arm_cos_q15.c

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/* ----------------------------------------------------------------------
* Copyright (C) 2010-2014 ARM Limited. All rights reserved.
*
* $Date:        07. September 2015
* $Revision:    V.1.4.5 a
*
* Project:      CMSIS DSP Library
* Title:        arm_cos_q15.c
*
* Description:  Fast cosine calculation for Q15 values.
*
* 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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*     notice, this list of conditions and the following disclaimer.
*   - Redistributions in binary form must reproduce the above copyright
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*     software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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* -------------------------------------------------------------------- */

#include "arm_math.h"
#include "arm_common_tables.h"

q15_t arm_cos_q15(
  q15_t x)
{
  q15_t cosVal;                                  /* Temporary variables for input, output */
  int32_t index;                                 /* Index variables */
  q15_t a, b;                                    /* Four nearest output values */
  q15_t fract;                                   /* Temporary values for fractional values */

  /* add 0.25 (pi/2) to read sine table */
  x = (uint16_t)x + 0x2000;
  if(x < 0)
  {   /* convert negative numbers to corresponding positive ones */
      x = (uint16_t)x + 0x8000;
  }

  /* Calculate the nearest index */
  index = (uint32_t)x >> FAST_MATH_Q15_SHIFT;

  /* Calculation of fractional value */
  fract = (x - (index << FAST_MATH_Q15_SHIFT)) << 9;

  /* Read two nearest values of input value from the sin table */
  a = sinTable_q15[index];
  b = sinTable_q15[index+1];

  /* Linear interpolation process */
  cosVal = (q31_t)(0x8000-fract)*a >> 16;
  cosVal = (q15_t)((((q31_t)cosVal << 16) + ((q31_t)fract*b)) >> 16);

  return cosVal << 1;
}