Fix RNG transmutation

enkou-shaders/src/rng.rs

@@ -14,9 +14,42 @@ use rand_xoshiro::Xoshiro256StarStar;
 /// This function assumes a properly-initialized state array;
 /// output may silently degenerate if the initial state is all zeros,
 /// so this module is private to the crate.
-pub(crate) fn xoshiro_from_state(
-    _rng_state: <Xoshiro256StarStar as SeedableRng>::Seed,
+pub(crate) fn xoshiro256starstar_from_seed(
+    rng_state: <Xoshiro256StarStar as SeedableRng>::Seed,
 ) -> Xoshiro256StarStar {
-    let rng_state_actual = [1u64, 2u64, 3u64, 4u64];
+    let mut rng_state_actual = [0u64; 4];
+
+    // NOTE: Bit shifting is bad, but we don't have great alternatives:
+    //  - `chunks_exact` has issues with pointer casting
+    //  - `u64::from_le_bytes` has issues with `OpBitcast` in SPIR-V validation
+    for i in 0..rng_state_actual.len() {
+        for j in 0..size_of::<u64>() {
+            rng_state_actual[i] |= (rng_state[i * size_of::<u64>() + j] as u64) << j * 8;
+        }
+    }
+
     unsafe { core::mem::transmute(rng_state_actual) }
 }
+
+#[cfg(test)]
+mod test {
+    use crate::rng::xoshiro256starstar_from_seed;
+    use core::iter::zip;
+    use rand::{RngExt, SeedableRng};
+    use rand_xoshiro::Xoshiro256StarStar;
+
+    #[test]
+    fn match_seeded() {
+        let mut seed: <Xoshiro256StarStar as SeedableRng>::Seed = [0u8; 32];
+        for i in 0..seed.len() {
+            seed[i] = i as u8;
+        }
+
+        let rng1 = Xoshiro256StarStar::from_seed(seed).random_iter::<u64>();
+        let rng2 = xoshiro256starstar_from_seed(seed).random_iter::<u64>();
+
+        zip(rng1, rng2)
+            .take(100)
+            .for_each(|(rng1_value, rng2_value)| assert_eq!(rng1_value, rng2_value));
+    }
+}