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PDP-10 Archives
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SRI_NIC_PERM_FS_1_19910112
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kcc-6/lib/math/atan2.c
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/*
* +++ NAME +++
*
* ATAN2 Double precision arc tangent of two arguments
*
* +++ INDEX +++
*
* ATAN2
* machine independent routines
* trigonometric functions
* math libraries
*
* +++ DESCRIPTION +++
*
* Returns double precision arc tangent of two
* double precision floating point arguments DATAN(Y/X).
*
* +++ USAGE +++
*
* double atan2(y, x)
* double y, x;
*
* +++ REFERENCES +++
*
* Fortran 77 user's guide, Digital Equipment Corp. pp B-4.
* VAX-11 C manual
* UNIX (4BSD) manual
*
* +++ RESTRICTIONS +++
*
* This is a modification of the original algorithm. Argument
* conventions now follow Vax-11C and Fortran-77.
*
* The arguments are Y followed by X and it computes atan Y/X.
* This makes it compatable with VAX-11 C and UNIX (and Fortran-77).
* (Modified 12-Apr-1984 by Andy Vesper, DEC, Maynard, MA, USA)
*
* For precision information refer to documentation of the
* other floating point library routines called.
*
* +++ PROGRAMMER +++
*
* Fred Fish
* Goodyear Aerospace Corp, Arizona Div.
* (602) 932-7000 work
* (602) 894-6881 home
*
* Modifications for inclusion in standard C library are
* (c) Copyright Ian Macky, SRI International 1985
* Additional modifications after v.6, 18-May-1987 are
* (c) Copyright Ken Harrenstien 1989
*
* This routine now conforms with the description of the atan2()
* function as defined in
* Harbison & Steele's "C: A Reference Manual", section 11.3.5
*
* +++ INTERNALS +++
*
* Computes atan(Y/X) from:
*
* 1. If X = 0 then
* atan(Y,X) = PI/2 * _sign(Y)
*
* 2. If X > 0 then
* atan(Y,X) = atan(Y/X)
*
* 3. If X < 0 then atan2(Y,X) =
* PI * _sign(Y) + atan(Y/X)
*
* ---
*/
#include <math.h>
#include <errno.h>
#include "pml.h"
double atan2(y, x)
double y, x;
{
if (x == 0) {
if (y == 0) {
errno = EDOM;
return 0.0;
}
return _sign(HALFPI, y);
}
if (x > 0)
return atan(y / x);
return atan(y / x) + _sign(PI, y);
}