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Added a lot of comments.

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Acrimon 2019-08-05 21:04:03 +02:00
parent 1c22f380b0
commit dcc19e2d04
1 changed files with 18 additions and 1 deletions
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19
world/src/util/seed_expan.rs
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@ -1,3 +1,7 @@
// This module contains a few functions and utilities for expanding a seed into more data for use in worldgen.
// Very standard substitution box. Takes one number and gives back another. This one works per byte.
// Standard component in diffusion functions.
static SBOX: [u8; 256] = [
206, 21, 212, 69, 54, 234, 13, 42, 184, 48, 92, 64, 196, 55, 225, 235, 229, 120, 135, 72, 32,
147, 74, 142, 197, 79, 139, 164, 110, 57, 176, 47, 192, 174, 178, 49, 193, 71, 78, 18, 237, 81,
@ -14,6 +18,7 @@ static SBOX: [u8; 256] = [
216, 33, 165, 86, 222, 199, 208, 201,
];
// Helper function to work with the box above. Takes a u64 and runs each byte through the box.
fn sbox(x: u64) -> u64 {
let mut bytes = x.to_ne_bytes();
for byte in &mut bytes {
@ -22,7 +27,8 @@ fn sbox(x: u64) -> u64 {
u64::from_ne_bytes(bytes)
}
// chaotic bijective function
// A bijective diffusion function with chaotic behaviour. It essentially mixes numbers.
// A 1 bit change somewhere will affect a large portion of the other bits after running through this function.
fn diffuse_rnd(mut x: u64) -> u64 {
x = x.wrapping_mul(0x6eed0e9da4d94a4f);
let a = x >> 32;
@ -32,6 +38,7 @@ fn diffuse_rnd(mut x: u64) -> u64 {
sbox(x)
}
// Helper for running diffuse_rnd 4 times, enough to mix the bits around properly.
fn diffuse(mut x: u64) -> u64 {
for _ in 0..4 {
x = diffuse_rnd(x);
@ -39,11 +46,13 @@ fn diffuse(mut x: u64) -> u64 {
x
}
// Expands a 32 bit state into at 64 bit state.
fn initial_expand(x: u32) -> u64 {
let f = (x as u64).wrapping_mul(0x2f72b4215a3d8caf);
f.wrapping_mul(f)
}
// Truncate a 64 bit state to a 32 bit seed.
fn truncate(x: u64) -> u32 {
let xb = x.to_ne_bytes();
let nb = [
@ -55,6 +64,8 @@ fn truncate(x: u64) -> u32 {
u32::from_ne_bytes(nb)
}
// Generates a sequence of diffused (mixed) numbers from one seed.
// Used for generating a lot of initial seeds for the noise algorithms in worldgen.
pub fn diffused_field(seed: u32, amount: u32) -> Vec<u32> {
let mut field = Vec::new();
for i in 0..=amount {
@ -64,14 +75,20 @@ pub fn diffused_field(seed: u32, amount: u32) -> Vec<u32> {
field
}
// Expand a 32 bit seed into a 32 byte state for the RNG used in worldgen.
pub fn expand_seed_to_rng(seed: u32) -> [u8; 32] {
// Create a new empty ChaChaRng state.
let mut r: [u64; 4] = [0; 4];
// Create a new empty internal state.
let mut state: u64 = initial_expand(seed);
// Fill the ChaChaRng state with random bits from repeatedly running mixing the state.
for i in 0..4 {
state = diffuse(state);
r[i] = state;
}
// Convert the ChaChaRng state to bytes. Uses unsafe here because the safe code for it would be much longer.
unsafe { std::mem::transmute(r) }
}