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lang/basic/lib/log.c

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+/*
+ * (c) copyright 1983 by the Vrije Universiteit, Amsterdam, The Netherlands.
+ *
+ *          This product is part of the Amsterdam Compiler Kit.
+ *
+ * Permission to use, sell, duplicate or disclose this software must be
+ * obtained in writing. Requests for such permissions may be sent to
+ *
+ *      Dr. Andrew S. Tanenbaum
+ *      Wiskundig Seminarium
+ *      Vrije Universiteit
+ *      Postbox 7161
+ *      1007 MC Amsterdam
+ *      The Netherlands
+ *
+ */
+
+/* $Header $ */
+
+/* Author: J.W. Stevenson */
+
+extern double	_fef();
+
+/*
+	log returns the natural logarithm of its floating
+	point argument.
+
+	The coefficients are #2705 from Hart & Cheney. (19.38D)
+
+	It calls _fef.
+*/
+
+#define	HUGE	1.701411733192644270e38
+
+static double log2	= 0.693147180559945309e0;
+static double sqrto2	= 0.707106781186547524e0;
+static double p0	= -.240139179559210510e2;
+static double p1	= 0.309572928215376501e2;
+static double p2	= -.963769093368686593e1;
+static double p3	= 0.421087371217979714e0;
+static double q0	= -.120069589779605255e2;
+static double q1	= 0.194809660700889731e2;
+static double q2	= -.891110902798312337e1;
+
+double
+_log(arg)
+double arg;
+{
+	double x,z, zsq, temp;
+	int exp;
+
+	if(arg <= 0) {
+		error(3);
+		return(-HUGE);
+	}
+	x = _fef(arg,&exp);
+	/*
+	while(x < 0.5) {
+		x =* 2;
+		exp--;
+	}
+	*/
+	if(x<sqrto2) {
+		x *= 2;
+		exp--;
+	}
+
+	z = (x-1)/(x+1);
+	zsq = z*z;
+
+	temp = ((p3*zsq + p2)*zsq + p1)*zsq + p0;
+	temp = temp/(((zsq + q2)*zsq + q1)*zsq + q0);
+	temp = temp*z + exp*log2;
+	return(temp);
+}