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  1. /* ----------------------------------------------------------------------
  2.  * Project:      CMSIS DSP Library
  3.  * Title:        arm_std_f32.c
  4.  * Description:  Standard deviation of the elements of a floating-point vector
  5.  *
  6.  * $Date:        27. January 2017
  7.  * $Revision:    V.1.5.1
  8.  *
  9.  * Target Processor: Cortex-M cores
  10.  * -------------------------------------------------------------------- */
  11. /*
  12.  * Copyright (C) 2010-2017 ARM Limited or its affiliates. All rights reserved.
  13.  *
  14.  * SPDX-License-Identifier: Apache-2.0
  15.  *
  16.  * Licensed under the Apache License, Version 2.0 (the License); you may
  17.  * not use this file except in compliance with the License.
  18.  * You may obtain a copy of the License at
  19.  *
  20.  * www.apache.org/licenses/LICENSE-2.0
  21.  *
  22.  * Unless required by applicable law or agreed to in writing, software
  23.  * distributed under the License is distributed on an AS IS BASIS, WITHOUT
  24.  * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  25.  * See the License for the specific language governing permissions and
  26.  * limitations under the License.
  27.  */
  28.  
  29. #include "arm_math.h"
  30.  
  31. /**
  32.  * @ingroup groupStats
  33.  */
  34.  
  35. /**
  36.  * @defgroup STD Standard deviation
  37.  *
  38.  * Calculates the standard deviation of the elements in the input vector.
  39.  * The underlying algorithm is used:
  40.  *
  41.  * <pre>
  42.  *   Result = sqrt((sumOfSquares - sum<sup>2</sup> / blockSize) / (blockSize - 1))
  43.  *
  44.  *     where, sumOfSquares = pSrc[0] * pSrc[0] + pSrc[1] * pSrc[1] + ... + pSrc[blockSize-1] * pSrc[blockSize-1]
  45.  *
  46.  *                     sum = pSrc[0] + pSrc[1] + pSrc[2] + ... + pSrc[blockSize-1]
  47.  * </pre>
  48.  *
  49.  * There are separate functions for floating point, Q31, and Q15 data types.
  50.  */
  51.  
  52. /**
  53.  * @addtogroup STD
  54.  * @{
  55.  */
  56.  
  57.  
  58. /**
  59.  * @brief Standard deviation of the elements of a floating-point vector.
  60.  * @param[in]       *pSrc points to the input vector
  61.  * @param[in]       blockSize length of the input vector
  62.  * @param[out]      *pResult standard deviation value returned here
  63.  * @return none.
  64.  */
  65.  
  66. void arm_std_f32(
  67.   float32_t * pSrc,
  68.   uint32_t blockSize,
  69.   float32_t * pResult)
  70. {
  71.   float32_t sum = 0.0f;                          /* Temporary result storage */
  72.   float32_t sumOfSquares = 0.0f;                 /* Sum of squares */
  73.   float32_t in;                                  /* input value */
  74.   uint32_t blkCnt;                               /* loop counter */
  75. #if defined (ARM_MATH_DSP)
  76.   float32_t meanOfSquares, mean, squareOfMean;   /* Temporary variables */
  77. #else
  78.   float32_t squareOfSum;                         /* Square of Sum */
  79.   float32_t var;                                 /* Temporary varaince storage */
  80. #endif
  81.  
  82.   if (blockSize == 1U)
  83.   {
  84.     *pResult = 0;
  85.     return;
  86.   }
  87.  
  88. #if defined (ARM_MATH_DSP)
  89.   /* Run the below code for Cortex-M4 and Cortex-M3 */
  90.  
  91.   /*loop Unrolling */
  92.   blkCnt = blockSize >> 2U;
  93.  
  94.   /* First part of the processing with loop unrolling.  Compute 4 outputs at a time.
  95.    ** a second loop below computes the remaining 1 to 3 samples. */
  96.   while (blkCnt > 0U)
  97.   {
  98.     /* C = (A[0] * A[0] + A[1] * A[1] + ... + A[blockSize-1] * A[blockSize-1])  */
  99.     /* Compute Sum of squares of the input samples
  100.      * and then store the result in a temporary variable, sum. */
  101.     in = *pSrc++;
  102.     sum += in;
  103.     sumOfSquares += in * in;
  104.     in = *pSrc++;
  105.     sum += in;
  106.     sumOfSquares += in * in;
  107.     in = *pSrc++;
  108.     sum += in;
  109.     sumOfSquares += in * in;
  110.     in = *pSrc++;
  111.     sum += in;
  112.     sumOfSquares += in * in;
  113.  
  114.     /* Decrement the loop counter */
  115.     blkCnt--;
  116.   }
  117.  
  118.   /* If the blockSize is not a multiple of 4, compute any remaining output samples here.
  119.    ** No loop unrolling is used. */
  120.   blkCnt = blockSize % 0x4U;
  121.  
  122.   while (blkCnt > 0U)
  123.   {
  124.     /* C = (A[0] * A[0] + A[1] * A[1] + ... + A[blockSize-1] * A[blockSize-1]) */
  125.     /* Compute Sum of squares of the input samples
  126.      * and then store the result in a temporary variable, sum. */
  127.     in = *pSrc++;
  128.     sum += in;
  129.     sumOfSquares += in * in;
  130.  
  131.     /* Decrement the loop counter */
  132.     blkCnt--;
  133.   }
  134.  
  135.   /* Compute Mean of squares of the input samples
  136.    * and then store the result in a temporary variable, meanOfSquares. */
  137.   meanOfSquares = sumOfSquares / ((float32_t) blockSize - 1.0f);
  138.  
  139.   /* Compute mean of all input values */
  140.   mean = sum / (float32_t) blockSize;
  141.  
  142.   /* Compute square of mean */
  143.   squareOfMean = (mean * mean) * (((float32_t) blockSize) /
  144.                                   ((float32_t) blockSize - 1.0f));
  145.  
  146.   /* Compute standard deviation and then store the result to the destination */
  147.   arm_sqrt_f32((meanOfSquares - squareOfMean), pResult);
  148.  
  149. #else
  150.   /* Run the below code for Cortex-M0 */
  151.  
  152.   /* Loop over blockSize number of values */
  153.   blkCnt = blockSize;
  154.  
  155.   while (blkCnt > 0U)
  156.   {
  157.     /* C = (A[0] * A[0] + A[1] * A[1] + ... + A[blockSize-1] * A[blockSize-1]) */
  158.     /* Compute Sum of squares of the input samples
  159.      * and then store the result in a temporary variable, sumOfSquares. */
  160.     in = *pSrc++;
  161.     sumOfSquares += in * in;
  162.  
  163.     /* C = (A[0] + A[1] + ... + A[blockSize-1]) */
  164.     /* Compute Sum of the input samples
  165.      * and then store the result in a temporary variable, sum. */
  166.     sum += in;
  167.  
  168.     /* Decrement the loop counter */
  169.     blkCnt--;
  170.   }
  171.  
  172.   /* Compute the square of sum */
  173.   squareOfSum = ((sum * sum) / (float32_t) blockSize);
  174.  
  175.   /* Compute the variance */
  176.   var = ((sumOfSquares - squareOfSum) / (float32_t) (blockSize - 1.0f));
  177.  
  178.   /* Compute standard deviation and then store the result to the destination */
  179.   arm_sqrt_f32(var, pResult);
  180.  
  181. #endif /* #if defined (ARM_MATH_DSP) */
  182. }
  183.  
  184. /**
  185.  * @} end of STD group
  186.  */
  187.