dft_precompute.c

#include <math.h>					//Required for cos and sin functions
typedef double IN_TYPE;		// Data type for the input signal
typedef double TEMP_TYPE; // Data type for the temporary variables
#define N 256							// DFT Size

void dft(IN_TYPE sample_real[N], IN_TYPE sample_imag[N]) {
	int i, j;
	TEMP_TYPE w;
	TEMP_TYPE c, s;

	// Temporary arrays to hold the intermediate frequency domain results
	TEMP_TYPE temp_real[N];
	TEMP_TYPE temp_imag[N];

	// Calculate each frequency domain sample iteratively
	for (i = 0; i < N; i += 1) {
		temp_real[i] = 0;
		temp_imag[i] = 0;

		// Calculate the jth frequency sample sequentially
		for (j = 0; j < N; j += 1) {
			// Fetch precomputed sine and cosine values
			c = cos_table[i * j];
			s = sin_table[i * j];

			// Multiply c and s with the appropriate input sample and keep running sum
			temp_real[i] += (sample_real[j] * c - sample_imag[j] * s);
			temp_imag[i] += (sample_real[j] * s + sample_imag[j] * c);
		}
	}

	// Perform an inplace DFT, i.e., copy result into the input arrays
	for (i = 0; i < N; i += 1) {
		sample_real[i] = temp_real[i];
		sample_imag[i] = temp_imag[i];
	}
}