arm_std_q15.c

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/* ----------------------------------------------------------------------    
* Copyright (C) 2010-2014 ARM Limited. All rights reserved.    
*    
* $Date:        19. March 2015
* $Revision:    V.1.4.5  
*    
* Project:      CMSIS DSP Library    
* Title:        arm_std_q15.c    
*    
* Description:  Standard deviation of an array of Q15 type.    
*    
* Target Processor: Cortex-M4/Cortex-M3/Cortex-M0
*  
* Redistribution and use in source and binary forms, with or without 
* modification, are permitted provided that the following conditions
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*   - Redistributions in binary form must reproduce the above copyright
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*     distribution.
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*     software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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* -------------------------------------------------------------------- */

#include "arm_math.h"

void arm_std_q15(
  q15_t * pSrc,
  uint32_t blockSize,
  q15_t * pResult)
{
  q31_t sum = 0;                                 /* Accumulator */
  q31_t meanOfSquares, squareOfMean;             /* square of mean and mean of square */
  uint32_t blkCnt;                               /* loop counter */
  q63_t sumOfSquares = 0;                        /* Accumulator */
   
#ifndef ARM_MATH_CM0_FAMILY

  /* Run the below code for Cortex-M4 and Cortex-M3 */

  q31_t in;                                      /* input value */
  q15_t in1;                                     /* input value */

    if(blockSize == 1)
    {
        *pResult = 0;
        return;
    }

  /*loop Unrolling */
  blkCnt = blockSize >> 2u;

  /* First part of the processing with loop unrolling.  Compute 4 outputs at a time.    
   ** a second loop below computes the remaining 1 to 3 samples. */
  while(blkCnt > 0u)
  {
    /* C = (A[0] * A[0] + A[1] * A[1] + ... + A[blockSize-1] * A[blockSize-1])  */
    /* Compute Sum of squares of the input samples    
     * and then store the result in a temporary variable, sum. */
    in = *__SIMD32(pSrc)++;
    sum += ((in << 16) >> 16);
    sum += (in >> 16);
    sumOfSquares = __SMLALD(in, in, sumOfSquares);
    in = *__SIMD32(pSrc)++;
    sum += ((in << 16) >> 16);
    sum += (in >> 16);
    sumOfSquares = __SMLALD(in, in, sumOfSquares);

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

  /* If the blockSize is not a multiple of 4, compute any remaining output samples here.    
   ** No loop unrolling is used. */
  blkCnt = blockSize % 0x4u;

  while(blkCnt > 0u)
  {
    /* C = (A[0] * A[0] + A[1] * A[1] + ... + A[blockSize-1] * A[blockSize-1]) */
    /* Compute Sum of squares of the input samples    
     * and then store the result in a temporary variable, sum. */
    in1 = *pSrc++;
    sumOfSquares = __SMLALD(in1, in1, sumOfSquares);
    sum += in1;

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

  /* Compute Mean of squares of the input samples    
   * and then store the result in a temporary variable, meanOfSquares. */
  meanOfSquares = (q31_t)(sumOfSquares / (q63_t)(blockSize - 1));

  /* Compute square of mean */
  squareOfMean = (q31_t) ((q63_t)sum * sum / (q63_t)(blockSize * (blockSize - 1)));

  /* mean of the squares minus the square of the mean. */
  /* Compute standard deviation and store the result to the destination */
  arm_sqrt_q15(__SSAT((meanOfSquares - squareOfMean) >> 15, 16u), pResult);

#else

  /* Run the below code for Cortex-M0 */
  q15_t in;                                      /* input value */

    if(blockSize == 1)
    {
        *pResult = 0;
        return;
    }

  /* Loop over blockSize number of values */
  blkCnt = blockSize;

  while(blkCnt > 0u)
  {
    /* C = (A[0] * A[0] + A[1] * A[1] + ... + A[blockSize-1] * A[blockSize-1]) */
    /* Compute Sum of squares of the input samples     
     * and then store the result in a temporary variable, sumOfSquares. */
    in = *pSrc++;
    sumOfSquares += (in * in);

    /* C = (A[0] + A[1] + A[2] + ... + A[blockSize-1]) */
    /* Compute sum of all input values and then store the result in a temporary variable, sum. */
    sum += in;

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

  /* Compute Mean of squares of the input samples     
   * and then store the result in a temporary variable, meanOfSquares. */
  meanOfSquares = (q31_t)(sumOfSquares / (q63_t)(blockSize - 1));

  /* Compute square of mean */
  squareOfMean = (q31_t) ((q63_t)sum * sum / (q63_t)(blockSize * (blockSize - 1)));

  /* mean of the squares minus the square of the mean. */
  /* Compute standard deviation and store the result to the destination */
  arm_sqrt_q15(__SSAT((meanOfSquares - squareOfMean) >> 15, 16u), pResult);

#endif /* #ifndef ARM_MATH_CM0_FAMILY */


}