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Introduce faster swap-or-not whole-list shuffle

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This commit is contained in:
Paul Hauner
2019-03-01 01:47:40 +11:00
parent ed032dddea
commit 8aa7f25bbc
3 changed files with 71 additions and 19 deletions
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46
eth2/utils/swap_or_not_shuffle/src/lib.rs
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@@ -4,6 +4,36 @@ use int_to_bytes::{int_to_bytes1, int_to_bytes4};
use std::cmp::max;
use std::io::Cursor;
pub fn get_permutated_list(
list: &[usize],
seed: &[u8],
shuffle_round_count: u8,
) -> Option<Vec<usize>> {
let list_size = list.len();
if list_size == 0 || list_size > usize::max_value() / 2 || list_size > 2_usize.pow(24) {
return None;
}
let mut pivots = Vec::with_capacity(shuffle_round_count as usize);
for round in 0..shuffle_round_count {
pivots.push(bytes_to_int64(&hash_with_round(seed, round)[..]) as usize % list_size);
}
let mut output = Vec::with_capacity(list_size);
for i in 0..list_size {
let mut index = i;
for round in 0..shuffle_round_count {
let pivot = pivots[round as usize];
index = do_round(seed, index, pivot, round, list_size)?;
}
output.push(list[index])
}
Some(output)
}
/// Return `p(index)` in a pseudorandom permutation `p` of `0...list_size-1` with ``seed`` as entropy.
///
/// Utilizes 'swap or not' shuffling found in
@@ -32,16 +62,20 @@ pub fn get_permutated_index(
let mut index = index;
for round in 0..shuffle_round_count {
let pivot = bytes_to_int64(&hash_with_round(seed, round)[..]) as usize % list_size;
let flip = (pivot + list_size - index) % list_size;
let position = max(index, flip);
let source = hash_with_round_and_position(seed, round, position)?;
let byte = source[(position % 256) / 8];
let bit = (byte >> (position % 8)) % 2;
index = if bit == 1 { flip } else { index }
index = do_round(seed, index, pivot, round, list_size)?;
}
Some(index)
}
fn do_round(seed: &[u8], index: usize, pivot: usize, round: u8, list_size: usize) -> Option<usize> {
let flip = (pivot + list_size - index) % list_size;
let position = max(index, flip);
let source = hash_with_round_and_position(seed, round, position)?;
let byte = source[(position % 256) / 8];
let bit = (byte >> (position % 8)) % 2;
Some(if bit == 1 { flip } else { index })
}
fn hash_with_round_and_position(seed: &[u8], round: u8, position: usize) -> Option<Vec<u8>> {
let mut seed = seed.to_vec();
seed.append(&mut int_to_bytes1(round));