/**
 * Deck creation and deterministic shuffling for the poker engine.
 * Pure: randomness is always injected; no module-level state.
 */

import type { Card, Rank, Suit } from "./types";

const SUITS: readonly Suit[] = ["s", "h", "d", "c"];
const MIN_RANK: Rank = 2;
const MAX_RANK: Rank = 14;

/** A fresh, ordered 52-card deck (order is deterministic before shuffling). */
export function createDeck(): Card[] {
  const deck: Card[] = [];
  for (const suit of SUITS) {
    for (let rank = MIN_RANK; rank <= MAX_RANK; rank += 1) {
      deck.push({ rank: rank as Rank, suit });
    }
  }
  return deck;
}

/**
 * Small, fast seeded PRNG (mulberry32). Used so a whole multi-hand game can
 * be replayed from a single integer seed stored in the table state.
 */
export function mulberry32(seed: number): () => number {
  let a = seed >>> 0;
  return () => {
    a = (a + 0x6d2b79f5) >>> 0;
    let t = a;
    t = Math.imul(t ^ (t >>> 15), t | 1);
    t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  };
}

/** Fisher-Yates shuffle; returns a new array, never mutates the input. */
export function shuffleDeck(cards: readonly Card[], rng: () => number): Card[] {
  const deck = [...cards];
  for (let i = deck.length - 1; i > 0; i -= 1) {
    const j = Math.floor(rng() * (i + 1));
    const tmp = deck[i];
    deck[i] = deck[j];
    deck[j] = tmp;
  }
  return deck;
}

const GOLDEN_RATIO_32 = 0x9e3779b9;

/** Per-hand seed derived from the table's base seed and the hand number. */
export function handSeed(baseSeed: number, handNumber: number): number {
  return (baseSeed + Math.imul(handNumber, GOLDEN_RATIO_32)) >>> 0;
}

/** The exact shuffled deck used for a given hand of a given game seed. */
export function createShuffledDeck(baseSeed: number, handNumber: number): Card[] {
  return shuffleDeck(createDeck(), mulberry32(handSeed(baseSeed, handNumber)));
}