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pc: add rev function

--- a/sys/man/1/pc

+++ b/sys/man/1/pc

@@ -84,6 +84,9 @@

 .I xtend(n,m)

 \fIn\fR truncated to \fIm\fR bits, with the highest bit interpreted as a sign bit.

 .TP

+.I rev(n,m)

+\fIn\fR truncated to \fIm\fR bits, with the order of bits reversed.

+.TP

 .I ubits(n)

 The minimum number of bits required to represent \fIn\fR as an unsigned number.

 .TP

--- a/sys/src/cmd/pc.y

+++ b/sys/src/cmd/pc.y

@@ -804,6 +804,38 @@

 	return a[0];

 }

+Num *

+fnrev(int, Num **a)

+{

+	mpdigit v, m;

+	int i, j, n;

+	

+	if(toint(a[1], &n, 1)){

+		numdecref(a[0]);

+		numdecref(a[1]);

+		return nil;

+	}

+	a[0] = nummod(a[0]);

+	mptrunc(a[0], n, a[0]);

+	for(i = 0; i < a[0]->top; i++){

+		v = a[0]->p[i];

+		m = -1;

+		for(j = sizeof(mpdigit) * 8; j >>= 1; ){

+			m ^= m << j;

+			v = v >> j & m | v << j & ~m;

+		}

+		a[0]->p[i] = v;

+	}

+	for(i = 0; i < a[0]->top / 2; i++){

+		v = a[0]->p[i];

+		a[0]->p[i] = a[0]->p[a[0]->top - 1 - i];

+		a[0]->p[a[0]->top - 1 - i] = v;

+	}

+	mpleft(a[0], n - a[0]->top * sizeof(mpdigit) * 8, a[0]);

+	numdecref(a[1]);

+	return a[0];

+}

+

 void

 main(int argc, char **argv)

 {

@@ -830,6 +862,7 @@

 	regfunc("gcd", fngcd, 2);

 	regfunc("minv", fnminv, 2);

 	regfunc("rand", fnrand, 1);

+	regfunc("rev", fnrev, 2);

 	prompt = 1;

 	ARGBEGIN{