();
        }, this.#cacheExpirationMS);
      }, 0);
    }
  }
}

/**
 * How often the DOM is updated with translations, in milliseconds.
 */
const DOM_UPDATE_INTERVAL_MS = 50;

/**
 * These tags are excluded from translation.
 */
const EXCLUDED_TAGS = new Set([
  // The following are elements that semantically should not be translated.
  "CODE",
  "KBD",
  "SAMP",
  "VAR",
  "ACRONYM",

  // The following are deprecated tags.
  "DIR",
  "APPLET",

  // The following are embedded elements, and are not supported (yet).
  "MATH",
  "EMBED",
  "OBJECT",
  "IFRAME",

  // This is an SVG tag that can contain arbitrary XML, ignore it.
  "METADATA",

  // These are elements that are treated as opaque by Firefox which causes their
  // innerHTML property to be just the raw text node behind it. Any text that is sent as
  // HTML must be valid, and there is no guarantee that the innerHTML is valid.
  "NOSCRIPT",
  "NOEMBED",
  "NOFRAMES",

  // The title is handled separately, and a HEAD tag should not be considered.
  "HEAD",

  // These are not user-visible tags.
  "STYLE",
  "SCRIPT",
  "TEMPLATE",

  // Textarea elements contain user content, which should not be translated.
  "TEXTAREA",
]);

/**
 * Attributes to be translated
 */
const TRANSLATABLE_ATTRIBUTES = ["title", "placeholder"];

/**
 * Selector to get all the attributes
 *  ["[attribute1]", "[attribute2]", ...];
 */
const TRANSLATABLE_ATTRIBUTES_SELECTOR = TRANSLATABLE_ATTRIBUTES.map(
  attribute => "[" + attribute + "]"
);

/**
 * Options used by the mutation observer
 */
const MUTATION_OBSERVER_OPTIONS = {
  characterData: true,
  childList: true,
  subtree: true,
  attributes: true,
  attributeFilter: TRANSLATABLE_ATTRIBUTES,
};

/**
 * This class manages the process of translating the DOM from one language to another.
 * A translateHTML and a translateText function are injected into the constructor. This
 * class is responsible for subdividing a Node into small enough pieces to where it
 * contains a reasonable amount of text and inline elements for the translations engine
 * to translate. Once a node has been identified as a small enough chunk, its innerHTML
 * is read, and sent for translation. The async translation result comes back as an HTML
 * string. The DOM node is updated with the new text and potentially changed DOM ordering.
 *
 * This class also handles mutations of the DOM and will translate nodes as they are added
 * to the page, or the when the node's text is changed by content scripts.
 */
export class TranslationsDocument {
  /**
   * The BCP 47 language tag that is used on the page.
   *
    @type {string} */
  documentLanguage;

  /**
   * The timeout between the first translation received and the call to update the DOM
   * with translations.
   */
  #updateTimeout = null;
  #attributeUpdateTimeout = null;

  /**
   * The nodes that need translations. They are queued when the document tree is walked,
   * and then they are dispatched for translation based on their visibility. The viewport
   * nodes are given the highest priority.
   *
   * @type {Map<Node, NodeVisibility>}
   */
  #queuedNodes = new Map();

  /**
   * The nodes that need Attribute translations. They are queued when the document tree is walked,
   * and then they are dispatched for translation based on their visibility. The viewport
   * nodes are given the highest priority.
   *
   * @type  {Map<Node, { attributeList: string[], visibility: NodeVisibility }>}
   */
  #queuedAttributeNodes = new Map();

  /**
   * The count of how many pending translations have been sent to the translations
   * engine.
   */
  #pendingTranslationsCount = 0;

  /**
   * The list of nodes that need updating with the translated HTML. These are batched
   * into an update.
   *
   * @type {Set<{ node: Node, translatedHTML: string }>}
   */
  #nodesWithTranslatedHTML = new Set();

  /**
   * The list of nodes that need updating with the translated Attribute HTML. These are batched
   * into an update.
   *
   * @type {Set<{ node: Node, translation: string, attribute: string }>}
   */
  #nodesWithTranslatedAttributes = new Set();

  /**
   * The set of nodes that have been subdivided and processed for translation. They
   * should not be submitted again unless their contents have been changed.
   *
   * @type {WeakSet<Node>}
   */
  #processedNodes = new WeakSet();

  /**
   * All root elements we're trying to translate. This should be the `document.body`
   * and the the `title` element.
   *
   * @type {Set<Node>}
   */
  #rootNodes = new Set();

  /**
   * This promise gets resolved when the initial viewport translations are done.
   * This is a key user-visible performance metric. It represents what the user
   * actually sees.
   *
   * @type {Promise<void> | null}
   */
  viewportTranslated = null;

  isDestroyed = false;

  /**
   * Construct a new TranslationsDocument. It is tied to a specific Document and cannot
   * be re-used. The translation functions are injected since this class shouldn't
   * manage the life cycle of the translations engines.
   *
   * @param {Document} document
   * @param {string} documentLanguage - The BCP 47 tag of the source language.
   * @param {string} toLanguage - The BCP 47 tag of the destination language.
   * @param {number} innerWindowId - This is used for better profiler marker reporting.
   * @param {MessagePort} port - The port to the translations engine.
   * @param {() => void} requestNewPort - Used when an engine times out and a new
   *                                      translation request comes in.
   * @param {number} translationsStart
   * @param {() => number} now
   * @param {LRUCache} translationsCache
   */
  constructor(
    document,
    documentLanguage,
    toLanguage,
    innerWindowId,
    port,
    requestNewPort,
    translationsStart,
    now,
    translationsCache
  ) {
    /**
     * The language of the document. If elements are found that do not match this language,
     * then they are skipped.
     *
     * @type {string}
     */
    this.documentLanguage = documentLanguage;
    if (documentLanguage.length !== 2) {
      throw new Error(
        "Expected the document language to be a valid 2 letter BCP 47 language tag: " +
          documentLanguage
      );
    }
    if (toLanguage.length !== 2) {
      throw new Error(
        "Expected the destination language to be a valid 2 letter BCP 47 language tag: " +
          toLanguage
      );
    }

    /** @type {QueuedTranslator} */
    this.translator = new QueuedTranslator(port, requestNewPort);

    /** @type {number} */
    this.innerWindowId = innerWindowId;

    /** @type {DOMParser} */
    this.domParser = new document.ownerGlobal.DOMParser();

    /** @type {Document} */
    this.document = document;

    /** @type {LRUCache} */
    this.translationsCache = translationsCache;

    /**
     * This selector runs to find child nodes that should be excluded. It should be
     * basically the same implementation of `isExcludedNode`, but as a selector.
     *
     * @type {string}
     */
    this.excludedNodeSelector = [
      // Use: [lang|=value] to match language codes.
      //
      // Per: https://developer.mozilla.org/en-US/docs/Web/CSS/Attribute_selectors
      //
      // The elements with an attribute name of attr whose value can be exactly
      // value or can begin with value immediately followed by a hyphen, - (U+002D).
      // It is often used for language subcode matches.
      `[lang]:not([lang|="${this.documentLanguage}"])`,
      `[translate=no]`,
      `.notranslate`,
      `[contenteditable="true"]`,
      `[contenteditable=""]`,
      [...EXCLUDED_TAGS].join(","),
    ].join(",");

    this.observer = new document.ownerGlobal.MutationObserver(mutationsList => {
      for (const mutation of mutationsList) {
        switch (mutation.type) {
          case "childList":
            for (const node of mutation.addedNodes) {
              this.#processedNodes.delete(node);
              this.subdivideNodeForTranslations(node);
              if (node.nodeType === Node.ELEMENT_NODE) {
                this.translateAttributes(node);
              }
            }
            break;
          case "characterData":
            this.#processedNodes.delete(mutation);
            this.subdivideNodeForTranslations(mutation.target);
            break;
          case "attributes":
            this.queueAttributeNodeForTranslation(mutation.target, [
              mutation.attributeName,
            ]);
            this.dispatchQueuedAttributeTranslations();
            break;
          default:
            break;
        }
      }
    });

    this.document.addEventListener(
      "visibilitychange",
      this.handleVisibilityChange
    );

    this.addRootElement(document.querySelector("title"));
    this.addRootElement(document.body, true /* reportWordsInViewport */);

    this.viewportTranslated?.then(() => {
      ChromeUtils.addProfilerMarker(
        "TranslationsChild",
        { innerWindowId, startTime: now() },
        "Viewport translations"
      );
      ChromeUtils.addProfilerMarker(
        "TranslationsChild",
        { innerWindowId, startTime: translationsStart },
        "Time to first translation"
      );
    });

    document.documentElement.lang = toLanguage;

    lazy.console.log(
      "Beginning to translate.",
      // The defaultView may not be there on tests.
      document.defaultView?.location.href
    );
  }

  /**
   * Queue a node for translation of attributes.
   *
   * @param {Node} node
   * @param {Array<string>} attributeList
   */
  queueAttributeNodeForTranslation(node, attributeList) {
    /** @type {NodeVisibility} */
    let visibility = "out-of-viewport";
    if (isNodeHidden(node)) {
      visibility = "hidden";
    } else if (isNodeInViewport(node)) {
      visibility = "in-viewport";
    }
    this.#queuedAttributeNodes.set(node, { attributeList, visibility });
  }

  /**
   * Start and stop the translator as the page is shown. For instance, this will
   * transition into "hidden" when the user tabs away from a document.
   */
  handleVisibilityChange = () => {
    if (this.document.visibilityState === "visible") {
      this.translator.showPage();
    } else {
      ChromeUtils.addProfilerMarker(
        "Translations",
        { innerWindowId: this.innerWindowId },
        "Pausing translations and discarding the port"
      );
      this.translator.hidePage();
    }
  };

  /**
   * Remove any dangling event handlers.
   */
  destroy() {
    this.isDestroyed = true;
    this.translator.destroy();
    this.stopMutationObserver();
    this.document.removeEventListener(
      "visibilitychange",
      this.handleVisibilityChange
    );
  }

  /**
   * Helper function for adding a new root to the mutation
   * observer.
   *
   * @param {Node} root
   */
  observeNewRoot(root) {
    this.#rootNodes.add(root);
    this.observer.observe(root, MUTATION_OBSERVER_OPTIONS);
  }

  /**
   * This function finds all sub shadow trees of node and
   * add the ShadowRoot of those subtrees to the mutation
   * observer.
   */
  addShadowRootsToObserver(node) {
    const nodeIterator = node.ownerDocument.createTreeWalker(
      node,
      NodeFilter.SHOW_ELEMENT,
      function (node) {
        return node.openOrClosedShadowRoot
          ? NodeFilter.FILTER_ACCEPT
          : NodeFilter.FILTER_SKIP;
      }
    );
    let currentNode;
    while ((currentNode = nodeIterator.nextNode())) {
      // Only shadow hosts are accepted nodes
      const shadowRoot = currentNode.openOrClosedShadowRoot;
      this.observeNewRoot(shadowRoot);
      this.addShadowRootsToObserver(shadowRoot);
    }
  }

  /**
   * Add a new element to start translating. This root is tracked for mutations and
   * kept up to date with translations. This will be the body element and title tag
   * for the document.
   *
   * @param {Element} [node]
   */
  addRootElement(node) {
    if (!node) {
      return;
    }

    if (node.nodeType !== Node.ELEMENT_NODE) {
      // This node is not an element, do not add it.
      return;
    }

    if (this.#rootNodes.has(node)) {
      // Exclude nodes that are already targetted.
      return;
    }

    this.#rootNodes.add(node);

    let viewportNodeTranslations = this.subdivideNodeForTranslations(node);
    let viewportAttributeTranslations = this.translateAttributes(node);

    if (!this.viewportTranslated) {
      this.viewportTranslated = Promise.allSettled([
        ...(viewportNodeTranslations ?? []),
        ...(viewportAttributeTranslations ?? []),
      ]);
    }

    this.observer.observe(node, MUTATION_OBSERVER_OPTIONS);
    this.addShadowRootsToObserver(node);
  }

  /**
   * Add qualified nodes to queueNodeForTranslation by recursively walk
   * through the DOM tree of node, including elements in Shadow DOM.
   *
   * @param {Element} [node]
   */
  processSubdivide(node) {
    const nodeIterator = node.ownerDocument.createTreeWalker(
      node,
      NodeFilter.SHOW_ELEMENT | NodeFilter.SHOW_TEXT,
      this.determineTranslationStatusForUnprocessedNodes
    );

    // This iterator will contain each node that has been subdivided enough to
    // be translated.
    let currentNode;
    while ((currentNode = nodeIterator.nextNode())) {
      const shadowRoot = currentNode.openOrClosedShadowRoot;
      if (shadowRoot) {
        this.processSubdivide(shadowRoot);
      } else {
        this.queueNodeForTranslation(currentNode);
      }
    }
  }

  /**
   * Start walking down through a node's subtree and decide which nodes to queue for
   * translation. This first node could be the root nodes of the DOM, such as the
   * document body, or the title element, or it could be a mutation target.
   *
   * The nodes go through a process of subdivision until an appropriate sized chunk
   * of inline text can be found.
   *
   * @param {Node} node
   */
  subdivideNodeForTranslations(node) {
    if (!this.#rootNodes.has(node)) {
      // This is a non-root node, which means it came from a mutation observer.
      // This new node could be a host element for shadow tree
      const shadowRoot = node.openOrClosedShadowRoot;
      if (shadowRoot && !this.#rootNodes.has(shadowRoot)) {
        this.observeNewRoot(shadowRoot);
      } else {
        // Ensure that it is a valid node to translate by checking all of its ancestors.
        for (let parent of getAncestorsIterator(node)) {
          // Parent is ShadowRoot. We can stop here since this is
          // the top ancestor of the shadow tree.
          if (parent.containingShadowRoot == parent) {
            break;
          }
          if (
            this.determineTranslationStatus(parent) ===
            NodeStatus.NOT_TRANSLATABLE
          ) {
            return;
          }
        }
      }
    }

    switch (this.determineTranslationStatusForUnprocessedNodes(node)) {
      case NodeStatus.NOT_TRANSLATABLE:
        // This node is rejected as it shouldn't be translated.
        return;

      // SHADOW_HOST and READY_TO_TRANSLATE both map to FILTER_ACCEPT
      case NodeStatus.SHADOW_HOST:
      case NodeStatus.READY_TO_TRANSLATE:
        const shadowRoot = node.openOrClosedShadowRoot;
        if (shadowRoot) {
          this.processSubdivide(shadowRoot);
        } else {
          // This node is ready for translating, and doesn't need to be subdivided. There
          // is no reason to run the TreeWalker, it can be directly submitted for
          // translation.
          this.queueNodeForTranslation(node);
        }
        break;

      case NodeStatus.SUBDIVIDE_FURTHER:
        // This node may be translatable, but it needs to be subdivided into smaller
        // pieces. Create a TreeWalker to walk the subtree, and find the subtrees/nodes
        // that contain enough inline elements to send to be translated.
        this.processSubdivide(node);
        break;
    }

    if (node.nodeName === "BODY") {
      this.reportWordsInViewport();
    }
    this.dispatchQueuedTranslations();
  }

  /**
   * Get all the nodes which have selected attributes
   * from the node/document and queue them.
   * Call the translate function on these nodes
   *
   * @param {Node} node
   * @returns {Array<Promise<void>> | null}
   */
  translateAttributes(node) {
    const attributeList = getTranslatableAttributes(node);
    if (attributeList.length) {
      // Queue the root node if it has any attributes
      // Because querySelectorAll searches only child nodes.
      this.queueAttributeNodeForTranslation(node, attributeList);
    }
    // Get all attributes in child nodes at once
    const nodesWithTranslatableAttributes = node.querySelectorAll(
      TRANSLATABLE_ATTRIBUTES_SELECTOR
    );
    for (const node of nodesWithTranslatableAttributes) {
      const attributeList = getTranslatableAttributes(node);
      this.queueAttributeNodeForTranslation(node, attributeList);
    }
    return this.dispatchQueuedAttributeTranslations();
  }

  /**
   * Test whether this is an element we do not want to translate. These are things like
   * <code> elements, elements with a different "lang" attribute, and elements that
   * have a `translate=no` attribute.
   *
   * @param {Node} node
   */
  isExcludedNode(node) {
    // Property access be expensive, so destructure required properties so they are
    // not accessed multiple times.
    const { nodeType } = node;

    if (nodeType === Node.TEXT_NODE) {
      // Text nodes are never excluded.
      return false;
    }
    if (nodeType !== Node.ELEMENT_NODE) {
      // Only elements and and text nodes should be considered.
      return true;
    }

    const { nodeName } = node;

    if (
      EXCLUDED_TAGS.has(
        // SVG tags can be lowercased, so ensure everything is uppercased.
        nodeName.toUpperCase()
      )
    ) {
      // This is an excluded tag.
      return true;
    }

    if (!this.matchesDocumentLanguage(node)) {
      // Exclude nodes that don't match the fromLanguage.
      return true;
    }

    if (node.getAttribute("translate") === "no") {
      // This element has a translate="no" attribute.
      // https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/translate
      return true;
    }

    if (node.classList.contains("notranslate")) {
      // Google Translate skips translations if the classList contains "notranslate"
      // https://cloud.google.com/translate/troubleshooting
      return true;
    }

    if (node.isContentEditable) {
      // This field is editable, and so exclude it similar to the way that form input
      // fields are excluded.
      return true;
    }

    return false;
  }

  /**
   * Runs `determineTranslationStatus`, but only on unprocessed nodes.
   *
   * @param {Node} node
   * @returns {number} - One of the NodeStatus values.
   */
  determineTranslationStatusForUnprocessedNodes = node => {
    if (this.#processedNodes.has(node)) {
      // Skip nodes that have already been processed.
      return NodeStatus.NOT_TRANSLATABLE;
    }

    return this.determineTranslationStatus(node);
  };

  /**
   * Determines if a node should be submitted for translation, not translatable, or if
   * it should be subdivided further. It doesn't check if the node has already been
   * processed.
   *
   * The return result works as a TreeWalker NodeFilter as well.
   *
   * @param {Node} node
   * @returns {number} - One of the `NodeStatus` values. See that object
   *   for documentation. These values match the filters for the TreeWalker.
   *   These values also work as a `NodeFilter` value.
   */
  determineTranslationStatus(node) {
    if (node.openOrClosedShadowRoot) {
      return NodeStatus.SHADOW_HOST;
    }

    if (isNodeQueued(node, this.#queuedNodes)) {
      // This node or its parent was already queued, reject it.
      return NodeStatus.NOT_TRANSLATABLE;
    }

    if (this.isExcludedNode(node)) {
      // This is an explicitly excluded node.
      return NodeStatus.NOT_TRANSLATABLE;
    }

    if (node.textContent.trim().length === 0) {
      // Do not use subtrees that are empty of text. This textContent call is fairly
      // expensive.
      return !node.hasChildNodes()
        ? NodeStatus.NOT_TRANSLATABLE
        : NodeStatus.SUBDIVIDE_FURTHER;
    }

    if (nodeNeedsSubdividing(node)) {
      // Skip this node, and dig deeper into its tree to cut off smaller pieces
      // to translate. It is presumed to be a wrapper of block elements.
      return NodeStatus.SUBDIVIDE_FURTHER;
    }

    if (
      containsExcludedNode(node, this.excludedNodeSelector) &&
      !hasTextNodes(node)
    ) {
      // Skip this node, and dig deeper into its tree to cut off smaller pieces
      // to translate.
      return NodeStatus.SUBDIVIDE_FURTHER;
    }

    // This node can be treated as entire block to submit for translation.
    return NodeStatus.READY_TO_TRANSLATE;
  }

  /**
   * Queue a node for translation.
   *
   * @param {Node} node
   */
  queueNodeForTranslation(node) {
    /** @type {NodeVisibility} */
    let visibility = "out-of-viewport";
    if (isNodeHidden(node)) {
      visibility = "hidden";
    } else if (isNodeInViewport(node)) {
      visibility = "in-viewport";
    }

    this.#queuedNodes.set(node, visibility);
  }

  /**
   * Submit the translations giving priority to nodes in the viewport.
   *
   * @returns {Array<Promise<void>> | null}
   */
  dispatchQueuedTranslations() {
    let inViewportCounts = 0;
    let outOfViewportCounts = 0;
    let hiddenCounts = 0;

    let inViewportTranslations = null;
    if (!this.viewportTranslated) {
      inViewportTranslations = [];
    }

    for (const [node, visibility] of this.#queuedNodes) {
      if (visibility === "in-viewport") {
        inViewportCounts++;
        const promise = this.submitTranslation(node);
        if (inViewportTranslations) {
          inViewportTranslations.push(promise);
        }
      }
    }
    for (const [node, visibility] of this.#queuedNodes) {
      if (visibility === "out-of-viewport") {
        outOfViewportCounts++;
        this.submitTranslation(node);
      }
    }
    for (const [node, visibility] of this.#queuedNodes) {
      if (visibility === "hidden") {
        hiddenCounts++;
        this.submitTranslation(node);
      }
    }

    ChromeUtils.addProfilerMarker(
      "Translations",
      { innerWindowId: this.innerWindowId },
      `Translate ${this.#queuedNodes.size} nodes.\n\n` +
        `In viewport: ${inViewportCounts}\n` +
        `Out of viewport: ${outOfViewportCounts}\n` +
        `Hidden: ${hiddenCounts}\n`
    );

    this.#queuedNodes.clear();
    return inViewportTranslations;
  }

  /**
   * Submit the Attribute translations giving priority to nodes in the viewport.
   *
   * @returns {Array<Promise<void>> | null}
   */
  dispatchQueuedAttributeTranslations() {
    let inViewportCounts = 0;
    let outOfViewportCounts = 0;
    let hiddenCounts = 0;

    let inViewportTranslations = null;
    if (!this.viewportTranslated) {
      inViewportTranslations = [];
    }
    // Submit the nodes with attrbutes to be translated.
    for (const [node, { attributeList, visibility }] of this
      .#queuedAttributeNodes) {
      if (visibility === "in-viewport") {
        inViewportCounts++;
        const promise = this.submitAttributeTranslation(node, attributeList);
        if (inViewportTranslations) {
          inViewportTranslations.push(promise);
        }
      }
    }
    for (const [node, { attributeList, visibility }] of this
      .#queuedAttributeNodes) {
      if (visibility === "out-of-viewport") {
        outOfViewportCounts++;
        this.submitAttributeTranslation(node, attributeList);
      }
    }
    for (const [node, { attributeList, visibility }] of this
      .#queuedAttributeNodes) {
      if (visibility === "hidden") {
        hiddenCounts++;
        this.submitAttributeTranslation(node, attributeList);
      }
    }

    ChromeUtils.addProfilerMarker(
      "Attribute Translations",
      { innerWindowId: this.innerWindowId },
      `Attribute Translate ${this.#queuedAttributeNodes.size} nodes.\n\n` +
        `In viewport: ${inViewportCounts}\n` +
        `Out of viewport: ${outOfViewportCounts}\n` +
        `Hidden: ${hiddenCounts}\n`
    );

    this.#queuedAttributeNodes.clear();

    return inViewportTranslations;
  }

  /**
   * Submit a node for Attribute translation to the translations engine.
   *
   * @param {Node} node
   * @returns {Promise<void>}
   */
  async submitAttributeTranslation(node, attributeList) {
    if (node.nodeType === Node.ELEMENT_NODE) {
      for (const attribute of attributeList) {
        const text = node.getAttribute(attribute);

        if (text.trim().length === 0) {
          continue;
        }
        const translation = await this.maybeTranslate(
          node,
          text,
          false /*isHTML*/
        );
        if (translation != null) {
          this.scheduleNodeUpdateWithTranslationAttribute(
            node,
            translation,
            attribute
          );
        }
      }
    }
  }

  /**
   * Schedule a node to be updated with a translation.
   *
   * @param {Node} node
   * @param {string} translation
   */
  scheduleNodeUpdateWithTranslationAttribute(node, translation, attribute) {
    // Add the nodes to be populated with the next translation update.
    this.#nodesWithTranslatedAttributes.add({
      node,
      translation,
      attribute,
    });

    if (this.#pendingTranslationsCount === 0) {
      // No translations are pending, update the node.
      this.updateNodesWithTranslationsAttributes();
    } else if (!this.#attributeUpdateTimeout) {
      // Schedule an update.
      this.#attributeUpdateTimeout = lazy.setTimeout(
        this.updateNodesWithTranslationsAttributes.bind(this),
        DOM_UPDATE_INTERVAL_MS
      );
    } else {
      // An update has been previously scheduled, do nothing here.
    }
  }

  /**
   * This is called every `DOM_UPDATE_INTERVAL_MS` ms with translations
   * for attributes in the nodes.
   *
   * This function is called asynchronously, so nodes may already be dead. Before
   * accessing a node make sure and run `Cu.isDeadWrapper` to check that it is alive.
   */
  updateNodesWithTranslationsAttributes() {
    // Stop the mutations so that the updates won't trigger observations.

    this.pauseMutationObserverAndRun(() => {
      for (const { node, translation, attribute } of this
        .#nodesWithTranslatedAttributes) {
        if (Cu.isDeadWrapper(node)) {
          // The node is no longer alive.
          ChromeUtils.addProfilerMarker(
            "Translations",
            { innerWindowId: this.innerWindowId },
            "Node is no long alive."
          );
          continue;
        }
        // Update the attribute of the node with translated attribute
        if (attribute) {
          node.setAttribute(attribute, translation);
        }
      }
      this.#nodesWithTranslatedAttributes.clear();
      this.#attributeUpdateTimeout = null;
    });
  }

  /**
   * Record how many words were in the viewport, as this is the most important
   * user-visible translation content.
   */
  reportWordsInViewport() {
    if (
      // This promise gets created for the first dispatchQueuedTranslations
      this.viewportTranslated ||
      this.#queuedNodes.size === 0
    ) {
      return;
    }

    // TODO(Bug 1814195) - Add telemetry.
    // TODO(Bug 1820618) - This whitespace regex will not work in CJK-like languages.
    // This requires a segmenter for a proper implementation.

    const whitespace = /\s+/;
    let wordCount = 0;
    for (const [node, visibility] of this.#queuedNodes) {
      if (visibility === "in-viewport") {
        wordCount += node.textContent.trim().split(whitespace).length;
      }
    }

    const message = wordCount + " words are in the viewport.";
    lazy.console.log(message);
    ChromeUtils.addProfilerMarker(
      "Translations",
      { innerWindowId: this.innerWindowId },
      message
    );
  }

  /**
   * Submit a node for translation to the translations engine.
   *
   * @param {Node} node
   * @returns {Promise<void>}
   */
  async submitTranslation(node) {
    // Give each element an id that gets passed through the translation so it can be
    // reunited later on.
    if (node.nodeType === Node.ELEMENT_NODE) {
      node.querySelectorAll("*").forEach((el, i) => {
        el.dataset.mozTranslationsId = i;
      });
    }

    /** @type {string} */
    let text;
    /** @type {boolean} */
    let isHTML;

    if (node.nodeType === Node.ELEMENT_NODE) {
      text = node.innerHTML;
      isHTML = true;
    } else {
      text = node.textContent;
      isHTML = false;
    }

    if (text.trim().length === 0) {
      return;
    }

    // Mark this node as not to be translated again unless the contents are changed
    // (which the observer will pick up on)
    this.#processedNodes.add(node);
    const translatedHTML = await this.maybeTranslate(node, text, isHTML);
    if (translatedHTML != null) {
      this.scheduleNodeUpdateWithTranslation(node, translatedHTML);
    }
  }

  /**
   * A single function to update pendingTranslationsCount while
   * calling the translate function
   *
   * @param {Node} node
   * @param {string} text
   * @property {boolean} isHTML
   * @returns {Promise<string | null>}
   */
  async maybeTranslate(node, text, isHTML) {
    this.#pendingTranslationsCount++;
    try {
      let translation = this.translationsCache.get(text, isHTML);
      if (translation === undefined) {
        translation = await this.translator.translate(node, text, isHTML);
        this.translationsCache.set(text, translation, isHTML);
      }

      return translation;
    } catch (error) {
      lazy.console.log("Translation failed", error);
    } finally {
      this.#pendingTranslationsCount--;
    }
    return null;
  }

  /**
   * Start the mutation observer, for instance after applying the translations to the DOM.
   */
  startMutationObserver() {
    if (Cu.isDeadWrapper(this.observer)) {
      // This observer is no longer alive.
      return;
    }
    for (const node of this.#rootNodes) {
      if (Cu.isDeadWrapper(node)) {
        // This node is no longer alive.
        continue;
      }
      this.observer.observe(node, MUTATION_OBSERVER_OPTIONS);
    }
  }

  /**
   * Stop the mutation observer, for instance to apply the translations to the DOM.
   */
  stopMutationObserver() {
    // Was the window already destroyed?
    if (!Cu.isDeadWrapper(this.observer)) {
      this.observer.disconnect();
    }
  }

  /**
   * This is called every `DOM_UPDATE_INTERVAL_MS` ms with translations for nodes.
   *
   * This function is called asynchronously, so nodes may already be dead. Before
   * accessing a node make sure and run `Cu.isDeadWrapper` to check that it is alive.
   */
  updateNodesWithTranslations() {
    // Stop the mutations so that the updates won't trigger observations.
    this.pauseMutationObserverAndRun(() => {
      for (const { node, translatedHTML } of this.#nodesWithTranslatedHTML) {
        if (Cu.isDeadWrapper(node)) {
          // The node is no longer alive.
          ChromeUtils.addProfilerMarker(
            "Translations",
            { innerWindowId: this.innerWindowId },
            "Node is no long alive."
          );
          continue;
        }
        switch (node.nodeType) {
          case Node.TEXT_NODE: {
            if (translatedHTML.trim().length !== 0) {
              // Only update the node if there is new text.
              node.textContent = translatedHTML;
            }
            break;
          }
          case Node.ELEMENT_NODE: {
            // TODO (Bug 1820625) - This is slow compared to the original implementation
            // in the addon which set the innerHTML directly. We can't set the innerHTML
            // here, but perhaps there is another way to get back some of the performance.
            const translationsDocument = this.domParser.parseFromString(
              `<!DOCTYPE html><div>${translatedHTML}</div>`,
              "text/html"
            );
            updateElement(translationsDocument, node);
            break;
          }
        }
      }

      this.#nodesWithTranslatedHTML.clear();
      this.#updateTimeout = null;
    });
  }

  /**
   * Stop the mutations so that the updates of the translations
   * in the nodes won't trigger observations.
   *
   * @param {Function} run The function to update translations
   */
  pauseMutationObserverAndRun(run) {
    this.stopMutationObserver();
    run();
    this.startMutationObserver();
  }

  /**
   * Schedule a node to be updated with a translation.
   *
   * @param {Node} node
   * @param {string} translatedHTML
   */
  scheduleNodeUpdateWithTranslation(node, translatedHTML) {
    // Add the nodes to be populated with the next translation update.
    this.#nodesWithTranslatedHTML.add({ node, translatedHTML });

    if (this.#pendingTranslationsCount === 0) {
      // No translations are pending, update the node.
      this.updateNodesWithTranslations();
    } else if (!this.#updateTimeout) {
      // Schedule an update.
      this.#updateTimeout = lazy.setTimeout(
        this.updateNodesWithTranslations.bind(this),
        DOM_UPDATE_INTERVAL_MS
      );
    } else {
      // An update has been previously scheduled, do nothing here.
    }
  }

  /**
   * Check to see if a language matches the document language.
   *
   * @param {Node} node
   */
  matchesDocumentLanguage(node) {
    if (!node.lang) {
      // No `lang` was present, so assume it matches the language.
      return true;
    }

    // First, cheaply check if language tags match, without canonicalizing.
    if (langTagsMatch(this.documentLanguage, node.lang)) {
      return true;
    }

    try {
      // Make sure the local is in the canonical form, and check again. This function
      // throws, so don't trust that the language tags are formatting correctly.
      const [language] = Intl.getCanonicalLocales(node.lang);

      return langTagsMatch(this.documentLanguage, language);
    } catch (_error) {
      return false;
    }
  }
}

/**
 * Get the list of attributes that need to be translated
 * in a given node.
 *
 * @returns {Array<string>}
 */
function getTranslatableAttributes(node) {
  if (node.nodeType !== Node.ELEMENT_NODE) {
    return [];
  }
  return TRANSLATABLE_ATTRIBUTES.filter(attribute =>
    node.hasAttribute(attribute)
  );
}

/**
 * This function needs to be fairly fast since it's used on many nodes when iterating
 * over the DOM to find nodes to translate.
 *
 * @param {Text | HTMLElement} node
 */
function isNodeHidden(node) {
  /** @type {HTMLElement} */
  const element = getElementForStyle(node);
  if (!element) {
    throw new Error("Unable to find the Element to compute the style for node");
  }

  // This flushes the style, which is a performance cost.
  const style = element.ownerGlobal.getComputedStyle(element);
  return style.display === "none" || style.visibility === "hidden";
}

/**
 * This function cheaply checks that language tags match.
 *
 * @param {string} knownLanguage
 * @param {string} otherLanguage
 */
function langTagsMatch(knownLanguage, otherLanguage) {
  if (knownLanguage === otherLanguage) {
    // A simple direct match.
    return true;
  }
  if (knownLanguage.length !== 2) {
    throw new Error("Expected the knownLanguage to be of length 2.");
  }
  // Check if the language tags part match, e.g. "en" and "en-US".
  return (
    knownLanguage[0] === otherLanguage[0] &&
    knownLanguage[1] === otherLanguage[1] &&
    otherLanguage[2] === "-"
  );
}

/**
 * This function returns the correct element to determine the
 * style of node.
 *
 * @param {Node} node
  @returns {HTMLElement} */
function getElementForStyle(node) {
  if (node.nodeType != Node.TEXT_NODE) {
    return node;
  }

  if (node.parentElement) {
    return node.parentElement;
  }

  // For cases like text node where its parent is ShadowRoot,
  // we'd like to use flattenedTreeParentNode
  if (node.flattenedTreeParentNode) {
    return node.flattenedTreeParentNode;
  }

  // If the text node is not connected or doesn't have a frame.
  return null;
}

/**
 * This function runs when walking the DOM, which means it is a hot function. It runs
 * fairly fast even though it is computing the bounding box. This is all done in a tight
 * loop, and it is done on mutations. Care should be taken with reflows caused by
 * getBoundingClientRect, as this is a common performance issue.
 *
 * The following are the counts of how often this is run on a news site:
 *
 * Given:
 *  1573 DOM nodes
 *  504 Text nodes
 *  1069 Elements
 *
 * There were:
 *  209 calls to get this funcion.
 *
 * @param {Node} node
 */
function isNodeInViewport(node) {
  const window = node.ownerGlobal;
  const document = node.ownerDocument;

  /** @type {HTMLElement} */
  const element = getElementForStyle(node);
  if (!element) {
    throw new Error("Unable to find the Element to compute the style for node");
  }

  const rect = element.getBoundingClientRect();
  return (
    rect.top >= 0 &&
    rect.left >= 0 &&
    rect.bottom <=
      (window.innerHeight || document.documentElement.clientHeight) &&
    rect.right <= (window.innerWidth || document.documentElement.clientWidth)
  );
}

/**
 * Actually perform the update of the element with the translated node. This step
 * will detach all of the "live" nodes, and match them up in the correct order as provided
 * by the translations engine.
 *
 * @param {Document} translationsDocument
 * @param {Element} element
 * @returns {void}
 */
function updateElement(translationsDocument, element) {
  // This text should have the same layout as the target, but it's not completely
  // guaranteed since the content page could change at any time, and the translation process is async.
  //
  // The document has the following structure:
  //
  // <html>
  //   <head>
  //   <body>{translated content}</body>
  // </html>

  const originalHTML = element.innerHTML;

  /**
   * The Set of translation IDs for nodes that have been cloned.
   *
   * @type {Set<number>}
   */
  const clonedNodes = new Set();

  merge(element, translationsDocument.body.firstChild);

  /**
   * Merge the live tree with the translated tree by re-using elements from the live tree.
   *
   * @param {Node} liveTree
   * @param {Node} translatedTree
   */
  function merge(liveTree, translatedTree) {
    /** @type {Map<number, Element>} */
    const liveElementsById = new Map();

    /** @type {Array<Text>} */
    const liveTextNodes = [];

    // Remove all the nodes from the liveTree, and categorize them by Text node or
    // Element node.
    let node;
    while ((node = liveTree.firstChild)) {
      node.remove();

      if (node.nodeType === Node.ELEMENT_NODE) {
        liveElementsById.set(node.dataset.mozTranslationsId, node);
      } else if (node.nodeType === Node.TEXT_NODE) {
        liveTextNodes.push(node);
      }
    }

    // The translated tree dictates the order.
    const translatedNodes = [...translatedTree.childNodes];
    for (
      let translatedIndex = 0;
      translatedIndex < translatedNodes.length;
      translatedIndex++
    ) {
      const translatedNode = translatedNodes[translatedIndex];

      if (translatedNode.nodeType === Node.TEXT_NODE) {
        // Copy the translated text to the original Text node and re-append it.
        let liveTextNode = liveTextNodes.shift();

        if (liveTextNode) {
          liveTextNode.data = translatedNode.data;
        } else {
          liveTextNode = translatedNode;
        }

        liveTree.appendChild(liveTextNode);
      } else if (translatedNode.nodeType === Node.ELEMENT_NODE) {
        const translationsId = translatedNode.dataset.mozTranslationsId;
        // Element nodes try to use the already existing DOM nodes.

        // Find the element in the live tree that matches the one in the translated tree.
        let liveElement = liveElementsById.get(translationsId);

        if (!liveElement) {
          lazy.console.warn("Could not find a corresponding live element", {
            path: createNodePath(translatedNode, translationsDocument.body),
            translationsId,
            liveElementsById,
            translatedNode,
          });
          continue;
        }

        // Has this element already been added to the list? Then duplicate it and re-add
        // it as a clone. The Translations Engine can sometimes duplicate HTML.
        if (liveElement.parentNode) {
          liveElement = liveElement.cloneNode(true /* deep clone */);
          clonedNodes.add(translationsId);
          lazy.console.warn(
            "Cloning a node because it was already inserted earlier",
            {
              path: createNodePath(translatedNode, translationsDocument.body),
              translatedNode,
              liveElement,
            }
          );
        }

        if (isNodeTextEmpty(translatedNode)) {
          // The original node had text, but the one that came out of translation
          // didn't have any text. This scenario might be caused by one of two causes:
          //
          //   1) The element was duplicated by translation but then not given text
          //      content. This happens on Wikipedia articles for example.
          //
          //   2) The translator messed up and could not translate the text. This
          //      happens on YouTube in the language selector. In that case, having the
          //      original text is much better than no text at all.
          //
          // To make sure it is case 1 and not case 2 check whether this is the only occurrence.
          for (let i = 0; i < translatedNodes.length; i++) {
            if (translatedIndex === i) {
              // This is the current node, not a sibling.
              continue;
            }
            const sibling = translatedNodes[i];
            if (
              // Only consider other element nodes.
              sibling.nodeType === Node.ELEMENT_NODE &&
              // If the sibling's translationsId matches, then use the sibling's
              // node instead.
              translationsId === sibling.dataset.mozTranslationsId
            ) {
              // This is case 1 from above. Remove this element's original text nodes,
              // since a sibling text node now has all of the text nodes.
              removeTextNodes(liveElement);
            }
          }

          // Report this issue to the console.
          lazy.console.warn(
            "The translated element has no text even though the original did.",
            {
              path: createNodePath(translatedNode, translationsDocument.body),
              translatedNode,
              liveElement,
            }
          );
        } else if (!isNodeTextEmpty(liveElement)) {
          // There are still text nodes to find and update, recursively merge.
          merge(liveElement, translatedNode);
        }

        // Put the live node back in the live branch. But now t has been synced with the
        // translated text and order.
        liveTree.appendChild(liveElement);
      }
    }

    const unhandledElements = [...liveElementsById].filter(
      ([, element]) => !element.parentNode
    );

    if (unhandledElements.length) {
      lazy.console.warn(
        `${createNodePath(
          translatedTree,
          translationsDocument.body
        )} Not all nodes unified`,
        {
          unhandledElements,
          clonedNodes,
          originalHTML,
          translatedHTML: translationsDocument.body.innerHTML,
          liveTree: liveTree.outerHTML,
          translatedTree: translatedTree.outerHTML,
        }
      );
    }
  }
}

/**
 * For debug purposes, compute a string path to an element.
 *
 * e.g. "div/div#header/p.bold.string/a"
 *
 * @param {Node} node
 * @param {Node | null} root
 */
function createNodePath(node, root) {
  if (root === null) {
    root = node.ownerDocument.body;
  }
  let path =
    node.parentNode && node.parentNode !== root
      ? createNodePath(node.parentNode)
      : "";
  path += `/${node.nodeName}`;
  if (node.id) {
    path += `#${node.id}`;
  } else if (node.className) {
    for (const className of node.classList) {
      path += "." + className;
    }
  }
  return path;
}

/**
 * @param {Node} node
 * @returns {boolean}
 */
function isNodeTextEmpty(node) {
  if ("innerText" in node) {
    return node.innerText.trim().length === 0;
  }
  if (node.nodeType === Node.TEXT_NODE && node.nodeValue) {
    return node.nodeValue.trim().length === 0;
  }
  return true;
}

/**
 * @param {Node} node
 */
function removeTextNodes(node) {
  for (const child of node.childNodes) {
    switch (child.nodeType) {
      case Node.TEXT_NODE:
        node.removeChild(child);
        break;
      case Node.ELEMENT_NODE:
        removeTextNodes(child);
        break;
      default:
        break;
    }
  }
}

/**
 * Test whether any of the direct child text nodes of are non-whitespace
 * text nodes.
 *
 * For example:
 *   - `<p>test</p>`: yes
 *   - `<p> </p>`: no
 *   - `<p><b>test</b></p>`: no
 *
 * @param {Node} node
 * @returns {boolean}
 */
function hasTextNodes(node) {
  if (node.nodeType !== Node.ELEMENT_NODE) {
    // Only check element nodes.
    return false;
  }

  for (const child of node.childNodes) {
    if (child.nodeType === Node.TEXT_NODE) {
      if (child.textContent.trim() === "") {
        // This is just whitespace.
        continue;
      }
      // A text node with content was found.
      return true;
    }
  }

  // No text nodes were found.
  return false;
}

/**
 * Like `isExcludedNode` but looks at the full subtree. Used to see whether
 * we can submit a subtree, or whether we should split it into smaller
 * branches first to try to exclude more of the non-translatable content.
 *
 * @param {Node} node
 * @param {string} excludedNodeSelector
 * @returns {boolean}
 */
function containsExcludedNode(node, excludedNodeSelector) {
  return (
    node.nodeType === Node.ELEMENT_NODE &&
    node.querySelector(excludedNodeSelector)
  );
}

/**
 * Check if this node has already been queued to be translated. This can be because
 * the node is itself is queued, or its parent node is queued.
 *
 * @param {Node} node
 * @param {Map<Node, any>} queuedNodes
 * @returns {boolean}
 */
function isNodeQueued(node, queuedNodes) {
  if (queuedNodes.has(node)) {
    return true;
  }

  // If the immediate parent is the body, it is allowed.
  if (node.parentNode === node.ownerDocument.body) {
    return false;
  }

  // Accessing the parentNode is expensive here according to performance profilling. This
  // is due to XrayWrappers. Minimize reading attributes by storing a reference to the
  // `parentNode` in a named variable, rather than re-accessing it.
  let parentNode;
  let lastNode = node;
  while ((parentNode = lastNode.parentNode)) {
    if (queuedNodes.has(parentNode)) {
      return parentNode;
    }
    lastNode = parentNode;
  }

  return false;
}

/**
 * Reads the elements computed style and determines if the element is a block-like
 * element or not. Every element that lays out like a block should be sent in as one
 * cohesive unit to be translated.
 *
 * @param {Element} element
 */
function getIsBlockLike(element) {
  const win = element.ownerGlobal;
  if (element.namespaceURI === "http://www.w3.org/2000/svg") {
    // SVG elements will report as inline, but there is no block layout in SVG.
    // Treat every SVG element as being block so that every node will be subdivided.
    return true;
  }
  const { display } = win.getComputedStyle(element);
  return display !== "inline" && display !== "none";
}

/**
 * Determine if this element is an inline element or a block element. Inline elements
 * should be sent as a contiguous chunk of text, while block elements should be further
 * subdivided before sending them in for translation.
 *
 * @param {Node} node
 * @returns {boolean}
 */
function nodeNeedsSubdividing(node) {
  if (node.nodeType === Node.TEXT_NODE) {
    // Text nodes are fully subdivided.
    return false;
  }

  if (!getIsBlockLike(node)) {
    // This element is inline, or not displayed.
    return false;
  }

  for (let child of node.childNodes) {
    switch (child.nodeType) {
      case Node.TEXT_NODE:
        // Keep checking for more inline or text nodes.
        continue;
      case Node.ELEMENT_NODE: {
        if (getIsBlockLike(child)) {
          // This node is a block node, so it needs further subdividing.
          return true;
        }
        // Keep checking for more inline or text nodes.
        continue;
      }
      default:
        return true;
    }
  }
  return false;
}

/**
 * Returns an iterator of a node's ancestors.
 *
 * @param {Node} node
 * @returns {Generator<ParentNode>}
 */
function* getAncestorsIterator(node) {
  const document = node.ownerDocument;
  for (
    let parent = node.parentNode;
    parent && parent !== document.documentElement;
    parent = parent.parentNode
  ) {
    yield parent;
  }
}

/**
 * This contains all of the information needed to perform a translation request.
 *
 * @typedef {object} TranslationRequest
 * @property {Node} node
 * @property {string} sourceText
 * @property {boolean} isHTML
 * @property {Function} resolve
 * @property {Function} reject
 */

/**
 * When a page is hidden, mutations may occur in the DOM. It doesn't make sense to
 * translate those elements while the page is hidden, especially as it may bring
 * a translations engine back to life, which can be quite expensive. Queue those
 * messages here.
 */
class QueuedTranslator {
  /**
   * @type {MessagePort | null}
   */
  #port = null;

  /**
   * @type {() => void}
   */
  #actorRequestNewPort;

  /**
   * An id for each message sent. This is used to match up the request and response.
   */
  #nextMessageId = 0;

  /**
   * Tie together a message id to a resolved response.
   *
   * @type {Map<number, TranslationRequest>}
   */
  #requests = new Map();

  /**
   * If the translations are paused, they are queued here. This Map is ordered by
   * from oldest to newest requests with stale requests being removed.
   *
   * @type {Map<Node, TranslationRequest>}
   */
  #queue = new Map();

  /**
   * @type {"uninitialized" | "ready" | "error" | "closed"}
   */
  engineStatus = "uninitialized";

  /**
   * @param {MessagePort} port
   * @param {() => void} actorRequestNewPort
   */
  constructor(port, actorRequestNewPort) {
    this.#actorRequestNewPort = actorRequestNewPort;

    this.acquirePort(port);
  }

  /**
   * When an engine gets closed while still in use, a new one will need to be requested.
   *
   * @type {{ promise: Promise<void>, resolve: Function, reject: Function } | null}
   */
  #portRequest = null;

  /**
   * Keep track if the page is shown or hidden. When the page is hidden, no translations
   * will be posted to the translations engine.
   */
  #isPageShown = true;

  /**
   * Note when a new port is being requested so we don't re-request it.
   */
  showPage() {
    this.#isPageShown = true;
    if (this.#port) {
      throw new Error(
        "Attempting to show the page when there is already port available"
      );
    }
    if (this.#queue.size) {
      // There are queued translations, request a new port. After the port is retrieved
      // the pending queue will be processed.
      this.#requestNewPort();
    }
  }

  /**
   * Hide the page, and move any outstanding translation requests to a queue.
   */
  hidePage() {
    this.#isPageShown = false;
    this.discardPort();

    if (this.#requests.size) {
      lazy.console.log(
        "Pausing translations with pending translation requests."
      );
    }
    this.#moveRequestsToQueue();
  }

  /**
   * Request a new port. The port will come in via `acquirePort`, and then resolved
   * through the `this.#portRequest.resolve`.
   *
   * @returns {Promise<void>}
   */
  #requestNewPort() {
    if (this.#portRequest) {
      // A port was already requested.
      return this.#portRequest.promise;
    }

    const portRequest = { promise: null, resolve: null, reject: null };
    portRequest.promise = new Promise((resolve, reject) => {
      portRequest.resolve = resolve;
      portRequest.reject = reject;
    });

    this.#portRequest = portRequest;

    // Send a request through the actor for a new port. The request response will
    // trigger the method `QueuedTranslator.prototype.acquirePort`
    this.#actorRequestNewPort();

    this.#portRequest.promise
      .then(
        () => {
          this.#portRequest = null;

          // Resume the queued translations.
          if (this.#queue.size) {
            lazy.console.log(
              `Resuming ${
                this.#queue.size
              } translations from the pending translation queue.`
            );

            const oldQueue = this.#queue;
            this.#queue = new Map();
            this.#repostTranslations(oldQueue);
          }
        },
        error => {
          lazy.console.error(error);
        }
      )
      .finally(() => {
        this.#portRequest = null;
      });

    return portRequest.promise;
  }

  /**
   * Send a request to translate text to the Translations Engine. If it returns `null`
   * then the request is stale. A rejection means there was an error in the translation.
   * This request may be queued.
   *
   * @param {Node} node
   * @param {string} sourceText
   * @param {boolean} isHTML
   */
  async translate(node, sourceText, isHTML) {
    if (this.#isPageShown && !this.#port) {
      try {
        await this.#requestNewPort();
      } catch {}
    }

    // At this point we don't know if the page is still shown, or if the attempt
    // to get a port was successful so check again.

    if (!this.#isPageShown || !this.#port) {
      // Queue the request while the page isn't shown.
      return new Promise((resolve, reject) => {
        const previousRequest = this.#queue.get(node);
        if (previousRequest) {
          // Previous requests get resolved as null, as this new one will replace it.
          previousRequest.resolve(null);
          // Delete the entry so that the order of the queue is maintained. The
          // new request will be put on the end.
          this.#queue.delete(node);
        }

        // This Promises's resolve and reject will be chained after the translation
        // request. For now add it to the queue along with the other arguments.
        this.#queue.set(node, { node, sourceText, isHTML, resolve, reject });
      });
    }

    return this.#postTranslationRequest(node, sourceText, isHTML);
  }

  /**
   * Posts the translation to the translations engine through the MessagePort.
   *
   * @param {Node} node
   * @param {string} sourceText
   * @param {boolean} isHTML
   * @returns {{ translateText: TranslationFunction, translateHTML: TranslationFunction}}
   */
  #postTranslationRequest(node, sourceText, isHTML) {
    return new Promise((resolve, reject) => {
      const messageId = this.#nextMessageId++;
      // Store the "resolve" for the promise. It will be matched back up with the
      // `messageId` in #handlePortMessage.
      this.#requests.set(messageId, {
        node,
        sourceText,
        isHTML,
        resolve,
        reject,
      });
      this.#port.postMessage({
        type: "TranslationsPort:TranslationRequest",
        messageId,
        sourceText,
        isHTML,
      });
    });
  }

  /**
   * Close the port and move any pending translations onto a queue.
   */
  discardPort() {
    if (this.#port) {
      this.#port.postMessage({ type: "TranslationsPort:DiscardTranslations" });
      this.#port.close();
      this.#port = null;
    }
    this.#moveRequestsToQueue();
    this.engineStatus = "uninitialized";
  }

  /**
   * Move any unfulfilled requests to the queue so they can be sent again when
   * the page is active again.
   */
  #moveRequestsToQueue() {
    if (this.#requests.size) {
      for (const request of this.#requests.values()) {
        this.#queue.set(request.node, request);
      }
      this.#requests = new Map();
    }
  }

  /**
   * Acquires a port, checks on the engine status, and then starts or resumes
   * translations.
   *
   * @param {MessagePort} port
   */
  acquirePort(port) {
    if (this.#port) {
      if (this.engineStatus === "ready") {
        lazy.console.error(
          "Received a new translation port while one already existed."
        );
      }
      this.discardPort();
    }

    this.#port = port;

    const portRequest = this.#portRequest;

    // Match up a response on the port to message that was sent.
    port.onmessage = ({ data }) => {
      switch (data.type) {
        case "TranslationsPort:TranslationResponse": {
          const { targetText, messageId } = data;
          // A request may not match match a messageId if there is a race during the pausing
          // and discarding of the queue.
          this.#requests.get(messageId)?.resolve(targetText);
          this.#requests.delete(messageId);
          break;
        }
        case "TranslationsPort:GetEngineStatusResponse": {
          if (portRequest) {
            const { resolve, reject } = portRequest;
            if (data.status === "ready") {
              resolve();
            } else {
              reject(new Error("The engine failed to load."));
            }
          }
          this.engineStatus = data.status;
          break;
        }
        case "TranslationsPort:EngineTerminated": {
          // The engine was terminated, and if a translation is needed a new port
          // will need to be requested.
          this.engineStatus = "closed";
          this.discardPort();
          if (this.#queue.size && this.#isPageShown) {
            this.#requestNewPort();
          }
          break;
        }
        default:
          lazy.console.error("Unknown translations port message: " + data.type);
          break;
      }
    };

    port.postMessage({ type: "TranslationsPort:GetEngineStatusRequest" });
  }

  /**
   * Re-send a list of translation requests.
   *
   * @param {Map<any, TranslationRequest>} mappedRequests
   *  This is either the this.#queue or this.#requests.
   */
  #repostTranslations(mappedRequests) {
    for (const value of mappedRequests.values()) {
      const { node, sourceText, isHTML, resolve, reject } = value;
      if (Cu.isDeadWrapper(node)) {
        // If the node is dead, resolve without any text. Do not reject as that
        // will be treated as an error.
        resolve(null);
      } else {
        this.#postTranslationRequest(node, sourceText, isHTML).then(
          resolve,
          reject
        );
      }
    }
  }

  /**
   * Close the port and remove any pending or queued requests.
   */
  destroy() {
    this.#port.close();
    this.#requests = new Map();
    this.#queue = new Map();
  }
}