function HtmlElementsPlugin(locations) {
  this.locations = locations;
}

HtmlElementsPlugin.prototype.apply = function(compiler) {
  var self = this;
  compiler.plugin('compilation', function(compilation) {
    compilation.options.htmlElements = compilation.options.htmlElements || {};

    compilation.plugin('html-webpack-plugin-before-html-generation', function(htmlPluginData, callback) {
      const locations = self.locations;

      if (locations) {
        const publicPath = htmlPluginData.assets.publicPath;

        Object.getOwnPropertyNames(locations).forEach(function(loc) {
          compilation.options.htmlElements[loc] = getHtmlElementString(locations[loc], publicPath);
        });
      }


      callback(null, htmlPluginData);
    });
  });

};

const RE_ENDS_WITH_BS = /\/$/;

/**
 * Create an HTML tag with attributes from a map.
 *
 * Example:
 * createTag('link', { rel: "manifest", href: "/assets/manifest.json" })
 * // <link rel="manifest" href="/assets/manifest.json">
 * @param tagName The name of the tag
 * @param attrMap A Map of attribute names (keys) and their values.
 * @param publicPath a path to add to eh start of static asset url
 * @returns {string}
 */
function createTag(tagName, attrMap, publicPath) {
  publicPath = publicPath || '';

  // add trailing slash if we have a publicPath and it doesn't have one.
  if (publicPath && !RE_ENDS_WITH_BS.test(publicPath)) publicPath += '/';

  const attributes = Object.getOwnPropertyNames(attrMap)
    .filter(function(name) { return name[0] !== '='; } )
    .map(function(name) {
      var value = attrMap[name];

      if (publicPath) {
        // check if we have explicit instruction, use it if so (e.g: =herf: false)
        // if no instruction, use public path if it's href attribute.
        const usePublicPath = attrMap.hasOwnProperty('=' + name) ? !!attrMap['=' + name] : name === 'href';

        if (usePublicPath) {
          // remove a starting trailing slash if the value has one so we wont have //
          value = publicPath + (value[0] === '/' ? value.substr(1) : value);
        }
      }

      return name + '="' + value + '"';
    });

  return '<' + tagName + ' ' + attributes.join(' ') + '>';
}

/**
 * Returns a string representing all html elements defined in a data source.
 *
 * Example:
 *
 *    const ds = {
 *      link: [
 *        { rel: "apple-touch-icon", sizes: "57x57", href: "/assets/icon/apple-icon-57x57.png" }
 *      ],
 *      meta: [
 *        { name: "msapplication-TileColor", content: "#00bcd4" }
 *      ]
 *    }
 *
 * getHeadTags(ds);
 * // "<link rel="apple-touch-icon" sizes="57x57" href="/assets/icon/apple-icon-57x57.png">"
 *    "<meta name="msapplication-TileColor" content="#00bcd4">"
 *
 * @returns {string}
 */
function getHtmlElementString(dataSource, publicPath) {
  return Object.getOwnPropertyNames(dataSource)
    .map(function(name) {
      if (Array.isArray(dataSource[name])) {
        return dataSource[name].map(function(attrs) { return createTag(name, attrs, publicPath); } );
      } else {
        return [ createTag(name, dataSource[name], publicPath) ];
      }
    })
    .reduce(function(arr, curr) {
      return arr.concat(curr);
    }, [])
    .join('\n\t');
}
module.exports = HtmlElementsPlugin;
