Add a "fork" of musl as //fusl.

fusl/src/math/atan2.c

+/* origin: FreeBSD /usr/src/lib/msun/src/e_atan2.c */
+/*
+ * ====================================================
+ * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+ *
+ * Developed at SunSoft, a Sun Microsystems, Inc. business.
+ * Permission to use, copy, modify, and distribute this
+ * software is freely granted, provided that this notice
+ * is preserved.
+ * ====================================================
+ *
+ */
+/* atan2(y,x)
+ * Method :
+ *      1. Reduce y to positive by atan2(y,x)=-atan2(-y,x).
+ *      2. Reduce x to positive by (if x and y are unexceptional):
+ *              ARG (x+iy) = arctan(y/x)           ... if x > 0,
+ *              ARG (x+iy) = pi - arctan[y/(-x)]   ... if x < 0,
+ *
+ * Special cases:
+ *
+ *      ATAN2((anything), NaN ) is NaN;
+ *      ATAN2(NAN , (anything) ) is NaN;
+ *      ATAN2(+-0, +(anything but NaN)) is +-0  ;
+ *      ATAN2(+-0, -(anything but NaN)) is +-pi ;
+ *      ATAN2(+-(anything but 0 and NaN), 0) is +-pi/2;
+ *      ATAN2(+-(anything but INF and NaN), +INF) is +-0 ;
+ *      ATAN2(+-(anything but INF and NaN), -INF) is +-pi;
+ *      ATAN2(+-INF,+INF ) is +-pi/4 ;
+ *      ATAN2(+-INF,-INF ) is +-3pi/4;
+ *      ATAN2(+-INF, (anything but,0,NaN, and INF)) is +-pi/2;
+ *
+ * Constants:
+ * The hexadecimal values are the intended ones for the following
+ * constants. The decimal values may be used, provided that the
+ * compiler will convert from decimal to binary accurately enough
+ * to produce the hexadecimal values shown.
+ */
+
+#include "libm.h"
+
+static const double
+pi     = 3.1415926535897931160E+00, /* 0x400921FB, 0x54442D18 */
+pi_lo  = 1.2246467991473531772E-16; /* 0x3CA1A626, 0x33145C07 */
+
+double atan2(double y, double x)
+{
+        double z;
+        uint32_t m,lx,ly,ix,iy;
+
+        if (isnan(x) || isnan(y))
+                return x+y;
+        EXTRACT_WORDS(ix, lx, x);
+        EXTRACT_WORDS(iy, ly, y);
+        if ((ix-0x3ff00000 | lx) == 0)  /* x = 1.0 */
+                return atan(y);
+        m = ((iy>>31)&1) | ((ix>>30)&2);  /* 2*sign(x)+sign(y) */
+        ix = ix & 0x7fffffff;
+        iy = iy & 0x7fffffff;
+
+        /* when y = 0 */
+        if ((iy|ly) == 0) {
+                switch(m) {
+                case 0:
+                case 1: return y;   /* atan(+-0,+anything)=+-0 */
+                case 2: return  pi; /* atan(+0,-anything) = pi */
+                case 3: return -pi; /* atan(-0,-anything) =-pi */
+                }
+        }
+        /* when x = 0 */
+        if ((ix|lx) == 0)
+                return m&1 ? -pi/2 : pi/2;
+        /* when x is INF */
+        if (ix == 0x7ff00000) {
+                if (iy == 0x7ff00000) {
+                        switch(m) {
+                        case 0: return  pi/4;   /* atan(+INF,+INF) */
+                        case 1: return -pi/4;   /* atan(-INF,+INF) */
+                        case 2: return  3*pi/4; /* atan(+INF,-INF) */
+                        case 3: return -3*pi/4; /* atan(-INF,-INF) */
+                        }
+                } else {
+                        switch(m) {
+                        case 0: return  0.0; /* atan(+...,+INF) */
+                        case 1: return -0.0; /* atan(-...,+INF) */
+                        case 2: return  pi;  /* atan(+...,-INF) */
+                        case 3: return -pi;  /* atan(-...,-INF) */
+                        }
+                }
+        }
+        /* |y/x| > 0x1p64 */
+        if (ix+(64<<20) < iy || iy == 0x7ff00000)
+                return m&1 ? -pi/2 : pi/2;
+
+        /* z = atan(|y/x|) without spurious underflow */
+        if ((m&2) && iy+(64<<20) < ix)  /* |y/x| < 0x1p-64, x<0 */
+                z = 0;
+        else
+                z = atan(fabs(y/x));
+        switch (m) {
+        case 0: return z;              /* atan(+,+) */
+        case 1: return -z;             /* atan(-,+) */
+        case 2: return pi - (z-pi_lo); /* atan(+,-) */
+        default: /* case 3 */
+                return (z-pi_lo) - pi; /* atan(-,-) */
+        }
+}