74 lines
1.4 KiB
C
74 lines
1.4 KiB
C
/*
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* (c) copyright 1988 by the Vrije Universiteit, Amsterdam, The Netherlands.
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* See the copyright notice in the ACK home directory, in the file "Copyright".
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*
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* Author: Ceriel J.H. Jacobs
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*/
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/* $Header$ */
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#include <math.h>
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#include <errno.h>
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extern int errno;
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extern double modf();
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double
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tan(x)
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double x;
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{
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/* Algorithm and coefficients from:
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"Software manual for the elementary functions"
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by W.J. Cody and W. Waite, Prentice-Hall, 1980
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*/
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int negative = x < 0;
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int invert = 0;
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double y;
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static double p[] = {
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1.0,
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-0.13338350006421960681e+0,
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0.34248878235890589960e-2,
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-0.17861707342254426711e-4
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};
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static double q[] = {
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1.0,
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-0.46671683339755294240e+0,
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0.25663832289440112864e-1,
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-0.31181531907010027307e-3,
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0.49819433993786512270e-6
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};
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if (negative) x = -x;
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/* ??? avoid loss of significance, error if x is too large ??? */
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y = x * M_2_PI + 0.5;
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/* Use extended precision to calculate reduced argument.
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Here we used 12 bits of the mantissa for a1.
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Also split x in integer part x1 and fraction part x2.
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*/
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#define A1 1.57080078125
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#define A2 -4.454455103380768678308e-6
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{
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double x1, x2;
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modf(y, &y);
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if (modf(0.5*y, &x1)) invert = 1;
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x2 = modf(x, &x1);
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x = x1 - y * A1;
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x += x2;
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x -= y * A2;
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#undef A1
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#undef A2
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}
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/* ??? avoid underflow ??? */
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y = x * x;
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x += x * y * POLYNOM2(y, p+1);
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y = POLYNOM4(y, q);
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if (negative) x = -x;
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return invert ? -y/x : x/y;
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}
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