FairBreak

// ── FairBreak v1 derivation ─────────────────────────────────────────────
// derivation: "sha256(entropy|rule_hash) → sfc32 → fisher-yates"

// sfc32 PRNG — Small Fast Counting, 128-bit state
// Algorithm by Chris Doty-Humphrey (Public Domain)
// All bit ops operate on signed 32-bit int; >>> 0 converts to uint32.
function sfc32(a, b, c, d) {
  return function next() {
    let t = (a + b | 0) + d | 0;
    d = d + 1 | 0;
    a = b ^ (b >>> 9);
    b = c + (c << 3) | 0;
    c = (c << 21 | c >>> 11);
    c = c + t | 0;
    return (t >>> 0) / 4294967296;  // returns float in [0, 1)
  };
}

// Parse first 16 bytes (= first 32 hex chars) of a SHA-256 hex string
// as 4 × uint32 big-endian for sfc32 seeds [a, b, c, d].
function hexToSfc32Seeds(hex) {
  return [0, 1, 2, 3].map(i => parseInt(hex.slice(i * 8, i * 8 + 8), 16) >>> 0);
}

// Standard Knuth/Durstenfeld Fisher-Yates in-place shuffle.
function fisherYates(arr, rng) {
  for (let i = arr.length - 1; i > 0; i--) {
    const j = Math.floor(rng() * (i + 1));
    [arr[i], arr[j]] = [arr[j], arr[i]];
  }
}

// SHA-256 via Web Crypto API (browser) — or node:crypto.subtle (Node ≥ 19)
async function sha256hex(text) {
  const buf = await crypto.subtle.digest(
    'SHA-256',
    new TextEncoder().encode(text)
  );
  return Array.from(new Uint8Array(buf))
    .map(b => b.toString(16).padStart(2, '0'))
    .join('');
}

// Full FairBreak v1 derivation.
// entropyHex : response.value from GET /v1/rng/reveal/{commitment_id}
// ruleHash   : SHA-256 of the canonical JSON (step 1 above)
// participants: deduped array in original entered order
async function derive(entropyHex, ruleHash, participants) {
  // seed = SHA-256(entropy_hex + "|" + rule_hash)
  const seed = await sha256hex(entropyHex + '|' + ruleHash);
  const [a, b, c, d] = hexToSfc32Seeds(seed);
  const rng = sfc32(a, b, c, d);
  const shuffled = [...participants];
  fisherYates(shuffled, rng);
  return { seed, shuffled };
  // Assign: spots[i] → shuffled[i]  (remaining spots unassigned if len(participants) < len(spots))
}