text: /* ----------------------------------------------------------------------
   * Project:      CMSIS DSP Library
   * Title:        arm_rms_q31.c
   * Description:  Root Mean Square of the elements of a Q31 vector
   *
   * $Date:        27. January 2017
   * $Revision:    V.1.5.1
   *
   * Target Processor: Cortex-M cores
   * -------------------------------------------------------------------- */
  /*
   * Copyright (C) 2010-2017 ARM Limited or its affiliates. All rights reserved.
   *
   * SPDX-License-Identifier: Apache-2.0
   *
   * Licensed under the Apache License, Version 2.0 (the License); you may
   * not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   * www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an AS IS BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  #include "CMSIS_DSP/cmsis_dsp.h"
  
  /**
   * @addtogroup RMS
   * @{
   */
  
  
  /**
   * @brief Root Mean Square of the elements of a Q31 vector.
   * @param[in]       *pSrc points to the input vector
   * @param[in]       blockSize length of the input vector
   * @param[out]      *pResult rms value returned here
   * @return none.
   *
   * @details
   * <b>Scaling and Overflow Behavior:</b>
   *
   *\par
   * The function is implemented using an internal 64-bit accumulator.
   * The input is represented in 1.31 format, and intermediate multiplication
   * yields a 2.62 format.
   * The accumulator maintains full precision of the intermediate multiplication results,
   * but provides only a single guard bit.
   * There is no saturation on intermediate additions.
   * If the accumulator overflows, it wraps around and distorts the result.
   * In order to avoid overflows completely, the input signal must be scaled down by
   * log2(blockSize) bits, as a total of blockSize additions are performed internally.
   * Finally, the 2.62 accumulator is right shifted by 31 bits to yield a 1.31 format value.
   *
   */
  
  void arm_rms_q31(
    q31_t * pSrc,
    uint32_t blockSize,
    q31_t * pResult)
  {
    q63_t sum = 0;                                 /* accumulator */
    q31_t in;                                      /* Temporary variable to store the input */
    uint32_t blkCnt;                               /* loop counter */
  
  #if defined (ARM_MATH_DSP)
    /* Run the below code for Cortex-M4 and Cortex-M3 */
  
    q31_t in1, in2, in3, in4;                      /* Temporary input variables */
  
    /*loop Unrolling */
    blkCnt = blockSize >> 2u;
  
    /* First part of the processing with loop unrolling.  Compute 8 outputs at a time.
     ** a second loop below computes the remaining 1 to 7 samples. */
    while (blkCnt > 0u)
    {
      /* C = A[0] * A[0] + A[1] * A[1] + A[2] * A[2] + ... + A[blockSize-1] * A[blockSize-1] */
      /* Compute sum of the squares and then store the result in a temporary variable, sum */
      /* read two samples from source buffer */
      in1 = pSrc[0];
      in2 = pSrc[1];
  
      /* calculate power and accumulate to accumulator */
      sum += (q63_t) in1 *in1;
      sum += (q63_t) in2 *in2;
  
      /* read two samples from source buffer */
      in3 = pSrc[2];
      in4 = pSrc[3];
  
      /* calculate power and accumulate to accumulator */
      sum += (q63_t) in3 *in3;
      sum += (q63_t) in4 *in4;
  
  
      /* update source buffer to process next samples */
      pSrc += 4u;
  
      /* Decrement the loop counter */
      blkCnt--;
    }
  
    /* If the blockSize is not a multiple of 8, compute any remaining output samples here.
     ** No loop unrolling is used. */
    blkCnt = blockSize % 0x4u;
  
  #else
    /* Run the below code for Cortex-M0 */
  
    blkCnt = blockSize;
  
  #endif /* #if defined (ARM_MATH_DSP) */
  
    while (blkCnt > 0u)
    {
      /* C = A[0] * A[0] + A[1] * A[1] + A[2] * A[2] + ... + A[blockSize-1] * A[blockSize-1] */
      /* Compute sum of the squares and then store the results in a temporary variable, sum */
      in = *pSrc++;
      sum += (q63_t) in *in;
  
      /* Decrement the loop counter */
      blkCnt--;
    }
  
    /* Convert data in 2.62 to 1.31 by 31 right shifts and saturate */
    /* Compute Rms and store the result in the destination vector */
    arm_sqrt_q31(clip_q63_to_q31((sum / (q63_t) blockSize) >> 31), pResult);
  }
  
  /**
   * @} end of RMS group
   */
  