/* ----------------------------------------------------------------------   
* Copyright (C) 2010 ARM Limited. All rights reserved.   
*   
* $Date:        15. July 2011  
* $Revision: 	V1.0.10  
*   
* Project: 	    CMSIS DSP Library   
* Title:	    arm_cfft_radix4_q31.c   
*   
* Description:	This file has function definition of Radix-4 FFT & IFFT function and   
*				In-place bit reversal using bit reversal table   
*   
* Target Processor: Cortex-M4/Cortex-M3/Cortex-M0
*  
* Version 1.0.10 2011/7/15 
*    Big Endian support added and Merged M0 and M3/M4 Source code.  
*   
* Version 1.0.3 2010/11/29  
*    Re-organized the CMSIS folders and updated documentation.   
*    
* Version 1.0.2 2010/11/11   
*    Documentation updated.    
*   
* Version 1.0.1 2010/10/05    
*    Production release and review comments incorporated.   
*   
* Version 1.0.0 2010/09/20    
*    Production release and review comments incorporated.   
*   
* Version 0.0.5  2010/04/26    
* 	 incorporated review comments and updated with latest CMSIS layer   
*   
* Version 0.0.3  2010/03/10    
*    Initial version   
* -------------------------------------------------------------------- */
#include "arm_math.h"


/**   
 * @ingroup groupTransforms   
 */

/**   
 * @addtogroup CFFT_CIFFT   
 * @{   
 */

/**   
 * @details   
 * @brief Processing function for the Q31 CFFT/CIFFT.   
 * @param[in]      *S    points to an instance of the Q31 CFFT/CIFFT structure.  
 * @param[in, out] *pSrc points to the complex data buffer of size <code>2*fftLen</code>. Processing occurs in-place.  
 * @return none.   
 *    
 * \par Input and output formats:   
 * \par   
 * Internally input is downscaled by 2 for every stage to avoid saturations inside CFFT/CIFFT process.  
 * Hence the output format is different for different FFT sizes.   
 * The input and output formats for different FFT sizes and number of bits to upscale are mentioned in the tables below for CFFT and CIFFT:  
 * \par  
 * \image html CFFTQ31.gif "Input and Output Formats for Q31 CFFT"   
 * \image html CIFFTQ31.gif "Input and Output Formats for Q31 CIFFT"   
 *   
 */

void arm_cfft_radix4_q31(
  const arm_cfft_radix4_instance_q31 * S,
  q31_t * pSrc)
{
  if(S->ifftFlag == 1u)
  {
    /* Complex IFFT radix-4 */
    arm_radix4_butterfly_inverse_q31(pSrc, S->fftLen, S->pTwiddle,
                                     S->twidCoefModifier);
  }
  else
  {
    /* Complex FFT radix-4 */
    arm_radix4_butterfly_q31(pSrc, S->fftLen, S->pTwiddle,
                             S->twidCoefModifier);
  }


  if(S->bitReverseFlag == 1u)
  {
    /*  Bit Reversal */
    arm_bitreversal_q31(pSrc, S->fftLen, S->bitRevFactor, S->pBitRevTable);
  }

}

/**   
 * @} end of CFFT_CIFFT group   
 */

/*   
* Radix-4 FFT algorithm used is :   
*   
* Input real and imaginary data:   
* x(n) = xa + j * ya   
* x(n+N/4 ) = xb + j * yb   
* x(n+N/2 ) = xc + j * yc   
* x(n+3N 4) = xd + j * yd   
*   
*   
* Output real and imaginary data:   
* x(4r) = xa'+ j * ya'   
* x(4r+1) = xb'+ j * yb'   
* x(4r+2) = xc'+ j * yc'   
* x(4r+3) = xd'+ j * yd'   
*   
*   
* Twiddle factors for radix-4 FFT:   
* Wn = co1 + j * (- si1)   
* W2n = co2 + j * (- si2)   
* W3n = co3 + j * (- si3)   
*   
*  Butterfly implementation:   
* xa' = xa + xb + xc + xd   
* ya' = ya + yb + yc + yd   
* xb' = (xa+yb-xc-yd)* co1 + (ya-xb-yc+xd)* (si1)   
* yb' = (ya-xb-yc+xd)* co1 - (xa+yb-xc-yd)* (si1)   
* xc' = (xa-xb+xc-xd)* co2 + (ya-yb+yc-yd)* (si2)   
* yc' = (ya-yb+yc-yd)* co2 - (xa-xb+xc-xd)* (si2)   
* xd' = (xa-yb-xc+yd)* co3 + (ya+xb-yc-xd)* (si3)   
* yd' = (ya+xb-yc-xd)* co3 - (xa-yb-xc+yd)* (si3)   
*   
*/

/**   
 * @brief  Core function for the Q31 CFFT butterfly process.  
 * @param[in, out] *pSrc            points to the in-place buffer of Q31 data type.  
 * @param[in]      fftLen           length of the FFT.  
 * @param[in]      *pCoef           points to twiddle coefficient buffer.  
 * @param[in]      twidCoefModifier twiddle coefficient modifier that supports different size FFTs with the same twiddle factor table.  
 * @return none.  
 */

void arm_radix4_butterfly_q31(
  q31_t * pSrc,
  uint32_t fftLen,
  q31_t * pCoef,
  uint32_t twidCoefModifier)
{
  uint32_t n1, n2, ia1, ia2, ia3, i0, i1, i2, i3, j, k;
  q31_t t1, t2, r1, r2, s1, s2, co1, co2, co3, si1, si2, si3;


  /* Total process is divided into three stages */

  /* process first stage, middle stages, & last stage */


  /* start of first stage process */

  /*  Initializations for the first stage */
  n2 = fftLen;
  n1 = n2;
  /* n2 = fftLen/4 */
  n2 >>= 2u;
  i0 = 0u;
  ia1 = 0u;

  j = n2;

  /*  Calculation of first stage */
  do
  {
    /*  index calculation for the input as, */
    /*  pSrc[i0 + 0], pSrc[i0 + fftLen/4], pSrc[i0 + fftLen/2u], pSrc[i0 + 3fftLen/4] */
    i1 = i0 + n2;
    i2 = i1 + n2;
    i3 = i2 + n2;

    /* input is in 1.31(q31) format and provide 4 guard bits for the input */

    /*  Butterfly implementation */
    /* xa + xc */
    r1 = (pSrc[(2u * i0)] >> 4u) + (pSrc[(2u * i2)] >> 4u);
    /* xa - xc */
    r2 = (pSrc[2u * i0] >> 4u) - (pSrc[2u * i2] >> 4u);

    /* ya + yc */
    s1 = (pSrc[(2u * i0) + 1u] >> 4u) + (pSrc[(2u * i2) + 1u] >> 4u);
    /* ya - yc */
    s2 = (pSrc[(2u * i0) + 1u] >> 4u) - (pSrc[(2u * i2) + 1u] >> 4u);

    /* xb + xd */
    t1 = (pSrc[2u * i1] >> 4u) + (pSrc[2u * i3] >> 4u);

    /* xa' = xa + xb + xc + xd */
    pSrc[2u * i0] = (r1 + t1);
    /* (xa + xc) - (xb + xd) */
    r1 = r1 - t1;
    /* yb + yd */
    t2 = (pSrc[(2u * i1) + 1u] >> 4u) + (pSrc[(2u * i3) + 1u] >> 4u);
    /* ya' = ya + yb + yc + yd */
    pSrc[(2u * i0) + 1u] = (s1 + t2);

    /* (ya + yc) - (yb + yd) */
    s1 = s1 - t2;

    /* yb - yd */
    t1 = (pSrc[(2u * i1) + 1u] >> 4u) - (pSrc[(2u * i3) + 1u] >> 4u);
    /* xb - xd */
    t2 = (pSrc[2u * i1] >> 4u) - (pSrc[2u * i3] >> 4u);

    /*  index calculation for the coefficients */
    ia2 = 2u * ia1;
    co2 = pCoef[ia2 * 2u];
    si2 = pCoef[(ia2 * 2u) + 1u];

    /* xc' = (xa-xb+xc-xd)co2 + (ya-yb+yc-yd)(si2) */
    pSrc[2u * i1] = (((int32_t) (((q63_t) r1 * co2) >> 32)) +
                     ((int32_t) (((q63_t) s1 * si2) >> 32))) << 1u;

    /* yc' = (ya-yb+yc-yd)co2 - (xa-xb+xc-xd)(si2) */
    pSrc[(2u * i1) + 1u] = (((int32_t) (((q63_t) s1 * co2) >> 32)) -
                            ((int32_t) (((q63_t) r1 * si2) >> 32))) << 1u;

    /* (xa - xc) + (yb - yd) */
    r1 = r2 + t1;
    /* (xa - xc) - (yb - yd) */
    r2 = r2 - t1;

    /* (ya - yc) - (xb - xd) */
    s1 = s2 - t2;
    /* (ya - yc) + (xb - xd) */
    s2 = s2 + t2;

    co1 = pCoef[ia1 * 2u];
    si1 = pCoef[(ia1 * 2u) + 1u];

    /* xb' = (xa+yb-xc-yd)co1 + (ya-xb-yc+xd)(si1) */
    pSrc[2u * i2] = (((int32_t) (((q63_t) r1 * co1) >> 32)) +
                     ((int32_t) (((q63_t) s1 * si1) >> 32))) << 1u;

    /* yb' = (ya-xb-yc+xd)co1 - (xa+yb-xc-yd)(si1) */
    pSrc[(2u * i2) + 1u] = (((int32_t) (((q63_t) s1 * co1) >> 32)) -
                            ((int32_t) (((q63_t) r1 * si1) >> 32))) << 1u;

    /*  index calculation for the coefficients */
    ia3 = 3u * ia1;
    co3 = pCoef[ia3 * 2u];
    si3 = pCoef[(ia3 * 2u) + 1u];

    /* xd' = (xa-yb-xc+yd)co3 + (ya+xb-yc-xd)(si3) */
    pSrc[2u * i3] = (((int32_t) (((q63_t) r2 * co3) >> 32)) +
                     ((int32_t) (((q63_t) s2 * si3) >> 32))) << 1u;

    /* yd' = (ya+xb-yc-xd)co3 - (xa-yb-xc+yd)(si3) */
    pSrc[(2u * i3) + 1u] = (((int32_t) (((q63_t) s2 * co3) >> 32)) -
                            ((int32_t) (((q63_t) r2 * si3) >> 32))) << 1u;

    /*  Twiddle coefficients index modifier */
    ia1 = ia1 + twidCoefModifier;

    /*  Updating input index */
    i0 = i0 + 1u;

  } while(--j);

  /* end of first stage process */

  /* data is in 5.27(q27) format */


  /* start of Middle stages process */


  /* each stage in middle stages provides two down scaling of the input */

  twidCoefModifier <<= 2u;


  for (k = fftLen / 4u; k > 4u; k >>= 2u)
  {
    /*  Initializations for the first stage */
    n1 = n2;
    n2 >>= 2u;
    ia1 = 0u;

    /*  Calculation of first stage */
    for (j = 0u; j <= (n2 - 1u); j++)
    {
      /*  index calculation for the coefficients */
      ia2 = ia1 + ia1;
      ia3 = ia2 + ia1;
      co1 = pCoef[ia1 * 2u];
      si1 = pCoef[(ia1 * 2u) + 1u];
      co2 = pCoef[ia2 * 2u];
      si2 = pCoef[(ia2 * 2u) + 1u];
      co3 = pCoef[ia3 * 2u];
      si3 = pCoef[(ia3 * 2u) + 1u];
      /*  Twiddle coefficients index modifier */
      ia1 = ia1 + twidCoefModifier;

      for (i0 = j; i0 < fftLen; i0 += n1)
      {
        /*  index calculation for the input as, */
        /*  pSrc[i0 + 0], pSrc[i0 + fftLen/4], pSrc[i0 + fftLen/2u], pSrc[i0 + 3fftLen/4] */
        i1 = i0 + n2;
        i2 = i1 + n2;
        i3 = i2 + n2;

        /*  Butterfly implementation */
        /* xa + xc */
        r1 = pSrc[2u * i0] + pSrc[2u * i2];
        /* xa - xc */
        r2 = pSrc[2u * i0] - pSrc[2u * i2];

        /* ya + yc */
        s1 = pSrc[(2u * i0) + 1u] + pSrc[(2u * i2) + 1u];
        /* ya - yc */
        s2 = pSrc[(2u * i0) + 1u] - pSrc[(2u * i2) + 1u];

        /* xb + xd */
        t1 = pSrc[2u * i1] + pSrc[2u * i3];

        /* xa' = xa + xb + xc + xd */
        pSrc[2u * i0] = (r1 + t1) >> 2u;
        /* xa + xc -(xb + xd) */
        r1 = r1 - t1;

        /* yb + yd */
        t2 = pSrc[(2u * i1) + 1u] + pSrc[(2u * i3) + 1u];
        /* ya' = ya + yb + yc + yd */
        pSrc[(2u * i0) + 1u] = (s1 + t2) >> 2u;

        /* (ya + yc) - (yb + yd) */
        s1 = s1 - t2;

        /* (yb - yd) */
        t1 = pSrc[(2u * i1) + 1u] - pSrc[(2u * i3) + 1u];
        /* (xb - xd) */
        t2 = pSrc[2u * i1] - pSrc[2u * i3];

        /* xc' = (xa-xb+xc-xd)co2 + (ya-yb+yc-yd)(si2) */
        pSrc[2u * i1] = (((int32_t) (((q63_t) r1 * co2) >> 32)) +
                         ((int32_t) (((q63_t) s1 * si2) >> 32))) >> 1u;

        /* yc' = (ya-yb+yc-yd)co2 - (xa-xb+xc-xd)(si2) */
        pSrc[(2u * i1) + 1u] = (((int32_t) (((q63_t) s1 * co2) >> 32)) -
                                ((int32_t) (((q63_t) r1 * si2) >> 32))) >> 1u;

        /* (xa - xc) + (yb - yd) */
        r1 = r2 + t1;
        /* (xa - xc) - (yb - yd) */
        r2 = r2 - t1;

        /* (ya - yc) -  (xb - xd) */
        s1 = s2 - t2;
        /* (ya - yc) +  (xb - xd) */
        s2 = s2 + t2;

        /* xb' = (xa+yb-xc-yd)co1 + (ya-xb-yc+xd)(si1) */
        pSrc[2u * i2] = (((int32_t) (((q63_t) r1 * co1) >> 32)) +
                         ((int32_t) (((q63_t) s1 * si1) >> 32))) >> 1u;

        /* yb' = (ya-xb-yc+xd)co1 - (xa+yb-xc-yd)(si1) */
        pSrc[(2u * i2) + 1u] = (((int32_t) (((q63_t) s1 * co1) >> 32)) -
                                ((int32_t) (((q63_t) r1 * si1) >> 32))) >> 1u;

        /* xd' = (xa-yb-xc+yd)co3 + (ya+xb-yc-xd)(si3) */
        pSrc[2u * i3] = (((int32_t) (((q63_t) r2 * co3) >> 32)) +
                         ((int32_t) (((q63_t) s2 * si3) >> 32))) >> 1u;

        /* yd' = (ya+xb-yc-xd)co3 - (xa-yb-xc+yd)(si3) */
        pSrc[(2u * i3) + 1u] = (((int32_t) (((q63_t) s2 * co3) >> 32)) -
                                ((int32_t) (((q63_t) r2 * si3) >> 32))) >> 1u;
      }
    }
    twidCoefModifier <<= 2u;
  }

  /* End of Middle stages process */

  /* data is in 11.21(q21) format for the 1024 point as there are 3 middle stages */
  /* data is in 9.23(q23) format for the 256 point as there are 2 middle stages */
  /* data is in 7.25(q25) format for the 64 point as there are 1 middle stage */
  /* data is in 5.27(q27) format for the 16 point as there are no middle stages */


  /* start of Last stage process */

  /*  Initializations of last stage */
  n1 = n2;
  n2 >>= 2u;

  /*  Calculations of last stage */
  for (i0 = 0u; i0 <= (fftLen - n1); i0 += n1)
  {
    /*  index calculation for the input as, */
    /*  pSrc[i0 + 0], pSrc[i0 + fftLen/4], pSrc[i0 + fftLen/2u], pSrc[i0 + 3fftLen/4] */
    i1 = i0 + n2;
    i2 = i1 + n2;
    i3 = i2 + n2;

    /*  Butterfly implementation */
    /* xa + xb */
    r1 = pSrc[2u * i0] + pSrc[2u * i2];
    /* xa - xb */
    r2 = pSrc[2u * i0] - pSrc[2u * i2];

    /* ya + yc */
    s1 = pSrc[(2u * i0) + 1u] + pSrc[(2u * i2) + 1u];
    /* ya - yc */
    s2 = pSrc[(2u * i0) + 1u] - pSrc[(2u * i2) + 1u];

    /* xc + xd */
    t1 = pSrc[2u * i1] + pSrc[2u * i3];
    /* xa' = xa + xb + xc + xd */
    pSrc[2u * i0] = (r1 + t1);
    /* (xa + xb) - (xc + xd) */
    r1 = r1 - t1;

    /* yb + yd */
    t2 = pSrc[(2u * i1) + 1u] + pSrc[(2u * i3) + 1u];
    /* ya' = ya + yb + yc + yd */
    pSrc[(2u * i0) + 1u] = (s1 + t2);
    /* (ya + yc) - (yb + yd) */
    s1 = s1 - t2;

    /* (yb-yd) */
    t1 = pSrc[(2u * i1) + 1u] - pSrc[(2u * i3) + 1u];
    /* (xb-xd) */
    t2 = pSrc[2u * i1] - pSrc[2u * i3];

    /* xc' = (xa-xb+xc-xd)co2 + (ya-yb+yc-yd)(si2) */
    pSrc[2u * i1] = r1;
    /* yc' = (ya-yb+yc-yd)co2 - (xa-xb+xc-xd)(si2) */
    pSrc[(2u * i1) + 1u] = s1;

    /* (xa+yb-xc-yd) */
    r1 = r2 + t1;
    /* (xa-yb-xc+yd) */
    r2 = r2 - t1;

    /* (ya-xb-yc+xd) */
    s1 = s2 - t2;
    /* (ya+xb-yc-xd) */
    s2 = s2 + t2;

    /* xb' = (xa+yb-xc-yd)co1 + (ya-xb-yc+xd)(si1) */
    pSrc[2u * i2] = r1;
    /* yb' = (ya-xb-yc+xd)co1 - (xa+yb-xc-yd)(si1) */
    pSrc[(2u * i2) + 1u] = s1;

    /* xd' = (xa-yb-xc+yd)co3 + (ya+xb-yc-xd)(si3) */
    pSrc[2u * i3] = r2;
    /* yd' = (ya+xb-yc-xd)co3 - (xa-yb-xc+yd)(si3) */
    pSrc[(2u * i3) + 1u] = s2;


  }

  /* output is in 11.21(q21) format for the 1024 point */
  /* output is in 9.23(q23) format for the 256 point */
  /* output is in 7.25(q25) format for the 64 point */
  /* output is in 5.27(q27) format for the 16 point */

  /* End of last stage process */

}


/**   
 * @brief  Core function for the Q31 CIFFT butterfly process.  
 * @param[in, out] *pSrc            points to the in-place buffer of Q31 data type.  
 * @param[in]      fftLen           length of the FFT.  
 * @param[in]      *pCoef           points to twiddle coefficient buffer.  
 * @param[in]      twidCoefModifier twiddle coefficient modifier that supports different size FFTs with the same twiddle factor table.  
 * @return none.  
 */


/*   
* Radix-4 IFFT algorithm used is :   
*   
* CIFFT uses same twiddle coefficients as CFFT Function   
*  x[k] = x[n] + (j)k * x[n + fftLen/4] + (-1)k * x[n+fftLen/2] + (-j)k * x[n+3*fftLen/4]   
*   
*   
* IFFT is implemented with following changes in equations from FFT   
*   
* Input real and imaginary data:   
* x(n) = xa + j * ya   
* x(n+N/4 ) = xb + j * yb   
* x(n+N/2 ) = xc + j * yc   
* x(n+3N 4) = xd + j * yd   
*   
*   
* Output real and imaginary data:   
* x(4r) = xa'+ j * ya'   
* x(4r+1) = xb'+ j * yb'   
* x(4r+2) = xc'+ j * yc'   
* x(4r+3) = xd'+ j * yd'   
*   
*   
* Twiddle factors for radix-4 IFFT:   
* Wn = co1 + j * (si1)   
* W2n = co2 + j * (si2)   
* W3n = co3 + j * (si3)   
   
* The real and imaginary output values for the radix-4 butterfly are   
* xa' = xa + xb + xc + xd   
* ya' = ya + yb + yc + yd   
* xb' = (xa-yb-xc+yd)* co1 - (ya+xb-yc-xd)* (si1)   
* yb' = (ya+xb-yc-xd)* co1 + (xa-yb-xc+yd)* (si1)   
* xc' = (xa-xb+xc-xd)* co2 - (ya-yb+yc-yd)* (si2)   
* yc' = (ya-yb+yc-yd)* co2 + (xa-xb+xc-xd)* (si2)   
* xd' = (xa+yb-xc-yd)* co3 - (ya-xb-yc+xd)* (si3)   
* yd' = (ya-xb-yc+xd)* co3 + (xa+yb-xc-yd)* (si3)   
*   
*/

void arm_radix4_butterfly_inverse_q31(
  q31_t * pSrc,
  uint32_t fftLen,
  q31_t * pCoef,
  uint32_t twidCoefModifier)
{
  uint32_t n1, n2, ia1, ia2, ia3, i0, i1, i2, i3, j, k;
  q31_t t1, t2, r1, r2, s1, s2, co1, co2, co3, si1, si2, si3;

  /* input is be 1.31(q31) format for all FFT sizes */
  /* Total process is divided into three stages */
  /* process first stage, middle stages, & last stage */

  /* Start of first stage process */

  /* Initializations for the first stage */
  n2 = fftLen;
  n1 = n2;
  /* n2 = fftLen/4 */
  n2 >>= 2u;
  i0 = 0u;
  ia1 = 0u;

  j = n2;

  do
  {

    /* input is in 1.31(q31) format and provide 4 guard bits for the input */

    /*  index calculation for the input as, */
    /*  pSrc[i0 + 0], pSrc[i0 + fftLen/4], pSrc[i0 + fftLen/2u], pSrc[i0 + 3fftLen/4] */
    i1 = i0 + n2;
    i2 = i1 + n2;
    i3 = i2 + n2;

    /*  Butterfly implementation */
    /* xa + xc */
    r1 = (pSrc[2u * i0] >> 4u) + (pSrc[2u * i2] >> 4u);
    /* xa - xc */
    r2 = (pSrc[2u * i0] >> 4u) - (pSrc[2u * i2] >> 4u);

    /* ya + yc */
    s1 = (pSrc[(2u * i0) + 1u] >> 4u) + (pSrc[(2u * i2) + 1u] >> 4u);
    /* ya - yc */
    s2 = (pSrc[(2u * i0) + 1u] >> 4u) - (pSrc[(2u * i2) + 1u] >> 4u);

    /* xb + xd */
    t1 = (pSrc[2u * i1] >> 4u) + (pSrc[2u * i3] >> 4u);

    /* xa' = xa + xb + xc + xd */
    pSrc[2u * i0] = (r1 + t1);
    /* (xa + xc) - (xb + xd) */
    r1 = r1 - t1;
    /* yb + yd */
    t2 = (pSrc[(2u * i1) + 1u] >> 4u) + (pSrc[(2u * i3) + 1u] >> 4u);
    /* ya' = ya + yb + yc + yd */
    pSrc[(2u * i0) + 1u] = (s1 + t2);

    /* (ya + yc) - (yb + yd) */
    s1 = s1 - t2;

    /* yb - yd */
    t1 = (pSrc[(2u * i1) + 1u] >> 4u) - (pSrc[(2u * i3) + 1u] >> 4u);
    /* xb - xd */
    t2 = (pSrc[2u * i1] >> 4u) - (pSrc[2u * i3] >> 4u);

    /*  index calculation for the coefficients */
    ia2 = 2u * ia1;
    co2 = pCoef[ia2 * 2u];
    si2 = pCoef[(ia2 * 2u) + 1u];

    /* xc' = (xa-xb+xc-xd)co2 - (ya-yb+yc-yd)(si2) */
    pSrc[2u * i1] = (((int32_t) (((q63_t) r1 * co2) >> 32)) -
                     ((int32_t) (((q63_t) s1 * si2) >> 32))) << 1u;

    /* yc' = (ya-yb+yc-yd)co2 + (xa-xb+xc-xd)(si2) */
    pSrc[2u * i1 + 1u] = (((int32_t) (((q63_t) s1 * co2) >> 32)) +
                          ((int32_t) (((q63_t) r1 * si2) >> 32))) << 1u;

    /* (xa - xc) - (yb - yd) */
    r1 = r2 - t1;
    /* (xa - xc) + (yb - yd) */
    r2 = r2 + t1;

    /* (ya - yc) + (xb - xd) */
    s1 = s2 + t2;
    /* (ya - yc) - (xb - xd) */
    s2 = s2 - t2;

    co1 = pCoef[ia1 * 2u];
    si1 = pCoef[(ia1 * 2u) + 1u];

    /* xb' = (xa+yb-xc-yd)co1 - (ya-xb-yc+xd)(si1) */
    pSrc[2u * i2] = (((int32_t) (((q63_t) r1 * co1) >> 32)) -
                     ((int32_t) (((q63_t) s1 * si1) >> 32))) << 1u;

    /* yb' = (ya-xb-yc+xd)co1 + (xa+yb-xc-yd)(si1) */
    pSrc[(2u * i2) + 1u] = (((int32_t) (((q63_t) s1 * co1) >> 32)) +
                            ((int32_t) (((q63_t) r1 * si1) >> 32))) << 1u;

    /*  index calculation for the coefficients */
    ia3 = 3u * ia1;
    co3 = pCoef[ia3 * 2u];
    si3 = pCoef[(ia3 * 2u) + 1u];

    /* xd' = (xa-yb-xc+yd)co3 - (ya+xb-yc-xd)(si3) */
    pSrc[2u * i3] = (((int32_t) (((q63_t) r2 * co3) >> 32)) -
                     ((int32_t) (((q63_t) s2 * si3) >> 32))) << 1u;

    /* yd' = (ya+xb-yc-xd)co3 + (xa-yb-xc+yd)(si3) */
    pSrc[(2u * i3) + 1u] = (((int32_t) (((q63_t) s2 * co3) >> 32)) +
                            ((int32_t) (((q63_t) r2 * si3) >> 32))) << 1u;

    /*  Twiddle coefficients index modifier */
    ia1 = ia1 + twidCoefModifier;

    /*  Updating input index */
    i0 = i0 + 1u;

  } while(--j);

  /* data is in 5.27(q27) format */
  /* each stage provides two down scaling of the input */


  /* Start of Middle stages process */

  twidCoefModifier <<= 2u;

  /*  Calculation of second stage to excluding last stage */
  for (k = fftLen / 4u; k > 4u; k >>= 2u)
  {
    /*  Initializations for the first stage */
    n1 = n2;
    n2 >>= 2u;
    ia1 = 0u;

    for (j = 0; j <= (n2 - 1u); j++)
    {
      /*  index calculation for the coefficients */
      ia2 = ia1 + ia1;
      ia3 = ia2 + ia1;
      co1 = pCoef[ia1 * 2u];
      si1 = pCoef[(ia1 * 2u) + 1u];
      co2 = pCoef[ia2 * 2u];
      si2 = pCoef[(ia2 * 2u) + 1u];
      co3 = pCoef[ia3 * 2u];
      si3 = pCoef[(ia3 * 2u) + 1u];
      /*  Twiddle coefficients index modifier */
      ia1 = ia1 + twidCoefModifier;

      for (i0 = j; i0 < fftLen; i0 += n1)
      {
        /*  index calculation for the input as, */
        /*  pSrc[i0 + 0], pSrc[i0 + fftLen/4], pSrc[i0 + fftLen/2u], pSrc[i0 + 3fftLen/4] */
        i1 = i0 + n2;
        i2 = i1 + n2;
        i3 = i2 + n2;

        /*  Butterfly implementation */
        /* xa + xc */
        r1 = pSrc[2u * i0] + pSrc[2u * i2];
        /* xa - xc */
        r2 = pSrc[2u * i0] - pSrc[2u * i2];

        /* ya + yc */
        s1 = pSrc[(2u * i0) + 1u] + pSrc[(2u * i2) + 1u];
        /* ya - yc */
        s2 = pSrc[(2u * i0) + 1u] - pSrc[(2u * i2) + 1u];

        /* xb + xd */
        t1 = pSrc[2u * i1] + pSrc[2u * i3];

        /* xa' = xa + xb + xc + xd */
        pSrc[2u * i0] = (r1 + t1) >> 2u;
        /* xa + xc -(xb + xd) */
        r1 = r1 - t1;
        /* yb + yd */
        t2 = pSrc[(2u * i1) + 1u] + pSrc[(2u * i3) + 1u];
        /* ya' = ya + yb + yc + yd */
        pSrc[(2u * i0) + 1u] = (s1 + t2) >> 2u;

        /* (ya + yc) - (yb + yd) */
        s1 = s1 - t2;

        /* (yb - yd) */
        t1 = pSrc[(2u * i1) + 1u] - pSrc[(2u * i3) + 1u];
        /* (xb - xd) */
        t2 = pSrc[2u * i1] - pSrc[2u * i3];

        /* xc' = (xa-xb+xc-xd)co2 - (ya-yb+yc-yd)(si2) */
        pSrc[2u * i1] = (((int32_t) (((q63_t) r1 * co2) >> 32u)) -
                         ((int32_t) (((q63_t) s1 * si2) >> 32u))) >> 1u;

        /* yc' = (ya-yb+yc-yd)co2 + (xa-xb+xc-xd)(si2) */
        pSrc[(2u * i1) + 1u] =
          (((int32_t) (((q63_t) s1 * co2) >> 32u)) +
           ((int32_t) (((q63_t) r1 * si2) >> 32u))) >> 1u;

        /* (xa - xc) - (yb - yd) */
        r1 = r2 - t1;
        /* (xa - xc) + (yb - yd) */
        r2 = r2 + t1;

        /* (ya - yc) +  (xb - xd) */
        s1 = s2 + t2;
        /* (ya - yc) -  (xb - xd) */
        s2 = s2 - t2;

        /* xb' = (xa+yb-xc-yd)co1 - (ya-xb-yc+xd)(si1) */
        pSrc[2u * i2] = (((int32_t) (((q63_t) r1 * co1) >> 32)) -
                         ((int32_t) (((q63_t) s1 * si1) >> 32))) >> 1u;

        /* yb' = (ya-xb-yc+xd)co1 + (xa+yb-xc-yd)(si1) */
        pSrc[(2u * i2) + 1u] = (((int32_t) (((q63_t) s1 * co1) >> 32)) +
                                ((int32_t) (((q63_t) r1 * si1) >> 32))) >> 1u;

        /* xd' = (xa-yb-xc+yd)co3 - (ya+xb-yc-xd)(si3) */
        pSrc[(2u * i3)] = (((int32_t) (((q63_t) r2 * co3) >> 32)) -
                           ((int32_t) (((q63_t) s2 * si3) >> 32))) >> 1u;

        /* yd' = (ya+xb-yc-xd)co3 + (xa-yb-xc+yd)(si3) */
        pSrc[(2u * i3) + 1u] = (((int32_t) (((q63_t) s2 * co3) >> 32)) +
                                ((int32_t) (((q63_t) r2 * si3) >> 32))) >> 1u;
      }
    }
    twidCoefModifier <<= 2u;
  }

  /* End of Middle stages process */

  /* data is in 11.21(q21) format for the 1024 point as there are 3 middle stages */
  /* data is in 9.23(q23) format for the 256 point as there are 2 middle stages */
  /* data is in 7.25(q25) format for the 64 point as there are 1 middle stage */
  /* data is in 5.27(q27) format for the 16 point as there are no middle stages */


  /* Start of last stage process */


  /*  Initializations of last stage */
  n1 = n2;
  n2 >>= 2u;

  /*  Calculations of last stage */
  for (i0 = 0u; i0 <= (fftLen - n1); i0 += n1)
  {
    /*  index calculation for the input as, */
    /*  pSrc[i0 + 0], pSrc[i0 + fftLen/4], pSrc[i0 + fftLen/2u], pSrc[i0 + 3fftLen/4] */
    i1 = i0 + n2;
    i2 = i1 + n2;
    i3 = i2 + n2;

    /*  Butterfly implementation */
    /* xa + xc */
    r1 = pSrc[2u * i0] + pSrc[2u * i2];
    /* xa - xc */
    r2 = pSrc[2u * i0] - pSrc[2u * i2];

    /* ya + yc */
    s1 = pSrc[(2u * i0) + 1u] + pSrc[(2u * i2) + 1u];
    /* ya - yc */
    s2 = pSrc[(2u * i0) + 1u] - pSrc[(2u * i2) + 1u];

    /* xc + xd */
    t1 = pSrc[2u * i1] + pSrc[2u * i3];
    /* xa' = xa + xb + xc + xd */
    pSrc[2u * i0] = (r1 + t1);
    /* (xa + xb) - (xc + xd) */
    r1 = r1 - t1;

    /* yb + yd */
    t2 = pSrc[(2u * i1) + 1u] + pSrc[(2u * i3) + 1u];
    /* ya' = ya + yb + yc + yd */
    pSrc[(2u * i0) + 1u] = (s1 + t2);
    /* (ya + yc) - (yb + yd) */
    s1 = s1 - t2;

    /* (yb-yd) */
    t1 = pSrc[(2u * i1) + 1u] - pSrc[(2u * i3) + 1u];
    /* (xb-xd) */
    t2 = pSrc[2u * i1] - pSrc[2u * i3];

    /* xc' = (xa-xb+xc-xd)co2 - (ya-yb+yc-yd)(si2) */
    pSrc[2u * i1] = r1;
    /* yc' = (ya-yb+yc-yd)co2 + (xa-xb+xc-xd)(si2) */
    pSrc[(2u * i1) + 1u] = s1;

    /* (xa - xc) - (yb-yd) */
    r1 = r2 - t1;

    /* (xa - xc) + (yb-yd) */
    r2 = r2 + t1;

    /* (ya - yc) + (xb-xd) */
    s1 = s2 + t2;

    /* (ya - yc) - (xb-xd) */
    s2 = s2 - t2;

    /* xb' = (xa+yb-xc-yd)co1 - (ya-xb-yc+xd)(si1) */
    pSrc[2u * i2] = r1;
    /* yb' = (ya-xb-yc+xd)co1 + (xa+yb-xc-yd)(si1) */
    pSrc[(2u * i2) + 1u] = s1;

    /* xd' = (xa-yb-xc+yd)co3 - (ya+xb-yc-xd)(si3) */
    pSrc[2u * i3] = r2;
    /* yd' = (ya+xb-yc-xd)co3 + (xa-yb-xc+yd)(si3) */
    pSrc[(2u * i3) + 1u] = s2;

  }

  /* output is in 11.21(q21) format for the 1024 point */
  /* output is in 9.23(q23) format for the 256 point */
  /* output is in 7.25(q25) format for the 64 point */
  /* output is in 5.27(q27) format for the 16 point */

  /* End of last stage process */
}


/*   
 * @brief  In-place bit reversal function.  
 * @param[in, out] *pSrc        points to the in-place buffer of Q31 data type.  
 * @param[in]      fftLen       length of the FFT.  
 * @param[in]      bitRevFactor bit reversal modifier that supports different size FFTs with the same bit reversal table  
 * @param[in]      *pBitRevTab  points to bit reversal table.  
 * @return none.  
 */

void arm_bitreversal_q31(
  q31_t * pSrc,
  uint32_t fftLen,
  uint16_t bitRevFactor,
  uint16_t * pBitRevTable)
{
  uint32_t fftLenBy2, fftLenBy2p1, i, j;
  q31_t in;

  /*  Initializations      */
  j = 0u;
  fftLenBy2 = fftLen / 2u;
  fftLenBy2p1 = (fftLen / 2u) + 1u;

  /* Bit Reversal Implementation */
  for (i = 0u; i <= (fftLenBy2 - 2u); i += 2u)
  {
    if(i < j)
    {
      /*  pSrc[i] <-> pSrc[j]; */
      in = pSrc[2u * i];
      pSrc[2u * i] = pSrc[2u * j];
      pSrc[2u * j] = in;

      /*  pSrc[i+1u] <-> pSrc[j+1u] */
      in = pSrc[(2u * i) + 1u];
      pSrc[(2u * i) + 1u] = pSrc[(2u * j) + 1u];
      pSrc[(2u * j) + 1u] = in;

      /*  pSrc[i+fftLenBy2p1] <-> pSrc[j+fftLenBy2p1] */
      in = pSrc[2u * (i + fftLenBy2p1)];
      pSrc[2u * (i + fftLenBy2p1)] = pSrc[2u * (j + fftLenBy2p1)];
      pSrc[2u * (j + fftLenBy2p1)] = in;

      /*  pSrc[i+fftLenBy2p1+1u] <-> pSrc[j+fftLenBy2p1+1u] */
      in = pSrc[(2u * (i + fftLenBy2p1)) + 1u];
      pSrc[(2u * (i + fftLenBy2p1)) + 1u] =
        pSrc[(2u * (j + fftLenBy2p1)) + 1u];
      pSrc[(2u * (j + fftLenBy2p1)) + 1u] = in;

    }

    /*  pSrc[i+1u] <-> pSrc[j+1u] */
    in = pSrc[2u * (i + 1u)];
    pSrc[2u * (i + 1u)] = pSrc[2u * (j + fftLenBy2)];
    pSrc[2u * (j + fftLenBy2)] = in;

    /*  pSrc[i+2u] <-> pSrc[j+2u] */
    in = pSrc[(2u * (i + 1u)) + 1u];
    pSrc[(2u * (i + 1u)) + 1u] = pSrc[(2u * (j + fftLenBy2)) + 1u];
    pSrc[(2u * (j + fftLenBy2)) + 1u] = in;

    /*  Reading the index for the bit reversal */
    j = *pBitRevTable;

    /*  Updating the bit reversal index depending on the fft length */
    pBitRevTable += bitRevFactor;
  }
}