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) ?
                              // Create a link wrapping the text node.
                              createLinkNode(filteredDocs[0], word) :
                              // this is just text so push a new text node
                              {type: 'text', value: word};
                        });

                // 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}]
    };
  }
};