summaryrefslogtreecommitdiff
path: root/src/math/s_cbrt.c
diff options
context:
space:
mode:
Diffstat (limited to 'src/math/s_cbrt.c')
-rw-r--r--src/math/s_cbrt.c77
1 files changed, 77 insertions, 0 deletions
diff --git a/src/math/s_cbrt.c b/src/math/s_cbrt.c
new file mode 100644
index 00000000..8adcb191
--- /dev/null
+++ b/src/math/s_cbrt.c
@@ -0,0 +1,77 @@
+/* @(#)s_cbrt.c 5.1 93/09/24 */
+/*
+ * ====================================================
+ * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+ *
+ * Developed at SunPro, a Sun Microsystems, Inc. business.
+ * Permission to use, copy, modify, and distribute this
+ * software is freely granted, provided that this notice
+ * is preserved.
+ * ====================================================
+ */
+
+#include <math.h>
+#include "math_private.h"
+
+/* cbrt(x)
+ * Return cube root of x
+ */
+static const uint32_t
+ B1 = 715094163, /* B1 = (682-0.03306235651)*2**20 */
+ B2 = 696219795; /* B2 = (664-0.03306235651)*2**20 */
+
+static const double
+C = 5.42857142857142815906e-01, /* 19/35 = 0x3FE15F15, 0xF15F15F1 */
+D = -7.05306122448979611050e-01, /* -864/1225 = 0xBFE691DE, 0x2532C834 */
+E = 1.41428571428571436819e+00, /* 99/70 = 0x3FF6A0EA, 0x0EA0EA0F */
+F = 1.60714285714285720630e+00, /* 45/28 = 0x3FF9B6DB, 0x6DB6DB6E */
+G = 3.57142857142857150787e-01; /* 5/14 = 0x3FD6DB6D, 0xB6DB6DB7 */
+
+double
+cbrt(double x)
+{
+ int32_t hx;
+ double r,s,t=0.0,w;
+ uint32_t sign;
+ uint32_t high,low;
+
+ GET_HIGH_WORD(hx,x);
+ sign=hx&0x80000000; /* sign= sign(x) */
+ hx ^=sign;
+ if(hx>=0x7ff00000) return(x+x); /* cbrt(NaN,INF) is itself */
+ GET_LOW_WORD(low,x);
+ if((hx|low)==0)
+ return(x); /* cbrt(0) is itself */
+
+ SET_HIGH_WORD(x,hx); /* x <- |x| */
+ /* rough cbrt to 5 bits */
+ if(hx<0x00100000) /* subnormal number */
+ {SET_HIGH_WORD(t,0x43500000); /* set t= 2**54 */
+ t*=x; GET_HIGH_WORD(high,t); SET_HIGH_WORD(t,high/3+B2);
+ }
+ else
+ SET_HIGH_WORD(t,hx/3+B1);
+
+
+ /* new cbrt to 23 bits, may be implemented in single precision */
+ r=t*t/x;
+ s=C+r*t;
+ t*=G+F/(s+E+D/s);
+
+ /* chopped to 20 bits and make it larger than cbrt(x) */
+ GET_HIGH_WORD(high,t);
+ INSERT_WORDS(t,high+0x00000001,0);
+
+
+ /* one step newton iteration to 53 bits with error less than 0.667 ulps */
+ s=t*t; /* t*t is exact */
+ r=x/s;
+ w=t+t;
+ r=(r-t)/(w+r); /* r-s is exact */
+ t=t+t*r;
+
+ /* retore the sign bit */
+ GET_HIGH_WORD(high,t);
+ SET_HIGH_WORD(t,high|sign);
+ return(t);
+}