/* ----------------------------------------------------------------------   
* Copyright (C) 2010 ARM Limited. All rights reserved.   
*   
* $Date:        15. July 2011  
* $Revision: 	V1.0.10  
*   
* Project: 	    CMSIS DSP Library   
* Title:	    arm_fir_decimate_fast_q31.c   
*   
* Description:	Fast Q31 FIR Decimator.   
*   
* Target Processor: Cortex-M4/Cortex-M3
*  
* 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.   
* -------------------------------------------------------------------- */

#include "arm_math.h"

/**   
 * @ingroup groupFilters   
 */

/**   
 * @addtogroup FIR_decimate   
 * @{   
 */

/**   
 * @brief Processing function for the Q31 FIR decimator (fast variant) for Cortex-M3 and Cortex-M4.   
 * @param[in] *S points to an instance of the Q31 FIR decimator structure.   
 * @param[in] *pSrc points to the block of input data.   
 * @param[out] *pDst points to the block of output data   
 * @param[in] blockSize number of input samples to process per call.   
 * @return none   
 *   
 * <b>Scaling and Overflow Behavior:</b>   
 *   
 * \par   
 * This function is optimized for speed at the expense of fixed-point precision and overflow protection.   
 * The result of each 1.31 x 1.31 multiplication is truncated to 2.30 format.   
 * These intermediate results are added to a 2.30 accumulator.   
 * Finally, the accumulator is saturated and converted to a 1.31 result.   
 * The fast version has the same overflow behavior as the standard version and provides less precision since it discards the low 32 bits of each multiplication result.   
 * In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits (where log2 is read as log to the base 2).   
 *   
 * \par   
 * Refer to the function <code>arm_fir_decimate_q31()</code> for a slower implementation of this function which uses a 64-bit accumulator to provide higher precision.   
 * Both the slow and the fast versions use the same instance structure.   
 * Use the function <code>arm_fir_decimate_init_q31()</code> to initialize the filter structure.   
 */

void arm_fir_decimate_fast_q31(
  arm_fir_decimate_instance_q31 * S,
  q31_t * pSrc,
  q31_t * pDst,
  uint32_t blockSize)
{
  q31_t *pState = S->pState;                     /* State pointer */
  q31_t *pCoeffs = S->pCoeffs;                   /* Coefficient pointer */
  q31_t *pStateCurnt;                            /* Points to the current sample of the state */
  q31_t x0, c0;                                  /* Temporary variables to hold state and coefficient values */
  q31_t *px;                                     /* Temporary pointers for state buffer */
  q31_t *pb;                                     /* Temporary pointers for coefficient buffer */
  q63_t sum0;                                    /* Accumulator */
  uint32_t numTaps = S->numTaps;                 /* Number of taps */
  uint32_t i, tapCnt, blkCnt, outBlockSize = blockSize / S->M;  /* Loop counters */


  /* S->pState buffer contains previous frame (numTaps - 1) samples */
  /* pStateCurnt points to the location where the new input data should be written */
  pStateCurnt = S->pState + (numTaps - 1u);

  /* Total number of output samples to be computed */
  blkCnt = outBlockSize;

  while(blkCnt > 0u)
  {
    /* Copy decimation factor number of new input samples into the state buffer */
    i = S->M;

    do
    {
      *pStateCurnt++ = *pSrc++;

    } while(--i);

    /* Set accumulator to zero */
    sum0 = 0;

    /* Initialize state pointer */
    px = pState;

    /* Initialize coeff pointer */
    pb = pCoeffs;

    /* Loop unrolling.  Process 4 taps at a time. */
    tapCnt = numTaps >> 2;

    /* Loop over the number of taps.  Unroll by a factor of 4.   
     ** Repeat until we've computed numTaps-4 coefficients. */
    while(tapCnt > 0u)
    {
      /* Read the b[numTaps-1] coefficient */
      c0 = *(pb++);

      /* Read x[n-numTaps-1] sample */
      x0 = *(px++);

      /* Perform the multiply-accumulate */
      sum0 = (q31_t) ((((q63_t) x0 * c0) + (sum0 << 32)) >> 32);

      /* Read the b[numTaps-2] coefficient */
      c0 = *(pb++);

      /* Read x[n-numTaps-2] sample */
      x0 = *(px++);

      /* Perform the multiply-accumulate */
      sum0 = (q31_t) ((((q63_t) x0 * c0) + (sum0 << 32)) >> 32);

      /* Read the b[numTaps-3] coefficient */
      c0 = *(pb++);

      /* Read x[n-numTaps-3] sample */
      x0 = *(px++);

      /* Perform the multiply-accumulate */
      sum0 = (q31_t) ((((q63_t) x0 * c0) + (sum0 << 32)) >> 32);

      /* Read the b[numTaps-4] coefficient */
      c0 = *(pb++);

      /* Read x[n-numTaps-4] sample */
      x0 = *(px++);

      /* Perform the multiply-accumulate */
      sum0 = (q31_t) ((((q63_t) x0 * c0) + (sum0 << 32)) >> 32);

      /* Decrement the loop counter */
      tapCnt--;
    }

    /* If the filter length is not a multiple of 4, compute the remaining filter taps */
    tapCnt = numTaps % 0x4u;

    while(tapCnt > 0u)
    {
      /* Read coefficients */
      c0 = *(pb++);

      /* Fetch 1 state variable */
      x0 = *(px++);

      /* Perform the multiply-accumulate */
      sum0 = (q31_t) ((((q63_t) x0 * c0) + (sum0 << 32)) >> 32);

      /* Decrement the loop counter */
      tapCnt--;
    }

    /* Advance the state pointer by the decimation factor   
     * to process the next group of decimation factor number samples */
    pState = pState + S->M;

    /* The result is in the accumulator, store in the destination buffer. */
    *pDst++ = (q31_t) (sum0 << 1);

    /* Decrement the loop counter */
    blkCnt--;
  }

  /* Processing is complete.   
   ** Now copy the last numTaps - 1 samples to the satrt of the state buffer.   
   ** This prepares the state buffer for the next function call. */

  /* Points to the start of the state buffer */
  pStateCurnt = S->pState;

  i = (numTaps - 1u) >> 2u;

  /* copy data */
  while(i > 0u)
  {
    *pStateCurnt++ = *pState++;
    *pStateCurnt++ = *pState++;
    *pStateCurnt++ = *pState++;
    *pStateCurnt++ = *pState++;

    /* Decrement the loop counter */
    i--;
  }

  i = (numTaps - 1u) % 0x04u;

  /* copy data */
  while(i > 0u)
  {
    *pStateCurnt++ = *pState++;

    /* Decrement the loop counter */
    i--;
  }
}

/**   
 * @} end of FIR_decimate group   
 */