ref: 1d891d163e814a93ee5346cf05b4fac297bf3a5c
dir: /sys/src/ape/lib/ap/math/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 <math.h> #define sq2p1 2.414213562373095048802e0 #define sq2m1 .414213562373095048802e0 #define pio2 1.570796326794896619231e0 #define pio4 .785398163397448309615e0 #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...]. */ static double xatan(double arg) { double argsq, value; /* get denormalized add in following if range arg**10 is much smaller than q1, so check for that case */ if(-.01 < arg && arg < .01) value = p0/q0; else { 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(1.0/arg); return pio4 + xatan((arg-1.0)/(arg+1.0)); } /* atan makes its argument positive and calls the inner routine satan. */ double atan(double arg) { if(arg > 0) return satan(arg); return -satan(-arg); }