Add more state objects.

src/crypto/feistel.rs

@@ -0,0 +1,35 @@
+/// Performs a deterministic pseudo-random shuffle of a 16-bit index.
+///
+/// # Arguments
+/// * `index` - The input value (0..65536).
+/// * `seed` - The 32-bit seed acting as the encryption key.
+///
+/// # Returns
+/// A unique 16-bit shuffled value.
+pub fn feistel_shuffle(index: u16, seed: u32) -> u16 {
+    // Split 16-bit index into two 8-bit halves
+    let mut l = ((index >> 8) & 0xFF) as u8;
+    let mut r = (index & 0xFF) as u8;
+
+    // Perform 4 rounds of Feistel mixing
+    for round in 0..4 {
+        // Derive sub-key: Rotate seed and add golden ratio constant
+        let rot_amount = (round * 5) % 32;
+        let sub_key = seed
+            .rotate_left(rot_amount)
+            .wrapping_add(round.wrapping_mul(0x9E3779B9));
+
+        // Round function F: Simple multiplicative hash mixing R and sub_key
+        // We cast to u32 for multiplication to avoid overflow, then mask back to 8 bits
+        let r_u32 = r as u32;
+        let hash_val = ((r_u32.wrapping_mul(sub_key)) ^ (r_u32 >> 4)) as u8 & 0xFF;
+
+        // Feistel step: New L = Old R, New R = Old L XOR F(R, key)
+        let temp = l;
+        l = r;
+        r = temp ^ hash_val;
+    }
+
+    // Recombine halves
+    ((l as u16) << 8) | (r as u16)
+}