85 lines
3.1 KiB
Rust
85 lines
3.1 KiB
Rust
// Crisp: a distortion plugin but not quite
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// Copyright (C) 2022-2023 Robbert van der Helm
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//
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// This program is free software: you can redistribute it and/or modify
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// it under the terms of the GNU General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program. If not, see <https://www.gnu.org/licenses/>.
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//! A minimal implementation of `pcg32i` PRNG from the PCG library. Implemented separately instead
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//! of using the rand crate implementation so we can adapt this for SIMD use.
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//!
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//! <https://github.com/imneme/pcg-c/blob/master/include/pcg_variants.h>
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//! <https://www.pcg-random.org/using-pcg-c.html>
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const PCG_DEFAULT_MULTIPLIER_32: u32 = 747796405;
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/// The `pcg32i` PRNG from PCG.
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#[derive(Copy, Clone)]
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pub struct Pcg32iState {
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state: u32,
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inc: u32,
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}
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impl Pcg32iState {
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/// Initialize the PRNG, aka `*_srandom()`.
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///
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/// <https://github.com/imneme/pcg-c/blob/83252d9c23df9c82ecb42210afed61a7b42402d7/include/pcg_variants.h#L757-L765>
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pub const fn new(state: u32, sequence: u32) -> Self {
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let mut rng = Self {
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state: 0,
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inc: (sequence << 1) | 1,
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};
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// https://github.com/imneme/pcg-c/blob/83252d9c23df9c82ecb42210afed61a7b42402d7/include/pcg_variants.h#L540-L543,
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// inlined so we can make this a const function
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rng.state = rng
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.state
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.wrapping_mul(PCG_DEFAULT_MULTIPLIER_32)
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.wrapping_add(rng.inc);
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rng.state += state;
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rng.state = rng
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.state
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.wrapping_mul(PCG_DEFAULT_MULTIPLIER_32)
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.wrapping_add(rng.inc);
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rng
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}
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/// Generate a new uniformly distirubted `u32` covering all possible values.
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///
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/// <https://github.com/imneme/pcg-c/blob/83252d9c23df9c82ecb42210afed61a7b42402d7/include/pcg_variants.h#L1711-L1717>
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#[inline]
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pub fn next_u32(&mut self) -> u32 {
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let old_state = self.state;
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self.state = self
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.state
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.wrapping_mul(PCG_DEFAULT_MULTIPLIER_32)
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.wrapping_add(self.inc);
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let word = ((old_state >> ((old_state >> 28) + 4)) ^ old_state).wrapping_mul(277803737);
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(word >> 22) ^ word
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}
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/// Generate a new `f32` value in the open `(0, 1)` range.
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#[inline]
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pub fn next_f32(&mut self) -> f32 {
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const FLOAT_SIZE: u32 = std::mem::size_of::<f32>() as u32 * 8;
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// Implementation from https://docs.rs/rand/0.8.4/rand/distributions/struct.Open01.html
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let value = self.next_u32();
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let fraction = value >> (FLOAT_SIZE - f32::MANTISSA_DIGITS - 1);
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let exponent_bits: u32 = ((f32::MAX_EXP - 1) as u32) << (f32::MANTISSA_DIGITS - 1);
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f32::from_bits(fraction | exponent_bits) - (1.0 - f32::EPSILON / 2.0)
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}
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}
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