Changing bit_reverse in fft.c

Author: Gathros

chapters/FFT/code/c/fft.c

@@ -4,14 +4,12 @@
 #include <stdlib.h>
 #include <time.h>
 
-#define PI 3.1415926535897932384626
-
 void dft(double complex *X, const size_t N) {
  double complex tmp[N];
  for (size_t i = 0; i < N; ++i) {
  tmp[i] = 0;
  for (size_t j = 0; j < N; ++j) {
- tmp[i] += X[j] * cexp(-2.0 * PI * I * j * i / N);
+ tmp[i] += X[j] * cexp(-2.0 * M_PI * I * j * i / N);
  }
  }
 
@@ -35,27 +33,30 @@ void cooley_tukey(double complex *X, const size_t N) {
  cooley_tukey(X + N / 2, N / 2);
 
  for (size_t i = 0; i < N / 2; ++i) {
- X[i + N / 2] = X[i] - cexp(-2.0 * I * PI * i / N) * X[i + N / 2];
+ X[i + N / 2] = X[i] - cexp(-2.0 * I * M_PI * i / N) * X[i + N / 2];
  X[i] -= (X[i + N / 2]-X[i]);
  }
  }
 }
 
 void bit_reverse(double complex *X, size_t N) {
- double complex temp;
- unsigned int b;
+ for (int i = 0; i < N; ++i) {
+ int n = i;
+ int a = i;
+ int count = (int)log2((double)N) - 1;
+
+ n >>= 1;
+ while (n > 0) {
+ a = (a << 1) | (n & 1);
+ count--;
+ n >>= 1;
+ }
+ n = (a << count) & ((1 << (int)log2((double)N)) - 1);
 
- for (unsigned int i = 0; i < N; ++i) {
- b = i;
- b = (((b & 0xaaaaaaaa) >> 1) | ((b & 0x55555555) << 1));
- b = (((b & 0xcccccccc) >> 2) | ((b & 0x33333333) << 2));
- b = (((b & 0xf0f0f0f0) >> 4) | ((b & 0x0f0f0f0f) << 4));
- b = (((b & 0xff00ff00) >> 8) | ((b & 0x00ff00ff) << 8));
- b = ((b >> 16) | (b << 16)) >> (32 - (unsigned int) log2((double)N));
- if (b > i) {
- temp = X[b];
- X[b] = X[i];
- X[i] = temp;
+ if (n > i) {
+ double complex tmp = X[i];
+ X[i] = X[n];
+ X[n] = tmp;
  }
  }
 }
@@ -65,7 +66,7 @@ void iterative_cooley_tukey(double complex *X, size_t N) {
 
  for (int i = 1; i <= log2((double)N); ++i) {
  int stride = pow(2, i);
- double complex w = cexp(-2.0 * I * PI / stride);
+ double complex w = cexp(-2.0 * I * M_PI / stride);
  for (size_t j = 0; j < N; j += stride) {
  double complex v = 1.0;
  for (size_t k = 0; k < stride / 2; ++k) {
@@ -93,8 +94,7 @@ int main() {
  }
 
  cooley_tukey(y, 64);
- //iterative_cooley_tukey(z, 64);
- dft(z, 64);
+ iterative_cooley_tukey(z, 64);
 
  approx(y, z, 64);