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const dctSize = 512; // Default DCT size, read from header
// --- Utility Functions for Audio Processing ---
// Hanning window for smooth audio transitions (JavaScript equivalent)
function hanningWindow(size) {
const window = new Float32Array(size);
const PI = Math.PI;
for (let i = 0; i < size; i++) {
window[i] = 0.5 * (1 - Math.cos((2 * PI * i) / (size - 1)));
}
return window;
}
const hanningWindowArray = hanningWindow(dctSize); // Pre-calculate window
// ============================================================================
// FFT-based DCT/IDCT Implementation
// ============================================================================
// Bit-reversal permutation (in-place)
function bitReversePermute(real, imag, N) {
let temp_bits = N;
let num_bits = 0;
while (temp_bits > 1) {
temp_bits >>= 1;
num_bits++;
}
for (let i = 0; i < N; i++) {
let j = 0;
let temp = i;
for (let b = 0; b < num_bits; b++) {
j = (j << 1) | (temp & 1);
temp >>= 1;
}
if (j > i) {
const tmp_real = real[i];
const tmp_imag = imag[i];
real[i] = real[j];
imag[i] = imag[j];
real[j] = tmp_real;
imag[j] = tmp_imag;
}
}
}
// In-place radix-2 FFT
function fftRadix2(real, imag, N, direction) {
const PI = Math.PI;
for (let stage_size = 2; stage_size <= N; stage_size *= 2) {
const half_stage = stage_size / 2;
const angle = direction * 2.0 * PI / stage_size;
let wr = 1.0;
let wi = 0.0;
const wr_delta = Math.cos(angle);
const wi_delta = Math.sin(angle);
for (let k = 0; k < half_stage; k++) {
for (let group_start = k; group_start < N; group_start += stage_size) {
const i = group_start;
const j = group_start + half_stage;
const temp_real = real[j] * wr - imag[j] * wi;
const temp_imag = real[j] * wi + imag[j] * wr;
real[j] = real[i] - temp_real;
imag[j] = imag[i] - temp_imag;
real[i] = real[i] + temp_real;
imag[i] = imag[i] + temp_imag;
}
const wr_old = wr;
wr = wr_old * wr_delta - wi * wi_delta;
wi = wr_old * wi_delta + wi * wr_delta;
}
}
}
function fftForward(real, imag, N) {
bitReversePermute(real, imag, N);
fftRadix2(real, imag, N, +1);
}
function fftInverse(real, imag, N) {
bitReversePermute(real, imag, N);
fftRadix2(real, imag, N, -1);
const scale = 1.0 / N;
for (let i = 0; i < N; i++) {
real[i] *= scale;
imag[i] *= scale;
}
}
// DCT-II via FFT using reordering method
function javascript_dct_fft(input, N) {
const PI = Math.PI;
const real = new Float32Array(N);
const imag = new Float32Array(N);
for (let i = 0; i < N / 2; i++) {
real[i] = input[2 * i];
real[N - 1 - i] = input[2 * i + 1];
}
fftForward(real, imag, N);
const output = new Float32Array(N);
for (let k = 0; k < N; k++) {
const angle = -PI * k / (2.0 * N);
const wr = Math.cos(angle);
const wi = Math.sin(angle);
const dct_value = real[k] * wr - imag[k] * wi;
if (k === 0) {
output[k] = dct_value * Math.sqrt(1.0 / N);
} else {
output[k] = dct_value * Math.sqrt(2.0 / N);
}
}
return output;
}
// IDCT (DCT-III) via FFT using reordering method
function javascript_idct_fft(input, N) {
const PI = Math.PI;
const real = new Float32Array(N);
const imag = new Float32Array(N);
for (let k = 0; k < N; k++) {
const angle = PI * k / (2.0 * N);
const wr = Math.cos(angle);
const wi = Math.sin(angle);
let scaled;
if (k === 0) {
scaled = input[k] / Math.sqrt(1.0 / N);
} else {
scaled = input[k] / Math.sqrt(2.0 / N) * 2.0;
}
real[k] = scaled * wr;
imag[k] = scaled * wi;
}
fftInverse(real, imag, N);
const output = new Float32Array(N);
for (let i = 0; i < N / 2; i++) {
output[2 * i] = real[i];
output[2 * i + 1] = real[N - 1 - i];
}
return output;
}
// Fast O(N log N) IDCT using FFT
function javascript_idct_512(input) {
return javascript_idct_fft(input, dctSize);
}
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