using System;

namespace XWorld.Framework
{
    // SplitMix64：跨平台确定性、无浮点、状态即种子（便于快照/复盘）
    public sealed class DeterministicRandom : IRandom
    {
        private ulong _state;

        public DeterministicRandom(ulong seed)
        {
            _state = seed;
        }

        // 当前内部状态；用它构造新实例可从"下一步"继续同一序列
        public ulong State => _state;

        private ulong NextU64()
        {
            _state += 0x9E3779B97F4A7C15UL;
            ulong z = _state;
            z = (z ^ (z >> 30)) * 0xBF58476D1CE4E5B9UL;
            z = (z ^ (z >> 27)) * 0x94D049BB133111EBUL;
            return z ^ (z >> 31);
        }

        public int Next(int maxExclusive)
        {
            if (maxExclusive <= 0)
                throw new ArgumentOutOfRangeException(nameof(maxExclusive), "maxExclusive 必须为正");

            ulong bound = (ulong)maxExclusive;
            // 拒绝采样，消除取模偏差
            ulong threshold = (ulong.MaxValue - bound + 1) % bound;
            while (true)
            {
                ulong r = NextU64();
                if (r >= threshold) return (int)(r % bound);
            }
        }
    }
}