?? log.c
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/* * (c) copyright 1988 by the Vrije Universiteit, Amsterdam, The Netherlands. * See the copyright notice in the ACK home directory, in the file "Copyright". * * Author: Ceriel J.H. Jacobs *//* $Header: /cvsup/minix/src/lib/ack/libp/log.c,v 1.1 2005/10/10 15:27:47 beng Exp $ */#define __NO_DEFS#include <math.h>#include <pc_err.h>#if __STDC__#include <pc_math.h>#include <float.h>#endif#undef HUGE#define HUGE 1e1000double_log(x) double x;{ /* Algorithm and coefficients from: "Software manual for the elementary functions" by W.J. Cody and W. Waite, Prentice-Hall, 1980 */ static double a[] = { -0.64124943423745581147e2, 0.16383943563021534222e2, -0.78956112887491257267e0 }; static double b[] = { -0.76949932108494879777e3, 0.31203222091924532844e3, -0.35667977739034646171e2, 1.0 }; extern double _fef(); double znum, zden, z, w; int exponent; if (x <= 0) { _trp(ELOG); return -HUGE; } x = _fef(x, &exponent); if (x > M_1_SQRT2) { znum = (x - 0.5) - 0.5; zden = x * 0.5 + 0.5; } else { znum = x - 0.5; zden = znum * 0.5 + 0.5; exponent--; } z = znum/zden; w = z * z; x = z + z * w * (POLYNOM2(w,a)/POLYNOM3(w,b)); z = exponent; x += z * (-2.121944400546905827679e-4); return x + z * 0.693359375;}
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