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Make the z-scrambling table smaller with bit fiddling.

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This gets it down to 24 bytes.
This commit is contained in:
Nathan Vegdahl 2022-07-21 04:14:06 -07:00
parent ec9a121e72
commit 6b7538e25f
1 changed files with 30 additions and 30 deletions
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60
src/scramble.rs
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@ -17,41 +17,41 @@ pub fn owen(n: u32, seed: u32) -> u32 {
/// of Pixels" by Ahmed et al., except using a more complete and /// of Pixels" by Ahmed et al., except using a more complete and
/// efficient algorithm based on the Owen scrambling function above. /// efficient algorithm based on the Owen scrambling function above.
pub fn z_scramble(n: u32, seed: u32) -> u32 { pub fn z_scramble(n: u32, seed: u32) -> u32 {
const PERM_TABLE: [u8; 24] = [
0 | (1 << 2) | (2 << 4) | (3 << 6), // [0, 1, 2, 3],
0 | (1 << 2) | (3 << 4) | (2 << 6), // [0, 1, 3, 2],
0 | (2 << 2) | (1 << 4) | (3 << 6), // [0, 2, 1, 3],
0 | (2 << 2) | (3 << 4) | (1 << 6), // [0, 2, 3, 1],
0 | (3 << 2) | (1 << 4) | (2 << 6), // [0, 3, 1, 2],
0 | (3 << 2) | (2 << 4) | (1 << 6), // [0, 3, 2, 1],
1 | (0 << 2) | (2 << 4) | (3 << 6), // [1, 0, 2, 3],
1 | (0 << 2) | (3 << 4) | (2 << 6), // [1, 0, 3, 2],
1 | (2 << 2) | (0 << 4) | (3 << 6), // [1, 2, 0, 3],
1 | (2 << 2) | (3 << 4) | (0 << 6), // [1, 2, 3, 0],
1 | (3 << 2) | (0 << 4) | (2 << 6), // [1, 3, 0, 2],
1 | (3 << 2) | (2 << 4) | (0 << 6), // [1, 3, 2, 0],
2 | (0 << 2) | (1 << 4) | (3 << 6), // [2, 0, 1, 3],
2 | (0 << 2) | (3 << 4) | (1 << 6), // [2, 0, 3, 1],
2 | (1 << 2) | (0 << 4) | (3 << 6), // [2, 1, 0, 3],
2 | (1 << 2) | (3 << 4) | (0 << 6), // [2, 1, 3, 0],
2 | (3 << 2) | (0 << 4) | (1 << 6), // [2, 3, 0, 1],
2 | (3 << 2) | (1 << 4) | (0 << 6), // [2, 3, 1, 0],
3 | (0 << 2) | (1 << 4) | (2 << 6), // [3, 0, 1, 2],
3 | (0 << 2) | (2 << 4) | (1 << 6), // [3, 0, 2, 1],
3 | (1 << 2) | (0 << 4) | (2 << 6), // [3, 1, 0, 2],
3 | (1 << 2) | (2 << 4) | (0 << 6), // [3, 1, 2, 0],
3 | (2 << 2) | (0 << 4) | (1 << 6), // [3, 2, 0, 1],
3 | (2 << 2) | (1 << 4) | (0 << 6), // [3, 2, 1, 0],
];
let mut result = 0; let mut result = 0;
for i in 0..16 { for i in 0..16 {
let mask = if i < 15 { (!0) << ((i + 1) * 2) } else { 0 }; let mask = if i < 15 { (!0) << ((i + 1) * 2) } else { 0 };
let entry = (hash_u32(n & mask, seed) % 24) as usize; let perm_entry = PERM_TABLE[(hash_u32(n & mask, seed) % 24) as usize];
let cell = ((n >> (i * 2)) & 0b11) as usize; let perm_cell_i = ((n >> (i * 2)) & 0b11) as usize;
result |= (Z_TABLE[entry][cell] as u32) << (i * 2); result |= (((perm_entry >> (perm_cell_i * 2)) & 0b11) as u32) << (i * 2);
} }
result result
} }
const Z_TABLE: &[[u8; 4]; 24] = &[
[0, 1, 2, 3],
[0, 1, 3, 2],
[0, 2, 1, 3],
[0, 2, 3, 1],
[0, 3, 1, 2],
[0, 3, 2, 1],
[1, 0, 2, 3],
[1, 0, 3, 2],
[1, 2, 0, 3],
[1, 2, 3, 0],
[1, 3, 0, 2],
[1, 3, 2, 0],
[2, 0, 1, 3],
[2, 0, 3, 1],
[2, 1, 0, 3],
[2, 1, 3, 0],
[2, 3, 0, 1],
[2, 3, 1, 0],
[3, 0, 1, 2],
[3, 0, 2, 1],
[3, 1, 0, 2],
[3, 1, 2, 0],
[3, 2, 0, 1],
[3, 2, 1, 0],
];