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switch gen_biguint to fill_bytes #53

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merged 3 commits into from
Jul 5, 2018
Merged

switch gen_biguint to fill_bytes #53

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af35fd3
switch gen_biguint to fill_bytes
May 31, 2018
b2c9577
rand benches
Jun 26, 2018
8b5a092
Add tests for PRNG value stability
cuviper Jul 2, 2018
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46 changes: 46 additions & 0 deletions benches/bigint.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -214,6 +214,52 @@ fn from_str_radix_36(b: &mut Bencher) {
from_str_radix_bench(b, 36);
}

fn rand_bench(b: &mut Bencher, bits: usize) {
let mut rng = get_rng();

b.iter(|| rng.gen_bigint(bits));
}

#[bench]
fn rand_64(b: &mut Bencher) {
rand_bench(b, 1 << 6);
}

#[bench]
fn rand_256(b: &mut Bencher) {
rand_bench(b, 1 << 8);
}

#[bench]
fn rand_1009(b: &mut Bencher) {
rand_bench(b, 1009);
}

#[bench]
fn rand_2048(b: &mut Bencher) {
rand_bench(b, 1 << 11);
}

#[bench]
fn rand_4096(b: &mut Bencher) {
rand_bench(b, 1 << 12);
}

#[bench]
fn rand_8192(b: &mut Bencher) {
rand_bench(b, 1 << 13);
}

#[bench]
fn rand_65536(b: &mut Bencher) {
rand_bench(b, 1 << 16);
}

#[bench]
fn rand_131072(b: &mut Bencher) {
rand_bench(b, 1 << 17);
}

#[bench]
fn shl(b: &mut Bencher) {
let n = BigUint::one() << 1000;
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15 changes: 9 additions & 6 deletions src/bigrand.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,6 +2,7 @@

use rand::prelude::*;
use rand::distributions::uniform::{SampleUniform, UniformSampler};
use rand::AsByteSliceMut;

use BigInt;
use BigUint;
Expand Down Expand Up @@ -39,13 +40,15 @@ impl<R: Rng + ?Sized> RandBigInt for R {
fn gen_biguint(&mut self, bit_size: usize) -> BigUint {
use super::big_digit::BITS;
let (digits, rem) = bit_size.div_rem(&BITS);
let mut data = Vec::with_capacity(digits + 1);
for _ in 0..digits {
data.push(self.gen());
}
let mut data = vec![BigDigit::default(); digits + (rem > 0) as usize];
// `fill_bytes` is faster than many `gen::<u32>` calls
self.fill_bytes(data[..].as_byte_slice_mut());
// Swap bytes per the `Rng::fill` source. This might be
// unnecessary if reproducibility across architectures is not
// desired.
data.to_le();
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This is an interesting question -- what guarantees should we make about reproducibility? Is this PR a breaking change altogether, since seeded RNGs may now produce a different BigUint? Maybe that's going too far to consider this across crate versions, but if we want it across architectures, we'll also need to be careful in the future about conditional BigDigit sizes, for instance. (Right now this is using the public BigUint::new though, which should always be u32-based even if the internals change.)

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This may be a value-breaking change (depending on the PRNG used), but I think will usually not be. (Rand does not specify that fill_bytes must output the same bits in the same order as repeated calls to next_u32, though I think this is the case for all current generators.)

if rem > 0 {
let final_digit: BigDigit = self.gen();
data.push(final_digit >> (BITS - rem));
data[digits] >>= BITS - rem;
}
BigUint::new(data)
}
Expand Down