36 lines
621 B
C
36 lines
621 B
C
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#ifdef sccsid
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static char sccsid[] = "@(#)log2.c 1.1 92/07/30 Copyr 1986 Sun Micro";
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#endif
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/*
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* Copyright (c) 1987 by Sun Microsystems, Inc.
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*/
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/*
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* LOG2(x)
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* RETURN THE BASE 2 LOGARITHM OF X
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*
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* Method:
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* purge off 0,INF, and NaN.
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* n = ilogb(x)
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* if(n<0) n+=1
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* z = scalbn(x,-n)
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* LOG2(x) = n + 1.4426950408889634074*log(x)
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*
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* where the constant 1.442.. is the inverse of ln2 rounded
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*/
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#include <math.h>
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double log2(x)
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double x;
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{
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int n;
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if(x==0.0||!finite(x)) return log(x);
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n = ilogb(x);
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if(n<0) n+=1;
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x = scalbn(x,-n); if(x==0.5) return n-1.0;
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return n+1.44269504088896340736*log(x);
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}
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