88 lines
1.9 KiB
C
88 lines
1.9 KiB
C
/*
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* This file is part of the UCB release of Plan 9. It is subject to the license
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* terms in the LICENSE file found in the top-level directory of this
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* distribution and at http://akaros.cs.berkeley.edu/files/Plan9License. No
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* part of the UCB release of Plan 9, including this file, may be copied,
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* modified, propagated, or distributed except according to the terms contained
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* in the LICENSE file.
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*/
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/*
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floating-point arctangent
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atan returns the value of the arctangent of its
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argument in the range [-pi/2,pi/2].
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atan2 returns the arctangent of arg1/arg2
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in the range [-pi,pi].
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there are no error returns.
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coefficients are #5077 from Hart & Cheney. (19.56D)
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*/
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#include <u.h>
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#include <libc.h>
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#define sq2p1 2.414213562373095048802e0
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#define sq2m1 .414213562373095048802e0
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#define p4 .161536412982230228262e2
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#define p3 .26842548195503973794141e3
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#define p2 .11530293515404850115428136e4
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#define p1 .178040631643319697105464587e4
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#define p0 .89678597403663861959987488e3
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#define q4 .5895697050844462222791e2
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#define q3 .536265374031215315104235e3
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#define q2 .16667838148816337184521798e4
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#define q1 .207933497444540981287275926e4
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#define q0 .89678597403663861962481162e3
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/*
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xatan evaluates a series valid in the
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range [-0.414...,+0.414...]. (jehanne_tan(pi/8))
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*/
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static
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double
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xatan(double arg)
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{
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double argsq, value;
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argsq = arg*arg;
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value = ((((p4*argsq + p3)*argsq + p2)*argsq + p1)*argsq + p0);
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value = value/(((((argsq + q4)*argsq + q3)*argsq + q2)*argsq + q1)*argsq + q0);
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return value*arg;
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}
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/*
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satan reduces its argument (known to be positive)
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to the range [0,0.414...] and calls xatan.
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*/
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static
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double
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satan(double arg)
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{
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if(arg < sq2m1)
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return xatan(arg);
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if(arg > sq2p1)
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return PIO2 - xatan(1/arg);
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return PIO2/2 + xatan((arg-1)/(arg+1));
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}
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/*
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atan makes its argument positive and
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calls the inner routine satan.
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*/
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double
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jehanne_atan(double arg)
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{
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if(arg > 0)
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return satan(arg);
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return -satan(-arg);
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}
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