From 76f0065d8b930aad3d9928fbfbfeac091db17fef Mon Sep 17 00:00:00 2001
From: Eryn Wells <eryn@erynwells.me>
Date: Sun, 4 Nov 2018 22:30:06 -0500
Subject: [PATCH] Generate some pseudo-random numbers with a PRNG class in the
 shaders.

---
 Terrain2/TerrainAlgorithms.metal | 42 +++++++++++++++++++++++++++++---
 1 file changed, 39 insertions(+), 3 deletions(-)

diff --git a/Terrain2/TerrainAlgorithms.metal b/Terrain2/TerrainAlgorithms.metal
index a355873..80deb6d 100644
--- a/Terrain2/TerrainAlgorithms.metal
+++ b/Terrain2/TerrainAlgorithms.metal
@@ -7,9 +7,42 @@
 //
 
 #include <metal_stdlib>
+#include <metal_types>
 #include "ShaderTypes.h"
 using namespace metal;
 
+/// A pseudo-random number generator providing several algorithms.
+/// - http://reedbeta.com/blog/quick-and-easy-gpu-random-numbers-in-d3d11/
+struct PRNG {
+    PRNG(uint seed) : mSeed(wangHash(seed)) { }
+
+    /// Generate a random unsigned integer using a linear congruential generator.
+    uint lcg() {
+        mSeed = 1664525 * mSeed + 1013904223;
+        return mSeed;
+    }
+
+    /// Generate a random unsigned integer using the Xorshift algorithm from George Marsaglia's paper.
+    uint xorShift() {
+        mSeed ^= (mSeed << 13);
+        mSeed ^= (mSeed >> 17);
+        mSeed ^= (mSeed << 5);
+        return mSeed;
+    }
+
+    uint wangHash(uint seed) {
+        seed = (seed ^ 61) ^ (seed >> 16);
+        seed *= 9;
+        seed = seed ^ (seed >> 4);
+        seed *= 0x27d4eb2d;
+        seed = seed ^ (seed >> 15);
+        return seed;
+    }
+
+private:
+    uint mSeed;
+};
+
 kernel void zeroKernel(texture2d<float, access::write> outTexture [[texture(GeneratorTextureIndexOut)]],
                        uint2 tid [[thread_position_in_grid]])
 {
@@ -17,8 +50,11 @@ kernel void zeroKernel(texture2d<float, access::write> outTexture [[texture(Gene
 }
 
 kernel void randomKernel(texture2d<float, access::write> outTexture [[texture(GeneratorTextureIndexOut)]],
-                         uint2 tid [[thread_position_in_grid]])
+                         uint2 tid [[thread_position_in_grid]],
+                         uint2 tgid [[threadgroup_position_in_grid]])
 {
-    float x = 2.0 * M_PI_F * (tid.x / 128.0);
-    outTexture.write(sin(x), tid);
+    PRNG rng(tid.x * tid.y);
+    uint r = rng.lcg();
+    float x = float(r * (1.0 / float(UINT_MAX))) * 0.5f;
+    outTexture.write(x, tid);
 }