/* ----------------------------------------------------------------------
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* Project: CMSIS DSP Library
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* Title: arm_fir_decimate_q31.c
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* Description: Q31 FIR Decimator
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*
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* $Date: 27. January 2017
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* $Revision: V.1.5.1
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*
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* Target Processor: Cortex-M cores
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* -------------------------------------------------------------------- */
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/*
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* Copyright (C) 2010-2017 ARM Limited or its affiliates. All rights reserved.
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*
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* SPDX-License-Identifier: Apache-2.0
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*
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* Licensed under the Apache License, Version 2.0 (the License); you may
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* not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an AS IS BASIS, WITHOUT
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* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include "arm_math.h"
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/**
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* @ingroup groupFilters
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*/
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/**
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* @addtogroup FIR_decimate
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* @{
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*/
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/**
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* @brief Processing function for the Q31 FIR decimator.
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* @param[in] *S points to an instance of the Q31 FIR decimator structure.
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* @param[in] *pSrc points to the block of input data.
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* @param[out] *pDst points to the block of output data
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* @param[in] blockSize number of input samples to process per call.
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* @return none
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*
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* <b>Scaling and Overflow Behavior:</b>
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* \par
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* The function is implemented using an internal 64-bit accumulator.
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* The accumulator has a 2.62 format and maintains full precision of the intermediate multiplication results but provides only a single guard bit.
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* Thus, if the accumulator result overflows it wraps around rather than clip.
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* In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits (where log2 is read as log to the base 2).
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* After all multiply-accumulates are performed, the 2.62 accumulator is truncated to 1.32 format and then saturated to 1.31 format.
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*
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* \par
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* Refer to the function <code>arm_fir_decimate_fast_q31()</code> for a faster but less precise implementation of this function for Cortex-M3 and Cortex-M4.
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*/
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void arm_fir_decimate_q31(
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const arm_fir_decimate_instance_q31 * S,
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q31_t * pSrc,
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q31_t * pDst,
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uint32_t blockSize)
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{
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q31_t *pState = S->pState; /* State pointer */
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q31_t *pCoeffs = S->pCoeffs; /* Coefficient pointer */
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q31_t *pStateCurnt; /* Points to the current sample of the state */
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q31_t x0, c0; /* Temporary variables to hold state and coefficient values */
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q31_t *px; /* Temporary pointers for state buffer */
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q31_t *pb; /* Temporary pointers for coefficient buffer */
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q63_t sum0; /* Accumulator */
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uint32_t numTaps = S->numTaps; /* Number of taps */
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uint32_t i, tapCnt, blkCnt, outBlockSize = blockSize / S->M; /* Loop counters */
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#if defined (ARM_MATH_DSP)
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/* Run the below code for Cortex-M4 and Cortex-M3 */
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/* S->pState buffer contains previous frame (numTaps - 1) samples */
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/* pStateCurnt points to the location where the new input data should be written */
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pStateCurnt = S->pState + (numTaps - 1U);
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/* Total number of output samples to be computed */
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blkCnt = outBlockSize;
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while (blkCnt > 0U)
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{
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/* Copy decimation factor number of new input samples into the state buffer */
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i = S->M;
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do
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{
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*pStateCurnt++ = *pSrc++;
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} while (--i);
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/* Set accumulator to zero */
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sum0 = 0;
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/* Initialize state pointer */
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px = pState;
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/* Initialize coeff pointer */
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pb = pCoeffs;
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/* Loop unrolling. Process 4 taps at a time. */
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tapCnt = numTaps >> 2;
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/* Loop over the number of taps. Unroll by a factor of 4.
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** Repeat until we've computed numTaps-4 coefficients. */
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while (tapCnt > 0U)
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{
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/* Read the b[numTaps-1] coefficient */
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c0 = *(pb++);
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/* Read x[n-numTaps-1] sample */
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x0 = *(px++);
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/* Perform the multiply-accumulate */
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sum0 += (q63_t) x0 *c0;
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/* Read the b[numTaps-2] coefficient */
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c0 = *(pb++);
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/* Read x[n-numTaps-2] sample */
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x0 = *(px++);
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/* Perform the multiply-accumulate */
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sum0 += (q63_t) x0 *c0;
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/* Read the b[numTaps-3] coefficient */
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c0 = *(pb++);
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/* Read x[n-numTaps-3] sample */
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x0 = *(px++);
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/* Perform the multiply-accumulate */
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sum0 += (q63_t) x0 *c0;
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/* Read the b[numTaps-4] coefficient */
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c0 = *(pb++);
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/* Read x[n-numTaps-4] sample */
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x0 = *(px++);
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/* Perform the multiply-accumulate */
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sum0 += (q63_t) x0 *c0;
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/* Decrement the loop counter */
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tapCnt--;
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}
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/* If the filter length is not a multiple of 4, compute the remaining filter taps */
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tapCnt = numTaps % 0x4U;
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while (tapCnt > 0U)
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{
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/* Read coefficients */
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c0 = *(pb++);
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/* Fetch 1 state variable */
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x0 = *(px++);
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/* Perform the multiply-accumulate */
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sum0 += (q63_t) x0 *c0;
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/* Decrement the loop counter */
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tapCnt--;
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}
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/* Advance the state pointer by the decimation factor
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* to process the next group of decimation factor number samples */
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pState = pState + S->M;
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/* The result is in the accumulator, store in the destination buffer. */
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*pDst++ = (q31_t) (sum0 >> 31);
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/* Decrement the loop counter */
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blkCnt--;
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}
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/* Processing is complete.
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** Now copy the last numTaps - 1 samples to the satrt of the state buffer.
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** This prepares the state buffer for the next function call. */
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/* Points to the start of the state buffer */
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pStateCurnt = S->pState;
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i = (numTaps - 1U) >> 2U;
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/* copy data */
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while (i > 0U)
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{
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*pStateCurnt++ = *pState++;
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*pStateCurnt++ = *pState++;
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*pStateCurnt++ = *pState++;
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*pStateCurnt++ = *pState++;
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/* Decrement the loop counter */
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i--;
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}
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i = (numTaps - 1U) % 0x04U;
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/* copy data */
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while (i > 0U)
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{
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*pStateCurnt++ = *pState++;
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/* Decrement the loop counter */
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i--;
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}
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#else
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/* Run the below code for Cortex-M0 */
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/* S->pState buffer contains previous frame (numTaps - 1) samples */
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/* pStateCurnt points to the location where the new input data should be written */
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pStateCurnt = S->pState + (numTaps - 1U);
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/* Total number of output samples to be computed */
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blkCnt = outBlockSize;
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while (blkCnt > 0U)
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{
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/* Copy decimation factor number of new input samples into the state buffer */
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i = S->M;
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do
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{
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*pStateCurnt++ = *pSrc++;
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} while (--i);
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/* Set accumulator to zero */
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sum0 = 0;
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/* Initialize state pointer */
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px = pState;
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/* Initialize coeff pointer */
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pb = pCoeffs;
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tapCnt = numTaps;
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while (tapCnt > 0U)
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{
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/* Read coefficients */
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c0 = *pb++;
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/* Fetch 1 state variable */
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x0 = *px++;
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/* Perform the multiply-accumulate */
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sum0 += (q63_t) x0 *c0;
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/* Decrement the loop counter */
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tapCnt--;
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}
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/* Advance the state pointer by the decimation factor
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* to process the next group of decimation factor number samples */
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pState = pState + S->M;
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/* The result is in the accumulator, store in the destination buffer. */
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*pDst++ = (q31_t) (sum0 >> 31);
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/* Decrement the loop counter */
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blkCnt--;
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}
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/* Processing is complete.
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** Now copy the last numTaps - 1 samples to the start of the state buffer.
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** This prepares the state buffer for the next function call. */
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/* Points to the start of the state buffer */
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pStateCurnt = S->pState;
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i = numTaps - 1U;
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/* copy data */
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while (i > 0U)
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{
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*pStateCurnt++ = *pState++;
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/* Decrement the loop counter */
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i--;
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}
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#endif /* #if defined (ARM_MATH_DSP) */
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}
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/**
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* @} end of FIR_decimate group
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*/
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