From 76781eb6393239e1ad516bb55354ad7ed57fb983 Mon Sep 17 00:00:00 2001
From: Nathan Vegdahl <cessen@cessen.com>
Date: Sat, 13 Aug 2022 13:02:47 -0700
Subject: [PATCH] Use new fast hash for base-4 Owen scrambling.

---
 src/renderer.rs |  4 ++--
 src/scramble.rs | 18 +++++++++++++++++-
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/src/renderer.rs b/src/renderer.rs
index 8c9bb4f..6fc18d4 100644
--- a/src/renderer.rs
+++ b/src/renderer.rs
@@ -18,7 +18,7 @@ use crate::{
     mis::power_heuristic,
     ray::Ray,
     scene::{Scene, SceneLightSample},
-    scramble::owen4,
+    scramble::owen4_fast,
     space_fill::{hilbert_spiral, morton},
     timer::Timer,
     tracer::Tracer,
@@ -233,7 +233,7 @@ impl<'a> Renderer<'a> {
                     // and golden ratio sampling, we do this up-front here rather than in
                     // the samplers themselves.
                     let si_offset =
-                        owen4(morton::encode(x, y), self.seed).wrapping_mul(self.spp as u32);
+                        owen4_fast(morton::encode(x, y), self.seed).wrapping_mul(self.spp as u32);
 
                     for si in 0..(self.spp as u32) {
                         let mut sample_gen = SampleGen::new(si_offset + si, self.seed);
diff --git a/src/scramble.rs b/src/scramble.rs
index 89c84e3..c59f0fe 100644
--- a/src/scramble.rs
+++ b/src/scramble.rs
@@ -16,7 +16,23 @@ pub fn owen2(n: u32, seed: u32) -> u32 {
     result
 }
 
-/// Performs a base-4 Owen scramble on an integer.
+#[inline(always)]
+pub fn owen4_fast(mut n: u32, seed: u32) -> u32 {
+    let scramble = hash(seed);
+
+    n = n.reverse_bits();
+
+    n ^= n.wrapping_mul(0x3d20adea);
+    n ^= (n >> 1) & (n << 1) & 0x55555555;
+    n = n.wrapping_add(scramble);
+    n = n.wrapping_mul((scramble >> 16) | 1);
+    n ^= (n >> 1) & (n << 1) & 0x55555555;
+    n ^= n.wrapping_mul(0x05526c56);
+    n ^= n.wrapping_mul(0x53a22864);
+
+    n.reverse_bits()
+}
+
 pub fn owen4(n: u32, seed: u32) -> u32 {
     // Bit-packed permutation table.
     const PERMUTATION_TABLE: [u8; 24] = [