File content as of revision 0:5e2a4b964485:

// http://www.mit.edu/~emin/source_code/fft/index.html

#define SIN_2PI_16 0.38268343236508978
#define SIN_4PI_16 0.707106781186547460
#define SIN_6PI_16 0.923879532511286740
#define C_P_S_2PI_16 1.30656296487637660
#define C_M_S_2PI_16 0.54119610014619690
#define C_P_S_6PI_16 1.3065629648763766
#define C_M_S_6PI_16 -0.54119610014619690

void fft1(float input[16],float output[16] ) {
  float temp, out0, out1, out2, out3, out4, out5, out6, out7, out8;
  float out9,out10,out11,out12,out13,out14,out15;

  out0=input[0]+input[8]; /* output[0 through 7] is the data that we */
  out1=input[1]+input[9]; /* take the 8 point real FFT of. */
  out2=input[2]+input[10];
  out3=input[3]+input[11];
  out4=input[4]+input[12];
  out5=input[5]+input[13];
  out6=input[6]+input[14];
  out7=input[7]+input[15];
  out8=input[0]-input[8];   /* inputs 8,9,10,11 are */
  out9=input[1]-input[9];   /* the Real part of the */
  out10=input[2]-input[10]; /* 4 point Complex FFT inputs.*/
  out11=input[3]-input[11]; 
  out12=input[12]-input[4]; /* outputs 12,13,14,15 are */
  out13=input[13]-input[5]; /* the Imaginary pars of  */
  out14=input[14]-input[6]; /* the 4 point Complex FFT inputs.*/
  out15=input[15]-input[7];
  temp=(out13-out9)*(SIN_2PI_16); 
  out9=out9*(C_P_S_2PI_16)+temp; 
  out13=out13*(C_M_S_2PI_16)+temp;
  out14*=(SIN_4PI_16);
  out10*=(SIN_4PI_16);
  out14=out14-out10;
  out10=out14+out10+out10;
  temp=(out15-out11)*(SIN_6PI_16);
  out11=out11*(C_P_S_6PI_16)+temp;
  out15=out15*(C_M_S_6PI_16)+temp;
  out8+=out10;
  out10=out8-out10-out10;
  out12+=out14;
  out14=out12-out14-out14;
  out9+=out11;
  out11=out9-out11-out11;
  out13+=out15;
  out15=out13-out15-out15;
  output[1]=out8+out9;
  output[7]=out8-out9;
  output[9]=out12+out13;
  output[15]=out13-out12;  
  output[5]=out10+out15;    /* implicit multiplies by */
  output[13]=out14-out11;   /* a twiddle factor of -j */                            
  output[3]=out10-out15;    /* implicit multiplies by */
  output[11]=-out14-out11;  /* a twiddle factor of -j */    
  out0=out0+out4;
  out4=out0-out4-out4;
  out1=out1+out5;
  out5=out1-out5-out5;
  out2+=out6;
  out6=out2-out6-out6;
  out3+=out7;
  out7=out3-out7-out7;  
  output[0]=out0+out2;
  output[4]=out0-out2;
  out1+=out3;
  output[12]=out3+out3-out1;
  output[0]+=out1;          /* Real Part of X[0] */
  output[8]=output[0]-out1-out1; /*Real Part of X[4] */
  out5*=SIN_4PI_16;
  out7*=SIN_4PI_16;
  out5=out5-out7;
  out7=out5+out7+out7;
  output[14]=out6-out7;     /* Imag Part of X[5] */
  output[2]=out5+out4;      /* Real Part of X[7] */
  output[6]=out4-out5;      /*Real Part of X[5] */
  output[10]=-out7-out6; /* Imag Part of X[7] */
}