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DSP_fft
4-104
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− */
/* Offset to next subtable of twiddle factors. With each iteration */
/* of the above block, six twiddle factors get read, s times, */
/* hence the offset into the twiddle factor array is advanced by */
/* this amount. */
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− */
t += 6 * s;
}
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− */
/* Get the magnitude of ”n”, so we know how many digits to reverse. */
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− */
for (i = 31, m = 1; (n & (1 << i)) == 0; i−−, m++) ;
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− */
/* Perform final stage with digit reversal. */
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− */
s = n >> 2;
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
/* One of the nice features, of this last stage are that, no multiplies */
/* are required. In addition, the data always strides by a fixed amount */
/* namely 8 elements. Since the data is stored as interleaved pairs, of */
/* real and imaginary data, the first eight elements contain the data */
/* for the first four complex inputs. These are the inputs to the first */
/* radix4 butterfly. */
/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/
#ifndef NOASSUME
#pragma MUST_ITERATE(4,,4);
#endif
for (i = 0; i < n; i += 4)
{
short x0r, x0i, x1r, x1i, x2r, x2i, x3r, x3i;
short y0r, y0i, y1r, y1i, y2r, y2i, y3r, y3i;
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− */
/* Read the four samples that are the input to this butterfly. */
/* −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− */