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/*
* Copyright (c) 2011 The WebRTC project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
/******************************************************************
iLBC Speech Coder ANSI-C Source Code
WebRtcIlbcfix_Chebyshev.c
******************************************************************/
#include "modules/audio_coding/codecs/ilbc/chebyshev.h"
#include "modules/audio_coding/codecs/ilbc/constants.h"
#include "modules/audio_coding/codecs/ilbc/defines.h"
/*------------------------------------------------------------------*
* Calculate the Chevyshev polynomial series
* F(w) = 2*exp(-j5w)*C(x)
* C(x) = (T_0(x) + f(1)T_1(x) + ... + f(4)T_1(x) + f(5)/2)
* T_i(x) is the i:th order Chebyshev polynomial
*------------------------------------------------------------------*/
int16_t WebRtcIlbcfix_Chebyshev(
/* (o) Result of C(x) */
int16_t x, /* (i) Value to the Chevyshev polynomial */
int16_t *f /* (i) The coefficients in the polynomial */
) {
int16_t b1_high, b1_low; /* Use the high, low format to increase the accuracy */
int32_t b2;
int32_t tmp1W32;
int32_t tmp2W32;
int i;
b2 = (int32_t)0x1000000; /* b2 = 1.0 (Q23) */
/* Calculate b1 = 2*x + f[1] */
tmp1W32 = (x << 10) + (f[1] << 14);
for (i = 2; i < 5; i++) {
tmp2W32 = tmp1W32;
/* Split b1 (in tmp1W32) into a high and low part */
b1_high = (int16_t)(tmp1W32 >> 16);
b1_low = (int16_t)((tmp1W32 - ((int32_t)b1_high << 16)) >> 1);
/* Calculate 2*x*b1-b2+f[i] */
tmp1W32 = ((b1_high * x + ((b1_low * x) >> 15)) << 2) - b2 + (f[i] << 14);
/* Update b2 for next round */
b2 = tmp2W32;
}
/* Split b1 (in tmp1W32) into a high and low part */
b1_high = (int16_t)(tmp1W32 >> 16);
b1_low = (int16_t)((tmp1W32 - ((int32_t)b1_high << 16)) >> 1);
/* tmp1W32 = x*b1 - b2 + f[i]/2 */
tmp1W32 = ((b1_high * x) << 1) + (((b1_low * x) >> 15) << 1) -
b2 + (f[i] << 13);
/* Handle overflows and set to maximum or minimum int16_t instead */
if (tmp1W32>((int32_t)33553408)) {
return(WEBRTC_SPL_WORD16_MAX);
} else if (tmp1W32<((int32_t)-33554432)) {
return(WEBRTC_SPL_WORD16_MIN);
} else {
return (int16_t)(tmp1W32 >> 10);
}
}