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/*- |
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* Copyright (c) 2005, Steven G. Kargl |
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* All rights reserved. |
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* |
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* Redistribution and use in source and binary forms, with or without |
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* modification, are permitted provided that the following conditions |
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* are met: |
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* 1. Redistributions of source code must retain the above copyright |
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* notice unmodified, this list of conditions, and the following |
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* disclaimer. |
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* 2. Redistributions in binary form must reproduce the above copyright |
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* notice, this list of conditions and the following disclaimer in the |
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* documentation and/or other materials provided with the distribution. |
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* |
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* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
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* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
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* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
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* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
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* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
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* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
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* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
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* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
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* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
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* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
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*/ |
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|
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/* |
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* Compute the natural logrithm of x by decomposing x into its base 2 |
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* representation. |
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* |
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* log(x) = log(f * 2**n) |
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* = log(f) + n * log(2) |
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* |
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* where f = [1/2,1). |
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* |
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* Use a Taylor's series about f0 = 0.75 to compute log(f). |
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* |
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* log(f) = log(f0) + sum{(1/n) * (-1)**(n-1) * ((f - f0)/f0)**n}. |
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*/ |
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|
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#include <math.h> |
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#include <float.h> |
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|
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#define LOGL2 6.931471805599453094e-1L |
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#define LOGLF0 -2.876820724517809274e-1L |
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|
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static long double zero = 0.0L; |
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|
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long double logl(long double x) { |
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int i, n; |
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long double f, t, val, oval; |
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|
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/* log(x) = sNaN if x < 0. */ |
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if (x < 0.0L) |
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return (x - x) / (x - x); |
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|
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/* log(+Inf) = +Inf */ |
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if (isinf(x)) |
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return x*x+x; |
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|
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/* log(+-0) = -Inf with signal */ |
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if (x == 0.0L) |
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return (- 1.0L / zero); |
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|
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/* log(NaN) = NaN with signal */ |
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if (isnan(x)) |
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return (x+x); |
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|
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/* |
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* Special case for values near log(1). Use the series expansion |
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* for log(1+x) = x - (1/2) * x**2 + (1/3) * x**3 + ... |
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*/ |
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if (0.95L < x && x < 1.05L) { |
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|
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f = t = x - 1.0L; |
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oval = val = 0.L; |
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|
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for (i = 1; ; i++) { |
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val += t / (long double) i; |
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t *= -f; |
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if (fabsl(oval - val) < LDBL_EPSILON) break; |
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oval = val; |
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} |
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|
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return (val); |
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} |
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|
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f = frexpl(x, &n); |
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f = t = (f - 0.75L) / 0.75L; |
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|
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oval = val = LOGLF0; |
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for (i = 1; ; i++) { |
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val += t / (long double) i; |
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t *= -f; |
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if (fabsl(oval-val) < LDBL_EPSILON) break; |
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oval = val; |
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} |
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return (val + n * LOGL2); |
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|
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} |