update: refactor deterministic number generation

README.md

@@ -106,6 +106,34 @@ Fixed64 sinValue = FixedTrigonometry.Sin(angle);
 Console.WriteLine(sinValue); // Output: ~0.707
 ```
 
+### Deterministic Random Generation
+
+Use `DeterministicRandom` when you need reproducible random values across runs, worlds, or features.  
+Streams are derived from a seed and remain deterministic regardless of threading or platform.
+
+```csharp
+// Simple constructor-based stream:
+var rng = new DeterministicRandom(42UL);
+
+// Deterministic integer:
+int value = rng.Next(1, 10); // [1,10)
+
+// Deterministic Fixed64 in [0,1):
+Fixed64 ratio = rng.NextFixed6401();
+
+// One stream per “feature” that’s stable for the same worldSeed + key:
+var rngOre = DeterministicRandom.FromWorldFeature(worldSeed: 123456789UL, featureKey: 0xORE);
+var rngRivers = DeterministicRandom.FromWorldFeature(123456789UL, 0xRIV, index: 0);
+
+// Deterministic Fixed64 draws:
+Fixed64 h = rngOre.NextFixed64(Fixed64.One);                      // [0, 1)
+Fixed64 size = rngOre.NextFixed64(Fixed64.Zero, 5 * Fixed64.One); // [0, 5)
+Fixed64 posX = rngRivers.NextFixed64(-Fixed64.One, Fixed64.One);  // [-1, 1)
+
+// Deterministic integers:
+int loot = rngOre.Next(1, 5); // [1,5)
+```
+
 ---
 
 ## 📦 Library Structure
@@ -116,6 +144,7 @@ Console.WriteLine(sinValue); // Output: ~0.707
 - **`IBound` Interface:** Standard interface for bounding shapes `BoundingBox`, `BoundingArea`, and `BoundingSphere`, each offering intersection, containment, and projection logic.
 - **`FixedMath` Static Class:** Provides common math and trigonometric functions using fixed-point math.
 - **`Fixed4x4` and `Fixed3x3`:** Support matrix operations for transformations.
+- **`DeterministicRandom` Struct:** Seedable, allocation-free RNG for repeatable procedural generation.  
 
 ### Fixed64 Struct
 

src/FixedMathSharp/Utility/DeterministicRandom.cs

@@ -0,0 +1,211 @@
+using System;
+using System.Runtime.CompilerServices;
+
+namespace FixedMathSharp.Utility
+{
+    /// <summary>
+    /// Fast, seedable, deterministic RNG suitable for lockstep sims and map gen.
+    /// Uses xoroshiro128++ with splitmix64 seeding. No allocations, no time/GUID.
+    /// </summary>
+    public struct DeterministicRandom
+    {
+        // xoroshiro128++ state
+        private ulong _s0;
+        private ulong _s1;
+
+        #region Construction / Seeding
+
+        public DeterministicRandom(ulong seed)
+        {
+            // Expand a single seed into two 64-bit state words via splitmix64.
+            _s0 = SplitMix64(ref seed);
+            _s1 = SplitMix64(ref seed);
+
+            // xoroshiro requires non-zero state; repair pathological seed.
+            if (_s0 == 0UL && _s1 == 0UL)
+                _s1 = 0x9E3779B97F4A7C15UL;
+        }
+
+        /// <summary>
+        /// Create a stream deterministically 
+        /// Derived from (worldSeed, featureKey[,index]).
+        /// </summary>
+        public static DeterministicRandom FromWorldFeature(ulong worldSeed, ulong featureKey, ulong index = 0)
+        {
+            // Simple reversible mix (swap for a stronger mix if required).
+            ulong seed = Mix64(worldSeed, featureKey);
+            seed = Mix64(seed, index);
+            return new DeterministicRandom(seed);
+        }
+
+        #endregion
+
+        #region Core PRNG
+
+        /// <summary>
+        /// xoroshiro128++ next 64 bits.
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public ulong NextU64()
+        {
+            ulong s0 = _s0, s1 = _s1;
+            ulong result = RotL(s0 + s1, 17) + s0;
+
+            s1 ^= s0;
+            _s0 = RotL(s0, 49) ^ s1 ^ (s1 << 21); // a,b
+            _s1 = RotL(s1, 28);                   // c
+
+            return result;
+        }
+
+        /// <summary>
+        /// Next non-negative Int32 in [0, int.MaxValue].
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public int Next()
+        {
+            // Take high bits for better quality; mask to 31 bits non-negative.
+            return (int)(NextU64() >> 33);
+        }
+
+        /// <summary>
+        /// Unbiased int in [0, maxExclusive).
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public int Next(int maxExclusive)
+        {
+            return maxExclusive <= 0 
+                ? throw new ArgumentOutOfRangeException(nameof(maxExclusive)) 
+                : (int)NextBounded((uint)maxExclusive);
+        }
+
+        /// <summary>
+        /// Unbiased int in [min, maxExclusive).
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public int Next(int minInclusive, int maxExclusive)
+        {
+            if (minInclusive >= maxExclusive) 
+                throw new ArgumentException("min >= max");
+            uint range = (uint)(maxExclusive - minInclusive);
+            return minInclusive + (int)NextBounded(range);
+        }
+
+        /// <summary>
+        /// Double in [0,1).
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public double NextDouble()
+        {
+            // 53 random bits -> [0,1)
+            return (NextU64() >> 11) * (1.0 / (1UL << 53));
+        }
+
+        /// <summary>
+        /// Fill span with random bytes.
+        /// </summary>
+        public void NextBytes(Span<byte> buffer)
+        {
+            int i = 0;
+            while (i + 8 <= buffer.Length)
+            {
+                ulong v = NextU64();
+                Unsafe.WriteUnaligned(ref buffer[i], v);
+                i += 8;
+            }
+            if (i < buffer.Length)
+            {
+                ulong v = NextU64();
+                while (i < buffer.Length)
+                {
+                    buffer[i++] = (byte)v;
+                    v >>= 8;
+                }
+            }
+        }
+
+        #endregion
+
+        #region Fixed64 helpers
+
+        /// <summary>
+        /// Random Fixed64 in [0,1).
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public Fixed64 NextFixed6401()
+        {
+            // Produce a raw value in [0, One.m_rawValue)
+            ulong rawOne = (ulong)Fixed64.One.m_rawValue;
+            ulong r = NextBounded(rawOne);
+            return Fixed64.FromRaw((long)r);
+        }
+
+        /// <summary>
+        /// Random Fixed64 in [0, maxExclusive).
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public Fixed64 NextFixed64(Fixed64 maxExclusive)
+        {
+            if (maxExclusive <= Fixed64.Zero)
+                throw new ArgumentOutOfRangeException(nameof(maxExclusive), "max must be > 0");
+            ulong rawMax = (ulong)maxExclusive.m_rawValue;
+            ulong r = NextBounded(rawMax);
+            return Fixed64.FromRaw((long)r);
+        }
+
+        /// <summary>
+        /// Random Fixed64 in [minInclusive, maxExclusive).
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public Fixed64 NextFixed64(Fixed64 minInclusive, Fixed64 maxExclusive)
+        {
+            if (minInclusive >= maxExclusive)
+                throw new ArgumentException("min >= max");
+            ulong span = (ulong)(maxExclusive.m_rawValue - minInclusive.m_rawValue);
+            ulong r = NextBounded(span);
+            return Fixed64.FromRaw((long)r + minInclusive.m_rawValue);
+        }
+
+        #endregion
+
+        #region Internals: unbiased range, splitmix64, mixing, rotations
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private ulong NextBounded(ulong bound)
+        {
+            // Rejection to avoid modulo bias.
+            // threshold = 2^64 % bound, but expressed as (-bound) % bound
+            ulong threshold = unchecked((ulong)-(long)bound) % bound;
+            while (true)
+            {
+                ulong r = NextU64();
+                if (r >= threshold)
+                    return r % bound;
+            }
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static ulong RotL(ulong x, int k) => (x << k) | (x >> (64 - k));
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static ulong SplitMix64(ref ulong state)
+        {
+            ulong z = (state += 0x9E3779B97F4A7C15UL);
+            z = (z ^ (z >> 30)) * 0xBF58476D1CE4E5B9UL;
+            z = (z ^ (z >> 27)) * 0x94D049BB133111EBUL;
+            return z ^ (z >> 31);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static ulong Mix64(ulong a, ulong b)
+        {
+            // Simple reversible mix (variant of splitmix finalizer).
+            ulong x = a ^ (b + 0x9E3779B97F4A7C15UL);
+            x = (x ^ (x >> 30)) * 0xBF58476D1CE4E5B9UL;
+            x = (x ^ (x >> 27)) * 0x94D049BB133111EBUL;
+            return x ^ (x >> 31);
+        }
+
+        #endregion
+    }
+}