File content as of revision 0:eedb7d567a5d:

/* ----------------------------------------------------------------------
 * Project:      CMSIS DSP Library
 * Title:        arm_sqrt_q31.c
 * Description:  Q31 square root function
 *
 * $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 "arm_math.h"
#include "arm_common_tables.h"

/**
 * @ingroup groupFastMath
 */

/**
 * @addtogroup SQRT
 * @{
 */

/**
 * @brief Q31 square root function.
 * @param[in]   in    input value.  The range of the input value is [0 +1) or 0x00000000 to 0x7FFFFFFF.
 * @param[out]  *pOut square root of input value.
 * @return The function returns ARM_MATH_SUCCESS if the input value is positive
 * and ARM_MATH_ARGUMENT_ERROR if the input is negative.  For
 * negative inputs, the function returns *pOut = 0.
 */

arm_status arm_sqrt_q31(
  q31_t in,
  q31_t * pOut)
{
  q31_t number, temp1, bits_val1, var1, signBits1, half;
  float32_t temp_float1;
  union
  {
      q31_t fracval;
      float32_t floatval;
  } tempconv;

  number = in;

  /* If the input is a positive number then compute the signBits. */
  if (number > 0)
  {
    signBits1 = __CLZ(number) - 1;

    /* Shift by the number of signBits1 */
    if ((signBits1 % 2) == 0)
    {
      number = number << signBits1;
    }
    else
    {
      number = number << (signBits1 - 1);
    }

    /* Calculate half value of the number */
    half = number >> 1;
    /* Store the number for later use */
    temp1 = number;

    /*Convert to float */
    temp_float1 = number * 4.6566128731e-010f;
    /*Store as integer */
    tempconv.floatval = temp_float1;
    bits_val1 = tempconv.fracval;
    /* Subtract the shifted value from the magic number to give intial guess */
    bits_val1 = 0x5f3759df - (bits_val1 >> 1);  /* gives initial guess */
    /* Store as float */
    tempconv.fracval = bits_val1;
    temp_float1 = tempconv.floatval;
    /* Convert to integer format */
    var1 = (q31_t) (temp_float1 * 1073741824);

    /* 1st iteration */
    var1 = ((q31_t) ((q63_t) var1 * (0x30000000 -
                                     ((q31_t)
                                      ((((q31_t)
                                         (((q63_t) var1 * var1) >> 31)) *
                                        (q63_t) half) >> 31))) >> 31)) << 2;
    /* 2nd iteration */
    var1 = ((q31_t) ((q63_t) var1 * (0x30000000 -
                                     ((q31_t)
                                      ((((q31_t)
                                         (((q63_t) var1 * var1) >> 31)) *
                                        (q63_t) half) >> 31))) >> 31)) << 2;
    /* 3rd iteration */
    var1 = ((q31_t) ((q63_t) var1 * (0x30000000 -
                                     ((q31_t)
                                      ((((q31_t)
                                         (((q63_t) var1 * var1) >> 31)) *
                                        (q63_t) half) >> 31))) >> 31)) << 2;

    /* Multiply the inverse square root with the original value */
    var1 = ((q31_t) (((q63_t) temp1 * var1) >> 31)) << 1;

    /* Shift the output down accordingly */
    if ((signBits1 % 2) == 0)
    {
      var1 = var1 >> (signBits1 / 2);
    }
    else
    {
      var1 = var1 >> ((signBits1 - 1) / 2);
    }
    *pOut = var1;

    return (ARM_MATH_SUCCESS);
  }
  /* If the number is a negative number then store zero as its square root value */
  else
  {
    *pOut = 0;
    return (ARM_MATH_ARGUMENT_ERROR);
  }
}

/**
 * @} end of SQRT group
 */