53 lines
1.5 KiB
Rust
53 lines
1.5 KiB
Rust
use cap_rand::RngCore;
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/// Implement `WasiRandom` using a deterministic cycle of bytes.
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pub struct Deterministic {
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cycle: std::iter::Cycle<std::vec::IntoIter<u8>>,
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}
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impl Deterministic {
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pub fn new(bytes: Vec<u8>) -> Self {
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Deterministic {
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cycle: bytes.into_iter().cycle(),
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}
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}
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}
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impl RngCore for Deterministic {
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fn next_u32(&mut self) -> u32 {
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let b0 = self.cycle.next().expect("infinite sequence");
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let b1 = self.cycle.next().expect("infinite sequence");
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let b2 = self.cycle.next().expect("infinite sequence");
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let b3 = self.cycle.next().expect("infinite sequence");
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((b0 as u32) << 24) + ((b1 as u32) << 16) + ((b2 as u32) << 8) + (b3 as u32)
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}
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fn next_u64(&mut self) -> u64 {
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let w0 = self.next_u32();
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let w1 = self.next_u32();
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((w0 as u64) << 32) + (w1 as u64)
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}
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fn fill_bytes(&mut self, buf: &mut [u8]) {
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for b in buf.iter_mut() {
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*b = self.cycle.next().expect("infinite sequence");
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}
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}
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fn try_fill_bytes(&mut self, buf: &mut [u8]) -> Result<(), cap_rand::Error> {
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self.fill_bytes(buf);
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Ok(())
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}
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}
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#[cfg(test)]
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mod test {
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use super::*;
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#[test]
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fn deterministic() {
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let mut det = Deterministic::new(vec![1, 2, 3, 4]);
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let mut buf = vec![0; 1024];
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det.try_fill_bytes(&mut buf).expect("get randomness");
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for (ix, b) in buf.iter().enumerate() {
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assert_eq!(*b, (ix % 4) as u8 + 1)
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}
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}
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}
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