Re: [9front] [patch] add log2 to ape

["Iruatã Souza" <iru.muzgo@gmail.com>]

On 06/12/2020 19:49, ori@eigenstate.org wrote:
> In error cases, this divides the error return
> by log2. I suspect we don't want to do that.
> 
> I'd also rather name all of the constants
> consistently. Maybe uppercase the initial
> character, instead of giving one of them
> an _ prefix.

The patch is simply wrong, it doesn't even compile correctly!

Sorry for the noise, here is a better one.

diff -r 3497ab777d56 amd64/include/ape/math.h
--- a/amd64/include/ape/math.h	Mon Dec 07 00:36:41 2020 +0100
+++ b/amd64/include/ape/math.h	Mon Dec 07 11:11:22 2020 +0100
@@ -26,6 +26,7 @@
 extern double ldexp(double, int);
 extern double log(double);
 extern double log10(double);
+extern double log2(double);
 extern double modf(double, double *);
 extern double pow(double, double);
 extern double sqrt(double);
diff -r 3497ab777d56 sys/src/ape/lib/ap/math/log.c
--- a/sys/src/ape/lib/ap/math/log.c	Mon Dec 07 00:36:41 2020 +0100
+++ b/sys/src/ape/lib/ap/math/log.c	Mon Dec 07 11:11:22 2020 +0100
@@ -10,16 +10,16 @@
 #include <math.h>
 #include <errno.h>

-#define	log2    0.693147180559945309e0
-#define	ln10o1  .4342944819032518276511
-#define	sqrto2  0.707106781186547524e0
-#define	p0      -.240139179559210510e2
-#define	p1      0.309572928215376501e2
-#define	p2      -.963769093377840513e1
-#define	p3      0.421087371217979714e0
-#define	q0      -.120069589779605255e2
-#define	q1      0.194809660700889731e2
-#define	q2      -.891110902798312337e1
+#define	Log2   0.693147180559945309e0
+#define	Ln10o1  .4342944819032518276511
+#define	Sqrto2  0.707106781186547524e0
+#define	P0      -.240139179559210510e2
+#define	P1      0.309572928215376501e2
+#define	P2      -.963769093377840513e1
+#define	P3      0.421087371217979714e0
+#define	Q0      -.120069589779605255e2
+#define	Q1      0.194809660700889731e2
+#define	Q2      -.891110902798312337e1

 double
 log(double arg)
@@ -36,7 +36,7 @@
 		x *= 2;
 		exp--;
 	}
-	if(x < sqrto2) {
+	if(x < Sqrto2) {
 		x *= 2;
 		exp--;
 	}
@@ -44,9 +44,9 @@
 	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;
+	temp = ((P3*zsq + P2)*zsq + P1)*zsq + P0;
+	temp = temp/(((zsq + Q2)*zsq + Q1)*zsq + Q0);
+	temp = temp*z + exp*Log2;
 	return temp;
 }

@@ -58,5 +58,16 @@
 		errno = (arg==0)? ERANGE : EDOM;
 		return -HUGE_VAL;
 	}
-	return log(arg) * ln10o1;
+	return log(arg) * Ln10o1;
 }
+
+double
+log2(double arg)
+{
+	if(arg <= 0) {
+		errno = (arg==0)? ERANGE : EDOM;
+		return -HUGE_VAL;
+	}
+
+	return log(arg)/Log2;
+}