From f1f6f4aa131cd90568ef8c022c3d41be5dd857fe Mon Sep 17 00:00:00 2001
From: Kostya Shishkov <kostya.shishkov@gmail.com>
Date: Thu, 11 Oct 2018 17:57:55 +0200
Subject: [PATCH] IMDCT implementation

---
 src/dsp/mdct.rs | 56 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/src/dsp/mdct.rs b/src/dsp/mdct.rs
index e69de29..b292739 100644
--- a/src/dsp/mdct.rs
+++ b/src/dsp/mdct.rs
@@ -0,0 +1,56 @@
+use std::f32::consts;
+use super::fft::*;
+
+pub struct IMDCT {
+    twiddle:    Vec<FFTComplex>,
+    fft:        FFT,
+    size:       usize,
+    z:          Vec<FFTComplex>,
+}
+
+/*
+fn imdct(src: &[f32], dst: &mut [f32], length: usize) {
+    for n in 0..length*2 {
+        dst[n] = 0.0;
+        for k in 0..length {
+            dst[n] += src[k] * (consts::PI / (length as f32) * ((n as f32) + 0.5 + ((length/2) as f32)) * ((k as f32) + 0.5)).cos();
+        }
+    }
+}*/
+
+impl IMDCT {
+    pub fn new(mode: FFTMode, size: usize) -> Self {
+        let mut twiddle: Vec<FFTComplex> = Vec::with_capacity(size / 4);
+        let factor = 2.0 * consts::PI / ((8 * size) as f32);
+        for k in 0..size/4 {
+            twiddle.push(FFTComplex::exp(factor * ((8 * k + 1) as f32)));
+        }
+        let fft = FFTBuilder::new_fft(mode, size/4);
+        let mut z: Vec<FFTComplex> = Vec::with_capacity(size / 2);
+        z.resize(size / 2, FFTC_ZERO);
+        IMDCT { twiddle, fft, size, z }
+    }
+    pub fn imdct(&mut self, src: &[f32], dst: &mut [f32]) {
+        let size2 = self.size / 2;
+        let size4 = self.size / 4;
+        let size8 = self.size / 8;
+        for k in 0..size4 {
+            let c = FFTComplex { re: src[size2 - 2 * k - 1], im: src[        2 * k] };
+            self.z[k] = c * self.twiddle[k];
+        }
+        self.fft.do_fft_inplace(&mut self.z, false);
+        for k in 0..size4 {
+            self.z[k] *= self.twiddle[k];
+        }
+        for n in 0..size8 {
+            dst[            2 * n]     = -self.z[size8 + n]    .im;
+            dst[            2 * n + 1] =  self.z[size8 - n - 1].re;
+            dst[    size4 + 2 * n]     = -self.z[        n]    .re;
+            dst[    size4 + 2 * n + 1] =  self.z[size4 - n - 1].im;
+            dst[    size2 + 2 * n]     = -self.z[size8 + n]    .re;
+            dst[    size2 + 2 * n + 1] =  self.z[size8 - n - 1].im;
+            dst[3 * size4 + 2 * n]     =  self.z[        n]    .im;
+            dst[3 * size4 + 2 * n + 1] = -self.z[size4 - n - 1].re;
+        }
+    }
+}
-- 
2.39.5