text: const visit = require('unist-util-visit-parents');
  const is = require('hast-util-is-element');
  const textContent = require('hast-util-to-string');
  
  /**
   * Automatically add in a link to the relevant document for code blocks.
   * E.g. `<code>MyClass</code>` becomes `<code><a href="path/to/myclass">MyClass</a></code>`
   *
   * @property docTypes an array of strings.
   * Only docs that have one of these docTypes will be linked to.
   * Usually set to the API exported docTypes, e.g. "class", "function", "directive", etc.
   *
   * @property customFilters array of functions `(docs, words, wordIndex) => docs` that will filter
   * out docs where a word should not link to a doc.
   *   - `docs` is the array of docs that match the link `word`
   *   - `words` is the collection of words parsed from the code text
   *   - `wordIndex` is the index of the current `word` for which we are finding a link
   *
   * @property codeElements an array of strings.
   * Only text contained in these elements will be linked to.
   * Usually set to "code" but also "code-example" for angular.io.
   */
  module.exports = function autoLinkCode(getDocFromAlias) {
    autoLinkCodeImpl.docTypes = [];
    autoLinkCodeImpl.customFilters = [];
    autoLinkCodeImpl.codeElements = ['code'];
    return autoLinkCodeImpl;
  
    function autoLinkCodeImpl()  {
      return (ast) => {
        visit(ast, 'element', (node, ancestors) => {
          // Only interested in code elements that are not inside links
          if (autoLinkCodeImpl.codeElements.some(elementType =>
              is(node, elementType)) &&
              (!node.properties.className || node.properties.className.indexOf('no-auto-link') === -1) &&
              ancestors.every(ancestor => !is(ancestor, 'a'))) {
            visit(node, 'text', (node, ancestors) => {
              // Only interested in text nodes that are not inside links
              if (ancestors.every(ancestor => !is(ancestor, 'a'))) {
  
                const parent = ancestors[ancestors.length - 1];
                const index = parent.children.indexOf(node);
  
                // Can we convert the whole text node into a doc link?
                const docs = getDocFromAlias(node.value);
                if (foundValidDoc(docs)) {
                  parent.children.splice(index, 1, createLinkNode(docs[0], node.value));
                } else {
                  // Parse the text for words that we can convert to links
                  const nodes = textContent(node).split(/([A-Za-z0-9_.-]+)/)
                    .filter(word => word.length)
                    .map((word, index, words) => {
                      // remove docs that fail the custom filter tests
                      const filteredDocs = autoLinkCodeImpl.customFilters.reduce((docs, filter) => filter(docs, words, index), getDocFromAlias(word));
                      return foundValidDoc(filteredDocs) ?
                                createLinkNode(filteredDocs[0], word) : // Create a link wrapping the text node.
                                { type: 'text', value: word };  // this is just text so push a new text node
                    });
  
                  // Replace the text node with the links and leftover text nodes
                  Array.prototype.splice.apply(parent.children, [index, 1].concat(nodes));
                }
              }
            });
          }
        });
      };
    }
  
    function foundValidDoc(docs) {
      return docs.length === 1 &&
             !docs[0].internal &&
             autoLinkCodeImpl.docTypes.indexOf(docs[0].docType) !== -1;
    }
  
    function createLinkNode(doc, text) {
      return {
        type: 'element',
        tagName: 'a',
        properties: { href: doc.path, class: 'code-anchor' },
        children: [{ type: 'text', value: text }]
      };
    }
  };
  