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72 lines
1.8 KiB
C
72 lines
1.8 KiB
C
/* Base 2 logarithm.
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Copyright (C) 2012-2026 Free Software Foundation, Inc.
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This file is free software: you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as
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published by the Free Software Foundation, either version 3 of the
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License, or (at your option) any later version.
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This file is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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#include <config.h>
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/* Specification. */
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#include <math.h>
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/* Best possible approximation of log(2) as a 'double'. */
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#define LOG2 0.693147180559945309417232121458176568075
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/* Best possible approximation of 1/log(2) as a 'double'. */
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#define LOG2_INVERSE 1.44269504088896340735992468100189213743
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/* sqrt(0.5). */
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#define SQRT_HALF 0.707106781186547524400844362104849039284
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double
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log2 (double x)
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{
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if (isnand (x))
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return x;
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if (x <= 0.0)
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{
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if (x == 0.0)
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/* Return -Infinity. */
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return - HUGE_VAL;
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else
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{
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/* Return NaN. */
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#if defined _MSC_VER
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static double zero;
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return zero / zero;
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#else
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return 0.0 / 0.0;
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#endif
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}
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}
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/* Decompose x into
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x = 2^e * y
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where
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e is an integer,
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1/2 < y < 2.
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Then log2(x) = e + log2(y) = e + log(y)/log(2). */
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{
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int e;
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double y = frexp (x, &e);
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if (y < SQRT_HALF)
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{
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y = 2.0 * y;
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e = e - 1;
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}
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return (double) e + log (y) * LOG2_INVERSE;
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}
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}
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