From 1077fa4d772832f77a677ce7fb7c2d513b959e3f Mon Sep 17 00:00:00 2001
From: Eric Andersen <andersen@codepoet.org>
Date: Thu, 10 May 2001 00:40:28 +0000
Subject: uClibc now has a math library.  muahahahaha!  -Erik

---
 libm/float/igamif.c | 112 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 112 insertions(+)
 create mode 100644 libm/float/igamif.c

(limited to 'libm/float/igamif.c')

diff --git a/libm/float/igamif.c b/libm/float/igamif.c
new file mode 100644
index 000000000..5a33b4982
--- /dev/null
+++ b/libm/float/igamif.c
@@ -0,0 +1,112 @@
+/*							igamif()
+ *
+ *      Inverse of complemented imcomplete gamma integral
+ *
+ *
+ *
+ * SYNOPSIS:
+ *
+ * float a, x, y, igamif();
+ *
+ * x = igamif( a, y );
+ *
+ *
+ *
+ * DESCRIPTION:
+ *
+ * Given y, the function finds x such that
+ *
+ *  igamc( a, x ) = y.
+ *
+ * Starting with the approximate value
+ *
+ *         3
+ *  x = a t
+ *
+ *  where
+ *
+ *  t = 1 - d - ndtri(y) sqrt(d)
+ * 
+ * and
+ *
+ *  d = 1/9a,
+ *
+ * the routine performs up to 10 Newton iterations to find the
+ * root of igamc(a,x) - y = 0.
+ *
+ *
+ * ACCURACY:
+ *
+ * Tested for a ranging from 0 to 100 and x from 0 to 1.
+ *
+ *                      Relative error:
+ * arithmetic   domain     # trials      peak         rms
+ *    IEEE      0,100         5000       1.0e-5      1.5e-6
+ *
+ */
+
+/*
+Cephes Math Library Release 2.2:  July, 1992
+Copyright 1984, 1987, 1992 by Stephen L. Moshier
+Direct inquiries to 30 Frost Street, Cambridge, MA 02140
+*/
+
+#include <math.h>
+
+extern float MACHEPF, MAXLOGF;
+
+#define fabsf(x) ( (x) < 0 ? -(x) : (x) )
+
+#ifdef ANSIC
+float igamcf(float, float);
+float ndtrif(float), expf(float), logf(float), sqrtf(float), lgamf(float);
+#else
+float igamcf();
+float ndtrif(), expf(), logf(), sqrtf(), lgamf();
+#endif
+
+
+float igamif( float aa, float yy0 )
+{
+float a, y0, d, y, x0, lgm;
+int i;
+
+a = aa;
+y0 = yy0;
+/* approximation to inverse function */
+d = 1.0/(9.0*a);
+y = ( 1.0 - d - ndtrif(y0) * sqrtf(d) );
+x0 = a * y * y * y;
+
+lgm = lgamf(a);
+
+for( i=0; i<10; i++ )
+	{
+	if( x0 <= 0.0 )
+		{
+		mtherr( "igamif", UNDERFLOW );
+		return(0.0);
+		}
+	y = igamcf(a,x0);
+/* compute the derivative of the function at this point */
+	d = (a - 1.0) * logf(x0) - x0 - lgm;
+	if( d < -MAXLOGF )
+		{
+		mtherr( "igamif", UNDERFLOW );
+		goto done;
+		}
+	d = -expf(d);
+/* compute the step to the next approximation of x */
+	if( d == 0.0 )
+		goto done;
+	d = (y - y0)/d;
+	x0 = x0 - d;
+	if( i < 3 )
+		continue;
+	if( fabsf(d/x0) < (2.0 * MACHEPF) )
+		goto done;
+	}
+
+done:
+return( x0 );
+}
-- 
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