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authorEric Andersen <andersen@codepoet.org>2001-05-10 00:40:28 +0000
committerEric Andersen <andersen@codepoet.org>2001-05-10 00:40:28 +0000
commit1077fa4d772832f77a677ce7fb7c2d513b959e3f (patch)
tree579bee13fb0b58d2800206366ec2caecbb15f3fc /libm/float/cbrtf.c
parent22358dd7ce7bb49792204b698f01a6f69b9c8e08 (diff)
uClibc now has a math library. muahahahaha!
-Erik
Diffstat (limited to 'libm/float/cbrtf.c')
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+/* cbrtf.c
+ *
+ * Cube root
+ *
+ *
+ *
+ * SYNOPSIS:
+ *
+ * float x, y, cbrtf();
+ *
+ * y = cbrtf( x );
+ *
+ *
+ *
+ * DESCRIPTION:
+ *
+ * Returns the cube root of the argument, which may be negative.
+ *
+ * Range reduction involves determining the power of 2 of
+ * the argument. A polynomial of degree 2 applied to the
+ * mantissa, and multiplication by the cube root of 1, 2, or 4
+ * approximates the root to within about 0.1%. Then Newton's
+ * iteration is used to converge to an accurate result.
+ *
+ *
+ *
+ * ACCURACY:
+ *
+ * Relative error:
+ * arithmetic domain # trials peak rms
+ * IEEE 0,1e38 100000 7.6e-8 2.7e-8
+ *
+ */
+ /* cbrt.c */
+
+/*
+Cephes Math Library Release 2.2: June, 1992
+Copyright 1984, 1987, 1988, 1992 by Stephen L. Moshier
+Direct inquiries to 30 Frost Street, Cambridge, MA 02140
+*/
+
+
+#include <math.h>
+
+static float CBRT2 = 1.25992104989487316477;
+static float CBRT4 = 1.58740105196819947475;
+
+
+float frexpf(float, int *), ldexpf(float, int);
+
+float cbrtf( float xx )
+{
+int e, rem, sign;
+float x, z;
+
+x = xx;
+if( x == 0 )
+ return( 0.0 );
+if( x > 0 )
+ sign = 1;
+else
+ {
+ sign = -1;
+ x = -x;
+ }
+
+z = x;
+/* extract power of 2, leaving
+ * mantissa between 0.5 and 1
+ */
+x = frexpf( x, &e );
+
+/* Approximate cube root of number between .5 and 1,
+ * peak relative error = 9.2e-6
+ */
+x = (((-0.13466110473359520655053 * x
+ + 0.54664601366395524503440 ) * x
+ - 0.95438224771509446525043 ) * x
+ + 1.1399983354717293273738 ) * x
+ + 0.40238979564544752126924;
+
+/* exponent divided by 3 */
+if( e >= 0 )
+ {
+ rem = e;
+ e /= 3;
+ rem -= 3*e;
+ if( rem == 1 )
+ x *= CBRT2;
+ else if( rem == 2 )
+ x *= CBRT4;
+ }
+
+
+/* argument less than 1 */
+
+else
+ {
+ e = -e;
+ rem = e;
+ e /= 3;
+ rem -= 3*e;
+ if( rem == 1 )
+ x /= CBRT2;
+ else if( rem == 2 )
+ x /= CBRT4;
+ e = -e;
+ }
+
+/* multiply by power of 2 */
+x = ldexpf( x, e );
+
+/* Newton iteration */
+x -= ( x - (z/(x*x)) ) * 0.333333333333;
+
+if( sign < 0 )
+ x = -x;
+return(x);
+}