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use crate::kiss_fft::{KissCpx, KissFftState, opus_fft_impl};
use std::f32::consts::PI;
const MAX_N2: usize = 960;
const MAX_N4: usize = 480;
pub struct MdctLookup {
pub n: usize,
pub max_lm: usize,
kfft: Vec<Option<KissFftState>>,
trig: Vec<f32>,
}
impl MdctLookup {
pub fn new(n: usize, max_lm: usize) -> Self {
let mut kfft = Vec::new();
let mut trig = Vec::new();
let mut curr_n = n;
for shift in 0..=max_lm {
let n4 = curr_n / 4;
if shift == 0 {
kfft.push(KissFftState::new(n4));
} else if let Some(base) = kfft.first().unwrap().as_ref() {
kfft.push(KissFftState::new_sub(base, n4));
} else {
kfft.push(None);
}
let n2 = curr_n / 2;
for i in 0..n2 {
let angle = 2.0 * PI * (i as f32 + 0.125) / curr_n as f32;
trig.push(angle.cos());
}
curr_n >>= 1;
}
Self {
n,
max_lm,
kfft,
trig,
}
}
fn get_trig(&self, shift: usize) -> (&[f32], usize) {
let mut offset = 0;
let mut curr_n = self.n;
for _ in 0..shift {
offset += curr_n / 2;
curr_n >>= 1;
}
(&self.trig[offset..offset + curr_n / 2], curr_n / 4)
}
pub fn get_trig_debug(&self, shift: usize) -> &[f32] {
let (trig, _) = self.get_trig(shift);
trig
}
#[inline]
pub fn forward(
&self,
input: &[f32],
output: &mut [f32],
window: &[f32],
overlap: usize,
shift: usize,
stride: usize,
) {
let st = self.kfft[shift]
.as_ref()
.expect("FFT state not initialized");
let n = self.n >> shift;
let n2 = n / 2;
let n4 = n / 4;
let scale = st.scale();
let (trig, _) = self.get_trig(shift);
let overlap2 = overlap / 2;
let mut f_buf = [0.0f32; MAX_N2];
let mut f2_buf = [KissCpx::new(0.0, 0.0); MAX_N4];
let f = &mut f_buf[..n2];
let f2 = &mut f2_buf[..n4];
// Assert caller invariants so LLVM can prove all loop accesses in-bounds
// and eliminate per-element conditional checks, enabling auto-vectorization.
// Max input index accessed = n/2 + overlap/2 - 1 (see loop analysis), so we need
// at least n/2 + overlap/2 elements. n + overlap is the theoretical over-estimate
// but the actual accesses stay within n/2 + overlap/2 due to the fold structure.
assert!(input.len() >= n2 + overlap2);
assert!(window.len() >= overlap);
{
let mut yp = 0usize;
let mut xp1 = overlap2;
let mut xp2 = n2 - 1 + overlap2;
let mut wp1 = overlap2;
// wp2 can underflow on the final post-loop decrement (value never read after),
// so saturating_sub is used only here; all other pointers stay non-negative.
let mut wp2 = overlap2.saturating_sub(1);
let limit = (overlap + 3) / 4;
let mid = if n4 > limit { n4 - limit } else { 0 };
// Loop 1: windowed fold (first overlap region).
// All indices proved valid when input.len()>=n+overlap, window.len()>=overlap.
let loop1_iters = limit.min(n4);
for _ in 0..loop1_iters {
let w1 = window[wp1];
let w2 = window[wp2];
f[yp] = input[xp1 + n2] * w2 + input[xp2] * w1;
yp += 1;
f[yp] = input[xp1] * w1 - input[xp2 - n2] * w2;
yp += 1;
xp1 += 2;
xp2 -= 2;
wp1 += 2;
wp2 = wp2.saturating_sub(2);
}
// Loop 2: no window (middle region, straight interleaved copy).
for _ in limit..mid {
f[yp] = input[xp2];
yp += 1;
f[yp] = input[xp1];
yp += 1;
xp1 += 2;
xp2 -= 2;
}
// Loop 3: windowed fold (second overlap region).
// At loop3 start, xp1 = n2 exactly (identity: overlap2 + 2*mid = n2).
let loop3_iters = if mid > limit { n4 - mid } else { 0 };
let mut wp1_l3 = 0usize;
let mut wp2_l3 = overlap.saturating_sub(1);
for _ in 0..loop3_iters {
let w1 = window[wp1_l3];
let w2 = window[wp2_l3];
f[yp] = -input[xp1 - n2] * w1 + input[xp2] * w2;
yp += 1;
f[yp] = input[xp1] * w2 + input[xp2 + n2] * w1;
yp += 1;
xp1 += 2;
xp2 -= 2;
wp1_l3 += 2;
wp2_l3 -= 2;
}
}
// Pre-rotation with bitrev indexing
for i in 0..n4 {
let re = f[2 * i];
let im = f[2 * i + 1];
let t0 = trig[i];
let t1 = trig[n4 + i];
let yr = re * t0 - im * t1;
let yi = im * t0 + re * t1;
f2[st.bitrev[i] as usize] = KissCpx::new(yr * scale, yi * scale);
}
opus_fft_impl(st, f2);
// Post-rotation
for i in 0..n4 {
let fp = &f2[i];
let t0 = trig[i];
let t1 = trig[n4 + i];
let yr = fp.i * t1 - fp.r * t0;
let yi = fp.r * t1 + fp.i * t0;
output[i * 2 * stride] = yr;
output[stride * (n2 - 1 - 2 * i)] = yi;
}
}
#[inline]
pub fn backward(
&self,
input: &[f32],
output: &mut [f32],
window: &[f32],
overlap: usize,
shift: usize,
stride: usize,
) {
let st = self.kfft[shift]
.as_ref()
.expect("FFT state not initialized");
let n = self.n >> shift;
let n2 = n / 2;
let n4 = n / 4;
let overlap2 = overlap / 2;
let (trig, _) = self.get_trig(shift);
let mut f2_buf = [KissCpx::new(0.0, 0.0); MAX_N4];
let f2 = &mut f2_buf[..n4];
for i in 0..n4 {
let rev = st.bitrev[i] as usize;
let x1 = input[2 * i * stride];
let x2 = input[stride * (n2 - 1 - 2 * i)];
let t0 = trig[i];
let t1 = trig[n4 + i];
let yr = x2 * t0 + x1 * t1;
let yi = x1 * t0 - x2 * t1;
f2[rev] = KissCpx::new(yi, yr);
}
opus_fft_impl(st, f2);
// Pre-rotate: Write to temp buffer starting at overlap2 (like C's out+(overlap>>1))
let mut temp = vec![0.0f32; n + overlap];
for i in 0..n4 {
temp[overlap2 + 2 * i] = f2[i].r;
temp[overlap2 + 2 * i + 1] = f2[i].i;
}
// Post-rotate from both ends
// C reads re=yp0[1], im=yp0[0] (swapped because using FFT instead of IFFT)
for i in 0..((n4 + 1) >> 1) {
let im0 = temp[overlap2 + 2 * i];
let re0 = temp[overlap2 + 2 * i + 1];
let t0_0 = trig[i];
let t1_0 = trig[n4 + i];
let yr0 = re0 * t0_0 + im0 * t1_0;
let yi0 = re0 * t1_0 - im0 * t0_0;
let im1 = temp[overlap2 + n2 - 2 - 2 * i];
let re1 = temp[overlap2 + n2 - 1 - 2 * i];
let t0_1 = trig[n4 - i - 1];
let t1_1 = trig[n2 - i - 1];
let yr1 = re1 * t0_1 + im1 * t1_1;
let yi1 = re1 * t1_1 - im1 * t0_1;
temp[overlap2 + 2 * i] = yr0;
temp[overlap2 + n2 - 1 - 2 * i] = yi0;
temp[overlap2 + n2 - 2 - 2 * i] = yr1;
temp[overlap2 + 2 * i + 1] = yi1;
}
// TDAC: Copy to output with windowing
// C code's TDAC reads from:
// yp1 = out[0..overlap/2) - previous frame's overlap data (preserved by caller)
// xp1 = out[overlap-1..overlap/2) - current frame's IMDCT output
// The caller must preserve overlap samples between frames for TDAC to work correctly.
// Copy post-rotated data to output[overlap/2..overlap/2+n2]
// This is where the IMDCT output goes (matching C's post-rotation output location)
for i in 0..n2 {
output[overlap2 + i] = temp[overlap2 + i];
}
// Apply TDAC to overlap region
// C code: xp1 = out+overlap-1, yp1 = out
// x1 = *xp1 (reads from out[overlap-1] down to out[overlap/2])
// x2 = *yp1 (reads from out[0] up to out[overlap/2-1])
// The key insight: x2 comes from the START of the output buffer,
// which contains the previous frame's overlap data (preserved by caller)
for i in 0..overlap2 {
// x1: current frame's IMDCT output at the end of overlap region
let x1 = output[overlap - 1 - i];
// x2: previous frame's overlap data at the start of buffer (or zeros for first frame)
let x2 = output[i];
let wp1 = window[i];
let wp2 = window[overlap - 1 - i];
output[i] = x2 * wp2 - x1 * wp1;
output[overlap - 1 - i] = x2 * wp1 + x1 * wp2;
}
}
}