mirror of
https://github.com/saymrwulf/xous-core.git
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597e24cd99
this is a space-optimized implementation of cosine. it's about 12k smaller than core lib's "proper" cosine. Gives about 9 bits amplitude resolution, and divides the waveform into 1024 time positions. Good enough for generating tones on a crappy speaker. Later on if anyone needs to "really" do cosine or sine, we'll have a footgun for them, but...let's hope Rust fixes issue https://github.com/rust-lang/rust/issues/105734 before then, and we can deprecate this library in favor of the mathematically accurate and canonical cosine implementation.
77 lines
No EOL
3.3 KiB
Rust
77 lines
No EOL
3.3 KiB
Rust
/*
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COS_TABLE generated with:
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#! /usr/bin/env python3
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import math
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print("const COS_TABLE: [u8; 256] = [", end="")
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for i in range(256):
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if i % 16 == 0:
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print("\n", end="")
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print("{:03}, ".format(int(255.0 * math.cos( ((3.14159265359 / 2.0) / 256) * float(i) ))), end="")
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print("];")
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*/
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/// One quarter of a cosine wave. The other four quarters are generated through symmetry.
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const COS_TABLE: [u8; 256] = [
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255, 254, 254, 254, 254, 254, 254, 254, 254, 254, 254, 254, 254, 254, 254, 253,
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253, 253, 253, 253, 253, 252, 252, 252, 252, 252, 251, 251, 251, 250, 250, 250,
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250, 249, 249, 249, 248, 248, 248, 247, 247, 246, 246, 246, 245, 245, 244, 244,
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244, 243, 243, 242, 242, 241, 241, 240, 240, 239, 239, 238, 237, 237, 236, 236,
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235, 234, 234, 233, 233, 232, 231, 231, 230, 229, 229, 228, 227, 227, 226, 225,
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224, 224, 223, 222, 221, 221, 220, 219, 218, 217, 217, 216, 215, 214, 213, 212,
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212, 211, 210, 209, 208, 207, 206, 205, 204, 203, 202, 201, 201, 200, 199, 198,
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197, 196, 195, 194, 193, 192, 191, 189, 188, 187, 186, 185, 184, 183, 182, 181,
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180, 179, 178, 176, 175, 174, 173, 172, 171, 170, 168, 167, 166, 165, 164, 162,
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161, 160, 159, 158, 156, 155, 154, 153, 151, 150, 149, 148, 146, 145, 144, 142,
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141, 140, 139, 137, 136, 135, 133, 132, 131, 129, 128, 127, 125, 124, 122, 121,
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120, 118, 117, 116, 114, 113, 111, 110, 109, 107, 106, 104, 103, 101, 100, 099,
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097, 096, 094, 093, 091, 090, 088, 087, 085, 084, 082, 081, 079, 078, 077, 075,
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074, 072, 071, 069, 068, 066, 064, 063, 061, 060, 058, 057, 055, 054, 052, 051,
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049, 048, 046, 045, 043, 042, 040, 038, 037, 035, 034, 032, 031, 029, 028, 026,
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024, 023, 021, 020, 018, 017, 015, 014, 012, 010, 009, 007, 006, 004, 003, 001,
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];
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use std::f32::consts::PI;
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enum Quadrant {
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FirstNormNorm,
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SecondReverseInvert,
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ThirdNormInvert,
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FourthReverseNorm,
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}
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/// LUT-based cosine computation. Has an `8-bit` resolution. Used to coarsely compute
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/// a cosine without relying on `std` functions. The concerns driving this implementation are:
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///
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/// - code size
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/// - xous-core issue #285 and https://github.com/rust-lang/rust/issues/105734
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///
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/// The issue above means any attempt to use `cos` (or any transcendental function) from
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/// Rust math library leads to a link time error. Until Rust can fix this regression we have
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/// to implement our own cosine function, or use `thin` LTO which adds about 7% to the size
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/// of the kernel, making it by far the single largest contributing factor to kernel bloat.
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pub fn cos(a: f32) -> f32 {
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let pos = a % (2.0 * PI);
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let q = if pos < (PI / 2.0) {
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Quadrant::FirstNormNorm
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} else if pos < (PI) {
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Quadrant::SecondReverseInvert
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} else if pos < (3.0 * PI / 2.0) {
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Quadrant::ThirdNormInvert
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} else {
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Quadrant::FourthReverseNorm
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};
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let idx = (256.0 * (a % (PI / 2.0)) / (PI / 2.0)) as usize;
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match q {
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Quadrant::FirstNormNorm => {
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(COS_TABLE[idx] as f32) / 255.0
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},
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Quadrant::SecondReverseInvert => {
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(COS_TABLE[255-idx] as f32) / -255.0
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},
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Quadrant::ThirdNormInvert => {
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(COS_TABLE[idx] as f32) / -255.0
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},
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Quadrant::FourthReverseNorm => {
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(COS_TABLE[255-idx] as f32) / 255.0
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}
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}
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} |