From 817ff72265efe20d6b511d2d6cd16cc6dfbff4f4 Mon Sep 17 00:00:00 2001
From: Robbert van der Helm <mail@robbertvanderhelm.nl>
Date: Tue, 8 Mar 2022 20:42:59 +0100
Subject: [PATCH] Add a PCG implementation for pcg32i and floats

---
 plugins/crisp/src/lib.rs |  2 +
 plugins/crisp/src/pcg.rs | 79 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 81 insertions(+)
 create mode 100644 plugins/crisp/src/pcg.rs

diff --git a/plugins/crisp/src/lib.rs b/plugins/crisp/src/lib.rs
index efd98808..6e11068a 100644
--- a/plugins/crisp/src/lib.rs
+++ b/plugins/crisp/src/lib.rs
@@ -17,6 +17,8 @@
 use nih_plug::prelude::*;
 use std::pin::Pin;
 
+mod pcg;
+
 /// Hardcoded to make SIMD-ifying this a bit easier in the future
 const NUM_CHANNELS: usize = 2;
 
diff --git a/plugins/crisp/src/pcg.rs b/plugins/crisp/src/pcg.rs
new file mode 100644
index 00000000..1ecc362c
--- /dev/null
+++ b/plugins/crisp/src/pcg.rs
@@ -0,0 +1,79 @@
+// Crisp: a distortion plugin but not quite
+// Copyright (C) 2021-2022 Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+//! A minimal implementation of `pcg32i` PRNG from the PCG library. Implemented separately instead
+//! of using the rand crate implementation so we can adapt this for SIMD use.
+//!
+//! <https://github.com/imneme/pcg-c/blob/master/include/pcg_variants.h>
+//! <https://www.pcg-random.org/using-pcg-c.html>
+
+const PCG_DEFAULT_MULTIPLIER_32: u32 = 747796405;
+
+/// The `pcg32i` PRNG from PCG.
+#[derive(Copy, Clone)]
+pub struct Pcg32iState {
+    state: u32,
+    inc: u32,
+}
+
+impl Pcg32iState {
+    /// Initialize the PRNG, aka `*_srandom()`.
+    ///
+    /// <https://github.com/imneme/pcg-c/blob/83252d9c23df9c82ecb42210afed61a7b42402d7/include/pcg_variants.h#L757-L765>
+    pub fn new(state: u32, sequence: u32) -> Self {
+        let mut rng = Self {
+            state: 0,
+            inc: (sequence << 1) | 1,
+        };
+
+        rng.step();
+        rng.state += state;
+        rng.step();
+
+        rng
+    }
+
+    /// Generate a new uniformly distirubted `u32` covering all possible values.
+    ///
+    /// <https://github.com/imneme/pcg-c/blob/83252d9c23df9c82ecb42210afed61a7b42402d7/include/pcg_variants.h#L1711-L1717>
+    #[inline]
+    pub fn next_u32(&mut self) -> u32 {
+        let old_state = self.state;
+        self.step();
+
+        let word = ((old_state >> ((old_state >> 28) + 4)) ^ old_state) * 277803737;
+        (word >> 22) ^ word
+    }
+
+    /// Generate a new `f32` value in the open `(0, 1)` range.
+    #[inline]
+    pub fn next_f32(&mut self) -> f32 {
+        const FLOAT_SIZE: u32 = std::mem::size_of::<f32>() as u32 * 8;
+
+        // Implementation from https://docs.rs/rand/0.8.4/rand/distributions/struct.Open01.html
+        let value = self.next_u32();
+        let fraction = value >> (FLOAT_SIZE - f32::MANTISSA_DIGITS - 1);
+
+        let exponent_bits: u32 = ((f32::MAX_EXP - 1) as u32) << (f32::MANTISSA_DIGITS - 1);
+        f32::from_bits(fraction | exponent_bits) - (1.0 - f32::EPSILON / 2.0)
+    }
+
+    /// <https://github.com/imneme/pcg-c/blob/83252d9c23df9c82ecb42210afed61a7b42402d7/include/pcg_variants.h#L540-L543>
+    #[inline]
+    fn step(&mut self) {
+        self.state = self.state * PCG_DEFAULT_MULTIPLIER_32 + self.inc;
+    }
+}