FastRNG.cs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * The Initial Developer of the Original Code is Rune Skovbo Johansen.
 * Portions created by the Initial Developer are Copyright (C) 2015
 * the Initial Developer. All Rights Reserved.
 */

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

public class XXHashRNG
{
	private int position;
	private readonly uint seed;

	public XXHashRNG() : this(Environment.TickCount) {}
	public XXHashRNG(int seed) => this.seed = (uint)seed;

	public bool TestOneIn(int odds) => Value * odds <= 1f;
	public int Range(int min, int max) => min + (int)(GetHash(position++) % (max - min));
	public float Range(float min, float max) => min + GetHash(position++) * (max - min) * (1.0f / uint.MaxValue);
	public float Value => new FloatUnion(0x3F800000U | (GetHash(position++) >> 9)).FloatVal - 1.0f;

	const uint PRIME32_2 = 2246822519U;
	const uint PRIME32_3 = 3266489917U;
	const uint PRIME32_4 = 668265263U;
	const uint PRIME32_5 = 374761393U;

	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	private uint GetHash(int buf)
	{
		uint h32 = seed + PRIME32_5;
		h32 += (uint)buf * PRIME32_3;
		h32 = ((h32 << 17) | (h32 >> 15)) * PRIME32_4;
		h32 ^= h32 >> 15;
		h32 *= PRIME32_2;
		h32 ^= h32 >> 13;
		h32 *= PRIME32_3;
		h32 ^= h32 >> 16;
		return h32;
	}

	[StructLayout(LayoutKind.Explicit)]
	private struct FloatUnion
	{
		[FieldOffset(0)] public readonly uint IntVal;
		[FieldOffset(0)] public readonly float FloatVal;

		public FloatUnion(uint intVal)
		{
			FloatVal = 0;
			IntVal = intVal;
		}
	}
}