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+
+/* @(#)e_hypot.c 1.3 95/01/18 */
+/*
+ * ====================================================
+ * 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.
+ * ====================================================
+ */
+
+//#include <sys/cdefs.h>
+//__FBSDID("$FreeBSD$");
+
+/* __ieee754_hypot(x,y)
+ *
+ * Method :
+ * If (assume round-to-nearest) z=x*x+y*y
+ * has error less than sqrt(2)/2 ulp, than
+ * sqrt(z) has error less than 1 ulp (exercise).
+ *
+ * So, compute sqrt(x*x+y*y) with some care as
+ * follows to get the error below 1 ulp:
+ *
+ * Assume x>y>0;
+ * (if possible, set rounding to round-to-nearest)
+ * 1. if x > 2y use
+ * x1*x1+(y*y+(x2*(x+x1))) for x*x+y*y
+ * where x1 = x with lower 32 bits cleared, x2 = x-x1; else
+ * 2. if x <= 2y use
+ * t1*y1+((x-y)*(x-y)+(t1*y2+t2*y))
+ * where t1 = 2x with lower 32 bits cleared, t2 = 2x-t1,
+ * y1= y with lower 32 bits chopped, y2 = y-y1.
+ *
+ * NOTE: scaling may be necessary if some argument is too
+ * large or too tiny
+ *
+ * Special cases:
+ * hypot(x,y) is INF if x or y is +INF or -INF; else
+ * hypot(x,y) is NAN if x or y is NAN.
+ *
+ * Accuracy:
+ * hypot(x,y) returns sqrt(x^2+y^2) with error less
+ * than 1 ulps (units in the last place)
+ */
+
+#include <cmath>
+#include <float.h>
+
+#include "math_private.h"
+
+double
+__ieee754_hypot(double x, double y)
+{
+ double a,b,t1,t2,y1,y2,w;
+ int32_t j,k,ha,hb;
+
+ GET_HIGH_WORD(ha,x);
+ ha &= 0x7fffffff;
+ GET_HIGH_WORD(hb,y);
+ hb &= 0x7fffffff;
+ if(hb > ha) {a=y;b=x;j=ha; ha=hb;hb=j;} else {a=x;b=y;}
+ a = fabs(a);
+ b = fabs(b);
+ if((ha-hb)>0x3c00000) {return a+b;} /* x/y > 2**60 */
+ k=0;
+ if(ha > 0x5f300000) { /* a>2**500 */
+ if(ha >= 0x7ff00000) { /* Inf or NaN */
+ u_int32_t low;
+ /* Use original arg order iff result is NaN; quieten sNaNs. */
+ w = fabsl(x+0.0L)-fabs(y+0);
+ GET_LOW_WORD(low,a);
+ if(((ha&0xfffff)|low)==0) w = a;
+ GET_LOW_WORD(low,b);
+ if(((hb^0x7ff00000)|low)==0) w = b;
+ return w;
+ }
+ /* scale a and b by 2**-600 */
+ ha -= 0x25800000; hb -= 0x25800000; k += 600;
+ SET_HIGH_WORD(a,ha);
+ SET_HIGH_WORD(b,hb);
+ }
+ if(hb < 0x20b00000) { /* b < 2**-500 */
+ if(hb <= 0x000fffff) { /* subnormal b or 0 */
+ u_int32_t low;
+ GET_LOW_WORD(low,b);
+ if((hb|low)==0) return a;
+ t1=0;
+ SET_HIGH_WORD(t1,0x7fd00000); /* t1=2^1022 */
+ b *= t1;
+ a *= t1;
+ k -= 1022;
+ } else { /* scale a and b by 2^600 */
+ ha += 0x25800000; /* a *= 2^600 */
+ hb += 0x25800000; /* b *= 2^600 */
+ k -= 600;
+ SET_HIGH_WORD(a,ha);
+ SET_HIGH_WORD(b,hb);
+ }
+ }
+ /* medium size a and b */
+ w = a-b;
+ if (w>b) {
+ t1 = 0;
+ SET_HIGH_WORD(t1,ha);
+ t2 = a-t1;
+ w = std::sqrt(t1*t1-(b*(-b)-t2*(a+t1)));
+ } else {
+ a = a+a;
+ y1 = 0;
+ SET_HIGH_WORD(y1,hb);
+ y2 = b - y1;
+ t1 = 0;
+ SET_HIGH_WORD(t1,ha+0x00100000);
+ t2 = a - t1;
+ w = std::sqrt(t1*y1-(w*(-w)-(t1*y2+t2*b)));
+ }
+ if(k!=0) {
+ t1 = 0.0;
+ SET_HIGH_WORD(t1,(1023+k)<<20);
+ return t1*w;
+ } else return w;
+}