class Island extends HTMLElement {
  static tagName = "is-land";
  static prefix = "is-land--";
  static attr = {
    template: "data-island",
    ready: "ready",
    defer: "defer-hydration",
  };

  static onceCache = new Map();
  static onReady = new Map();

  static fallback = {
    ":not(is-land,:defined,[defer-hydration])": (readyPromise, node, prefix) => {
      // remove from document to prevent web component init
      let cloned = document.createElement(prefix + node.localName);
      for(let attr of node.getAttributeNames()) {
        cloned.setAttribute(attr, node.getAttribute(attr));
      }
    
      // Declarative Shadow DOM (with polyfill)
      let shadowroot = node.shadowRoot;
      if(!shadowroot) {
        let tmpl = node.querySelector(":scope > template:is([shadowrootmode], [shadowroot])");
        if(tmpl) {
          let mode = tmpl.getAttribute("shadowrootmode") || tmpl.getAttribute("shadowroot") || "closed";
          shadowroot = node.attachShadow({ mode }); // default is closed
          shadowroot.appendChild(tmpl.content.cloneNode(true));
        }
      }
    
      // Cheers to https://gist.github.com/developit/45c85e9be01e8c3f1a0ec073d600d01e
      if(shadowroot) {
        cloned.attachShadow({ mode: shadowroot.mode }).append(...shadowroot.childNodes);
      }
    
      // Keep *same* child nodes to preserve state of children (e.g. details->summary)
      cloned.append(...node.childNodes);
      node.replaceWith(cloned);
    
      return readyPromise.then(() => {
        // Restore original children and shadow DOM
        if(cloned.shadowRoot) {
          node.shadowRoot.append(...cloned.shadowRoot.childNodes);
        }
        node.append(...cloned.childNodes);
        cloned.replaceWith(node);
      });
    }
  }

  constructor() {
    super();

    // Internal promises
    this.ready = new Promise(resolve => {
      this.readyResolve = resolve;
    });
  }

  // any parents of `el` that are <is-land> (with conditions)
  static getParents(el, stopAt = false) {
    let nodes = [];
    while(el) {
      if(el.matches && el.matches(Island.tagName)) {
        if(stopAt && el === stopAt) {
          break;
        }

        if(Conditions.hasConditions(el)) {
          nodes.push(el);
        }
      }
      el = el.parentNode;
    }
    return nodes;
  }

  static async ready(el, parents) {
    if(!parents) {
      parents = Island.getParents(el);
    }
    if(parents.length === 0) {
      return;
    }
    let imports = await Promise.all(parents.map(p => p.wait()));
    // return innermost module import
    if(imports.length) {
      return imports[0];
    }
  }

  forceFallback() {
    if(window.Island) {
      Object.assign(Island.fallback, window.Island.fallback);
    }

    for(let selector in Island.fallback) {
      // Reverse here as a cheap way to get the deepest nodes first
      let components = Array.from(this.querySelectorAll(selector)).reverse();

      // with thanks to https://gist.github.com/cowboy/938767
      for(let node of components) {
        if(!node.isConnected) {
          continue;
        }

        let parents = Island.getParents(node);
        // must be in a leaf island (not nested deep)
        if(parents.length === 1) {
          let p = Island.ready(node, parents);
          Island.fallback[selector](p, node, Island.prefix);
        }
      }
    }
  }

  wait() {
    return this.ready;
  }

  async connectedCallback() {
    // Only use fallback content with loading conditions
    if(Conditions.hasConditions(this)) {
      // Keep fallback content without initializing the components
      this.forceFallback();
    }

    await this.hydrate();
  }

  getTemplates() {
    return this.querySelectorAll(`template[${Island.attr.template}]`);
  }

  replaceTemplates(templates) {
    // replace <template> with the live content
    for(let node of templates) {
      // if the template is nested inside another child <is-land> inside, skip
      if(Island.getParents(node, this).length > 0) {
        continue;
      }

      let value = node.getAttribute(Island.attr.template);
      // get rid of the rest of the content on the island
      if(value === "replace") {
        let children = Array.from(this.childNodes);
        for(let child of children) {
          this.removeChild(child);
        }
        this.appendChild(node.content);
        break;
      } else {
        let html = node.innerHTML;
        if(value === "once" && html) {
          if(Island.onceCache.has(html)) {
            node.remove();
            return;
          }

          Island.onceCache.set(html, true);
        }

        node.replaceWith(node.content);
      }
    }
  }

  async hydrate() {
    let conditions = [];
    if(this.parentNode) {
      // wait for all parents before hydrating
      conditions.push(Island.ready(this.parentNode));
    }

    let attrs = Conditions.getConditions(this);
    for(let condition in attrs) {
      if(Conditions.map[condition]) {
        conditions.push(Conditions.map[condition](attrs[condition], this));
      }
    }

    // Loading conditions must finish before dependencies are loaded
    await Promise.all(conditions);

    this.replaceTemplates(this.getTemplates());

    for(let fn of Island.onReady.values()) {
      await fn.call(this, Island);
    }

    this.readyResolve();

    this.setAttribute(Island.attr.ready, "");

    // Remove [defer-hydration]
    this.querySelectorAll(`[${Island.attr.defer}]`).forEach(node => node.removeAttribute(Island.attr.defer));
  }
}

class Conditions {
  static map = {
    visible: Conditions.visible,
    idle: Conditions.idle,
    interaction: Conditions.interaction,
    media: Conditions.media,
    "save-data": Conditions.saveData,
  };

  static hasConditions(node) {
    return Object.keys(Conditions.getConditions(node)).length > 0;
  }

  static getConditions(node) {
    let map = {};
    for(let key of Object.keys(Conditions.map)) {
      if(node.hasAttribute(`on:${key}`)) {
        map[key] = node.getAttribute(`on:${key}`);
      }
    }

    return map;
  }

  static visible(noop, el) {
    if(!('IntersectionObserver' in window)) {
      // runs immediately
      return;
    }

    return new Promise(resolve => {
      let observer = new IntersectionObserver(entries => {
        let [entry] = entries;
        if(entry.isIntersecting) {
          observer.unobserve(entry.target);
          resolve();
        }
      });

      observer.observe(el);
    });
  }

  // Warning: on:idle is not very useful with other conditions as it may resolve long before.
  static idle() {
    let onload = new Promise(resolve => {
      if(document.readyState !== "complete") {
        window.addEventListener("load", () => resolve(), { once: true });
      } else {
        resolve();
      }
    });

    if(!("requestIdleCallback" in window)) {
      // run immediately
      return onload;
    }

    // both idle and onload
    return Promise.all([
      new Promise(resolve => {
        requestIdleCallback(() => {
          resolve();
        });
      }),
      onload,
    ]);
  }

  static interaction(eventOverrides, el) {
    let events = ["click", "touchstart"];
    // event overrides e.g. on:interaction="mouseenter"
    if(eventOverrides) {
      events = (eventOverrides || "").split(",").map(entry => entry.trim());
    }

    return new Promise(resolve => {
      function resolveFn(e) {
        resolve();

        // cleanup the other event handlers
        for(let name of events) {
          el.removeEventListener(name, resolveFn);
        }
      }

      for(let name of events) {
        el.addEventListener(name, resolveFn, { once: true });
      }
    });
  }

  static media(query) {
    let mm = {
      matches: true
    };

    if(query && ("matchMedia" in window)) {
      mm = window.matchMedia(query);
    }

    if(mm.matches) {
      return;
    }

    return new Promise(resolve => {
      mm.addListener(e => {
        if(e.matches) {
          resolve();
        }
      });
    });
  }

  static saveData(expects) {
    // return early if API does not exist
    if(!("connection" in navigator) || navigator.connection.saveData === (expects !== "false")) {
      return;
    }

    // dangly promise
    return new Promise(() => {});
  }
}

// Should this auto define? Folks can redefine later using { component } export
if("customElements" in window) {
  window.customElements.define(Island.tagName, Island);
  window.Island = Island;
}

export {
  Island,
  Island as component, // Backwards compat only: recommend `Island` export
};

// TODO remove in 4.0
export const ready = Island.ready; // Backwards compat only: recommend `Island` export