Make erosion cell hashing cross-device deterministic

THIRD-PARTY-NOTICES.md

@@ -15,7 +15,10 @@ Rune Skovbo Johansen's **Advanced Terrain Erosion Filter**:
 The CultMath files retain the MPL notice and separate the faithful planar
 filter from CultMath's spherical adaptation. Modifications include C#/HLSL
 type lowering, explicit bounded octave counts, finite zero-rounding behavior,
-and CultMath naming.
+and CultMath naming. CultMath replaces the reference floating hash with a
+shared integer avalanche hash so C# and GPU evaluation select identical cell
+pivots across devices; this changes the stochastic pattern, not the filter
+mechanics.
 
 The technique builds on earlier work credited by the upstream author to Fewes
 and Clay John. The hash form follows the public shader math of Inigo Quilez.

shaders/AdvancedErosionFilter.hlsl

@@ -19,11 +19,16 @@ struct CultMathAdvancedErosionResult
     float3 delta; float magnitude; float ridge_map; float fade_target;
 };
 
+float cultmath_erosion_hash_unit(int2 cell, uint salt)
+{
+    uint value = (uint)cell.x * 0x8da6b343u ^ (uint)cell.y * 0xd8163841u ^ salt;
+    value ^= value >> 16; value *= 0x7feb352du; value ^= value >> 15; value *= 0x846ca68bu; value ^= value >> 16;
+    return (value & 0x00ffffffu) * (1.0 / 16777216.0);
+}
 float2 cultmath_erosion_hash2(float2 input)
 {
-    float2 k = float2(0.3183099, 0.3678794);
-    float2 x = input * k + k.yx;
-    return -1.0 + 2.0 * frac(16.0 * k * frac(x.x * x.y * (x.x + x.y)));
+    int2 cell = (int2)input;
+    return float2(cultmath_erosion_hash_unit(cell, 0x68bc21ebu), cultmath_erosion_hash_unit(cell, 0x02e5be93u)) * 2.0 - 1.0;
 }
 
 float4 cultmath_phacelle_noise(float2 position, float2 normal_direction, float frequency, float offset_cycles, float normalization)

src/CultMath/AdvancedErosionFilter.cs

@@ -109,10 +109,16 @@ public static class AdvancedErosionFilter
 
     private static float2 Hash2(float2 input)
     {
-        var k = new float2(0.3183099f, 0.3678794f);
-        var x = input * k + new float2(k.y, k.x);
-        var scalar = math.frac(x.x * x.y * (x.x + x.y));
-        return -float2.one + 2.0f * math.frac(16.0f * k * scalar);
+        var x = (int)input.x;
+        var y = (int)input.y;
+        return new float2(HashUnit(x, y, 0x68bc21ebu), HashUnit(x, y, 0x02e5be93u)) * 2.0f - float2.one;
+    }
+
+    private static float HashUnit(int x, int y, uint salt)
+    {
+        var value = unchecked((uint)x * 0x8da6b343u ^ (uint)y * 0xd8163841u ^ salt);
+        value ^= value >> 16; value *= 0x7feb352du; value ^= value >> 15; value *= 0x846ca68bu; value ^= value >> 16;
+        return (value & 0x00ffffffu) * (1.0f / 16777216.0f);
     }
 
     private static float2 SafeNormalize(float2 value)