114 lines
4.5 KiB
C
114 lines
4.5 KiB
C
/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
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│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8 :vi│
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╞══════════════════════════════════════════════════════════════════════════════╡
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│ Copyright 2020 Justine Alexandra Roberts Tunney │
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│ │
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│ This program is free software; you can redistribute it and/or modify │
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│ it under the terms of the GNU General Public License as published by │
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│ the Free Software Foundation; version 2 of the License. │
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│ │
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│ This program is distributed in the hope that it will be useful, but │
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│ WITHOUT ANY WARRANTY; without even the implied warranty of │
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│ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU │
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│ General Public License for more details. │
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│ │
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│ You should have received a copy of the GNU General Public License │
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│ along with this program; if not, write to the Free Software │
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│ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA │
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│ 02110-1301 USA │
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╚─────────────────────────────────────────────────────────────────────────────*/
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#include "dsp/core/q.h"
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#include "libc/assert.h"
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#include "libc/dce.h"
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#include "libc/limits.h"
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#include "libc/macros.h"
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#include "libc/math.h"
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#include "libc/str/str.h"
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/**
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* Precomputes integers that can replace floating-point operands.
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*
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* “G-d made the integers, all else is the work of man.
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* — Leopold Kronecker
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*
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* This function shifts the decimal point to the left:
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*
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* 𝑛ᵢ ← ROUND[𝑐ᵢ × 2ᵐ] + φᵢ
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*
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* With extra effort to compute φ which is normally all zeroes but gives
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* us better rounding when it isn't. It's assumed optimized coefficients
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* will be used like this:
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*
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* (Σᵢ𝑥ᵢ𝑛ᵢ + 2⁽ᵐ⁻¹⁾) / 2ᵐ where 𝑥∈[𝐿,𝐻] and 𝑖∈[0,6)
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*
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* Intended to compute this
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*
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* ROUND[Σᵢ𝑥ᵢ𝑐ᵢ]
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*
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* As accurately or approximately as you want it to be. Popular scaling
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* factors are 7, 15, 16, 22, and 31. Building this code under MODE=tiny
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* will DCE the math.
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*
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* @param N receives optimized integers
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* @param C provides ideal coefficients
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* @param M is log₂ scaling factor, e.g. 7
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* @param L is minimum input data size, e.g. 0
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* @param H is maximum input data size, e.g. 255
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* @return sum of errors for all inputs
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* @see en.wikipedia.org/wiki/Binary_scaling
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* @see o/tool/build/coefficients.com
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* @cost ~300ns
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*/
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long GetIntegerCoefficients(long N[static 6], const double C[static 6], long M,
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long L, long H) {
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int i;
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int j[6], J[6];
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int O[6] = {0};
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int S[3] = {0, -1, +1};
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double R[6], K[6], D[6], HM, HL, least, error;
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least = 1;
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HM = 1L << M;
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HL = H - L + 1;
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assert(H >= L);
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assert(HL <= HM);
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for (i = 0; i < 6; ++i) {
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least *= HL;
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if (fabs(C[i]) > DBL_MIN) {
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J[i] = ARRAYLEN(S);
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R[i] = C[i] * HM;
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K[i] = rint(R[i]);
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N[i] = K[i];
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} else {
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J[i] = 1;
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R[i] = 0;
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K[i] = 0;
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N[i] = 0;
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}
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}
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if (!IsTiny() && least > 1) {
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for (j[0] = 0; j[0] < J[0]; ++j[0]) {
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for (j[1] = 0; j[1] < J[1]; ++j[1]) {
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for (j[2] = 0; j[2] < J[2]; ++j[2]) {
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for (j[3] = 0; j[3] < J[3]; ++j[3]) {
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for (j[4] = 0; j[4] < J[4]; ++j[4]) {
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for (j[5] = 0; j[5] < J[5]; ++j[5]) {
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for (i = 0; i < ARRAYLEN(J); ++i) {
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D[i] = S[j[i]] + K[i] - R[i];
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}
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if ((error = DifferSumSq(D, L, H) / HM) < least) {
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least = error;
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memcpy(O, j, sizeof(j));
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}
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}
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}
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}
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}
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}
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}
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for (i = 0; i < 6; ++i) {
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N[i] += S[O[i]];
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}
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}
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return lround(least);
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}
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