// Deterministic RNG (mulberry32). The generator state lives inside GameState so
// that replaying the same commands from the same seed reproduces the game
// exactly — the foundation for async play, spectating, and replays.

export interface RngState {
  /** Current 32-bit mulberry32 state; advances by one per value drawn. */
  readonly a: number;
}

export function createRng(seed: number): RngState {
  return { a: seed >>> 0 };
}

/** Draw the next float in [0, 1). Returns the value and the advanced state. */
export function nextFloat(state: RngState): [number, RngState] {
  const a = (state.a + 0x6d2b79f5) | 0;
  let t = Math.imul(a ^ (a >>> 15), 1 | a);
  t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
  const value = ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  return [value, { a: a >>> 0 }];
}

/** Draw an integer in [0, n). */
export function nextInt(state: RngState, n: number): [number, RngState] {
  const [f, next] = nextFloat(state);
  return [Math.floor(f * n), next];
}

/** Roll the Wiz-War die (numbered 1-4). */
export function rollDie(state: RngState): [number, RngState] {
  const [i, next] = nextInt(state, 4);
  return [i + 1, next];
}

/** Fisher-Yates shuffle. Returns a new array and the advanced state. */
export function shuffle<T>(state: RngState, items: readonly T[]): [T[], RngState] {
  const arr = [...items];
  let rng = state;
  for (let i = arr.length - 1; i > 0; i--) {
    const [j, next] = nextInt(rng, i + 1);
    rng = next;
    [arr[i], arr[j]] = [arr[j]!, arr[i]!];
  }
  return [arr, rng];
}