// used to retrieve template content
const templates = new WeakMap;

// used to retrieve node updates
const updates = new WeakMap;

// hyperHTML, the nitty gritty
function hyperHTML(chunks, ...interpolations) {

  // if the static chunks are unknown
  // (once per application)
  if (!templates.has(chunks)) {
    // use modern HTMLTemplateElement ...
    const template = document.createElement('template');
    // ... to reliably inject content joined by a unique comment
    template.innerHTML = chunks.join('<!--⚡️-->');
    // associate unique chunks to ...
    templates.set(chunks, {
      // ... the DocumentFragment within the template ...
      content: template.content,
      // ... and a list of paths to retrieve nodes to update
      paths: [].filter.call(
        template.content.childNodes,
        // consider only comments with the unique content
        node => node.nodeType === Node.COMMENT_NODE &&
                node.textContent === '⚡️'
      ).map(node => {
        // a comment is no layout, but it's handy to have it around
        // what we want though, is a text node to show our content
        node = node.parentNode.insertBefore(
          node.ownerDocument.createTextNode(''),
          node
        );
        // we can now create a path to find the node
        // which is used once on first template setup
        // [0, 1] for <p>Hello <!--⚡️--></p>
        const path = [];
        do {
          path.unshift(path.indexOf.call(node.parentNode.childNodes, node));
          node = node.parentNode;
        } while(node !== template.content);
        return path;
      })
    });
  }

  // if the current context has no updates available ...
  // (once per node that uses this specific template)
  if (!updates.has(this) || updates.get(this).chunks !== chunks) {
    // ... grab related info
    const info = templates.get(chunks);
    // ... clean up the node
    this.textContent = '';
    // ... append a deep clone of the template content ...
    this.appendChild(info.content.cloneNode(true));
    // ... and save details for the next update
    updates.set(this, {
      // used to check the node is rendering the expected template
      chunks,
      // find all nodes that need to be updated
      // save it as callback to invoke per each update
      fns: info.paths.map(path => {
        const node = path.reduce((p, i) => p.childNodes[i], this);
        return text => node.textContent = text;
      })
    });
  }

  // we are now sure the template is known
  // the content of the context is the epxected one
  // and there is a list of updates to invoke
  updates.get(this).fns.forEach((fn, i) => fn(interpolations[i]));
  return this;
}

// example
// bind a node to hyperHTML
const render = hyperHTML.bind(document.body);

// update at speed light its content
(function showTime() {
  requestAnimationFrame(showTime);
  render`about the time ${
    (new Date).toISOString().replace(/^.+T|Z$/g, '')
  }`;
}());