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Change hand-picked RNGs back to SmallRng

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While `Xoshiro256+` is faster on 64-bit, it has low linear complexity in the
lower three bits, which *are* used when generating dither.

Also, while `Xoshiro128StarStar` access one less variable from the heap,
multiplication is generally slower than addition in hardware.
This commit is contained in:
Roderick van Domburg 2021-10-21 19:31:58 +02:00
parent 4c89a721ee
commit ff3648434b
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3 changed files with 7 additions and 34 deletions
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28
playback/src/dither.rs
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@ -1,3 +1,4 @@
use rand::rngs::SmallRng;
use rand::SeedableRng;
use rand_distr::{Distribution, Normal, Triangular, Uniform};
use std::fmt;
@ -41,29 +42,12 @@ impl fmt::Display for dyn Ditherer {
}
}
// `SmallRng` is 33% faster than `ThreadRng`, but we can do even better.
// `SmallRng` defaults to `Xoshiro256PlusPlus` on 64-bit platforms and
// `Xoshiro128PlusPlus` on 32-bit platforms. These are excellent for the
// general case. In our case of just 64-bit floating points, we can make
// some optimizations. Compared to `SmallRng`, these hand-picked generators
// improve performance by another 9% on 64-bit platforms and 2% on 32-bit
// platforms.
//
// For reference, see https://prng.di.unimi.it. Note that we do not use
// `Xoroshiro128Plus` or `Xoshiro128Plus` because they display low linear
// complexity in the lower four bits, which is not what we want:
// linearization is the very point of dithering.
#[cfg(target_pointer_width = "64")]
type Rng = rand_xoshiro::Xoshiro256Plus;
#[cfg(not(target_pointer_width = "64"))]
type Rng = rand_xoshiro::Xoshiro128StarStar;
fn create_rng() -> Rng {
Rng::from_entropy()
fn create_rng() -> SmallRng {
SmallRng::from_entropy()
}
pub struct TriangularDitherer {
cached_rng: Rng,
cached_rng: SmallRng,
distribution: Triangular<f64>,
}
@ -90,7 +74,7 @@ impl TriangularDitherer {
}
pub struct GaussianDitherer {
cached_rng: Rng,
cached_rng: SmallRng,
distribution: Normal<f64>,
}
@ -119,7 +103,7 @@ impl GaussianDitherer {
pub struct HighPassDitherer {
active_channel: usize,
previous_noises: [f64; NUM_CHANNELS],
cached_rng: Rng,
cached_rng: SmallRng,
distribution: Uniform<f64>,
}