safe_shuffle/
lib.rs

1use rand_core::{CryptoRng, RngCore};
2
3pub struct SafeShuffler<R: CryptoRng + RngCore> {
4    rng: R,
5}
6
7impl<R: CryptoRng + RngCore> SafeShuffler<R> {
8    pub fn new(rng: R) -> Self {
9        SafeShuffler { rng }
10    }
11
12    pub fn shuffle<T>(&mut self, mut seq: Vec<T>) -> Vec<T> {
13        for i in 0..seq.len() {
14            let index = self.safe_range(seq.len());
15            seq.swap(i, index as usize);
16        }
17        seq
18    }
19
20    fn safe_range(&mut self, upper_bound: usize) -> usize {
21        let _word_size = usize::BITS / 8;
22
23        let bit_length = usize::MAX.count_ones() - upper_bound.leading_zeros();
24        let min_byte_length = (bit_length + 7) / 8;
25
26        let mut buffer: [u8; 8] = [0; 8];
27        let mut num: usize;
28
29        loop {
30            self.rng.fill_bytes(&mut buffer);
31
32            let byte_mask = (1 << (min_byte_length * 8)) - 1;
33            let msb_offset = 8 - (bit_length & 7);
34            let bit_mask = byte_mask >> msb_offset;
35
36            let unmasked = usize::from_be_bytes(buffer);
37            num = unmasked & bit_mask;
38
39            if num < upper_bound {
40                break;
41            }
42        }
43        num
44    }
45}
safe_shuffle/lib.rs

safe_shuffle/
lib.rs
