link.js

// TileLoom link play: the entire game state travels inside the URL fragment,
// compressed with deflate and base64url-encoded. No server involved.

'use strict';

const LinkGame = (() => {
  function b64encode(bytes) {
    let bin = '';
    for (let i = 0; i < bytes.length; i += 0x8000) {
      bin += String.fromCharCode.apply(null, bytes.subarray(i, i + 0x8000));
    }
    return btoa(bin).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
  }

  function b64decode(str) {
    const bin = atob(str.replace(/-/g, '+').replace(/_/g, '/'));
    const bytes = new Uint8Array(bin.length);
    for (let i = 0; i < bin.length; i++) bytes[i] = bin.charCodeAt(i);
    return bytes;
  }

  async function compress(text) {
    const stream = new Blob([text]).stream().pipeThrough(new CompressionStream('deflate-raw'));
    return new Uint8Array(await new Response(stream).arrayBuffer());
  }

  async function decompress(bytes) {
    const stream = new Blob([bytes]).stream().pipeThrough(new DecompressionStream('deflate-raw'));
    return new Response(stream).text();
  }

  // Returns a payload string: 'c<base64url>' (compressed) or 'r<base64url>'
  async function encodeState(state) {
    const json = JSON.stringify(state);
    if (typeof CompressionStream !== 'undefined') {
      try {
        return 'c' + b64encode(await compress(json));
      } catch (e) { /* fall through to raw */ }
    }
    return 'r' + b64encode(new TextEncoder().encode(json));
  }

  // Returns the decoded state object, or null if the payload is invalid
  async function decodeState(payload) {
    try {
      const bytes = b64decode(payload.slice(1));
      const json = payload[0] === 'c'
        ? await decompress(bytes)
        : new TextDecoder().decode(bytes);
      const s = JSON.parse(json);
      if (!s || !Array.isArray(s.board) || !Array.isArray(s.players) ||
          s.players.length !== 2) return null;
      return s;
    } catch (e) {
      return null;
    }
  }

  function urlFor(payload) {
    return location.origin + location.pathname + '#g=' + payload;
  }

  function payloadFromLocation() {
    const m = location.hash.match(/^#g=(.+)$/);
    return m ? m[1] : null;
  }

  return { encodeState, decodeState, urlFor, payloadFromLocation };
})();