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ape: apply infinite recursion in fmod() fix (thanks jxy and ality)

Apply changeset 2880:cab2b9d13a73 to ape's fmod() implementation.

Remove the unused math/fmod.c copy.

--- a/sys/src/ape/lib/ap/math/fmod.c

+++ /dev/null

@@ -1,27 +1,0 @@

-/* floating-point mod function without infinity or NaN checking */

-#include <math.h>

-double

-fmod (double x, double y)

-{

-	int sign = 0, yexp;

-	double r, yfr;

-

-	if (y == 0)

-		return 0;

-	if (y < 0)

-		y = -y;

-	yfr = frexp (y, &yexp);

-	if (x < 0) {

-		sign = 1;

-		r = -x;

-	} else

-		r = x;

-	while (r >= y) {

-		int rexp;

-		double rfr = frexp (r, &rexp);

-		r -= ldexp (y, rexp - yexp - (rfr < yfr));

-	}

-	if (sign)

-		r = -r;

-	return r;

-}

--- a/sys/src/ape/lib/ap/plan9/frexp.c

+++ b/sys/src/ape/lib/ap/plan9/frexp.c

@@ -73,6 +73,16 @@

 	Cheat x;

 	int e;

+	x.d = d;

+	e = (x.ms >> SHIFT) & MASK;

+	if(e == MASK){

+		*ip = d;

+		if(x.ls != 0 || (x.ms & 0xfffffL) != 0)	/* NaN */

+			return d;

+		/* ±Inf */

+		x.ms &= 0x80000000L;

+		return x.d;

+	}

 	if(d < 1) {

 		if(d < 0) {

 			f = modf(-d, ip);

@@ -82,8 +92,7 @@

 		*ip = 0;

 		return d;

 	}

-	x.d = d;

-	e = ((x.ms >> SHIFT) & MASK) - BIAS;

+	e -= BIAS;

 	if(e <= SHIFT+1) {

 		x.ms &= ~(0x1fffffL >> e);

 		x.ls = 0;