ref: 53fa826a8a695bcfade5c47f636f719c71e96b98
dir: /sys/src/libc/port/atan.c/
/* floating-point arctangent atan returns the value of the arctangent of its argument in the range [-pi/2,pi/2]. atan2 returns the arctangent of arg1/arg2 in the range [-pi,pi]. there are no error returns. coefficients are #5077 from Hart & Cheney. (19.56D) */ #include <u.h> #include <libc.h> #define sq2p1 2.414213562373095048802e0 #define sq2m1 .414213562373095048802e0 #define p4 .161536412982230228262e2 #define p3 .26842548195503973794141e3 #define p2 .11530293515404850115428136e4 #define p1 .178040631643319697105464587e4 #define p0 .89678597403663861959987488e3 #define q4 .5895697050844462222791e2 #define q3 .536265374031215315104235e3 #define q2 .16667838148816337184521798e4 #define q1 .207933497444540981287275926e4 #define q0 .89678597403663861962481162e3 /* xatan evaluates a series valid in the range [-0.414...,+0.414...]. (tan(pi/8)) */ static double xatan(double arg) { double argsq, value; argsq = arg*arg; value = ((((p4*argsq + p3)*argsq + p2)*argsq + p1)*argsq + p0); value = value/(((((argsq + q4)*argsq + q3)*argsq + q2)*argsq + q1)*argsq + q0); return value*arg; } /* satan reduces its argument (known to be positive) to the range [0,0.414...] and calls xatan. */ static double satan(double arg) { if(arg < sq2m1) return xatan(arg); if(arg > sq2p1) return PIO2 - xatan(1/arg); return PIO2/2 + xatan((arg-1)/(arg+1)); } /* atan makes its argument positive and calls the inner routine satan. */ double atan(double arg) { if(arg > 0) return satan(arg); return -satan(-arg); }