a

libmp/mpleft.c

@@ -0,0 +1,46 @@
+#include "os.h"
+#include <mp.h>
+#include "dat.h"
+
+// res = b << shift
+void
+mpleft(mpint *b, int shift, mpint *res)
+{
+	int d, l, r, i, otop;
+	mpdigit this, last;
+
+	// a negative left shift is a right shift
+	if(shift < 0){
+		mpright(b, -shift, res);
+		return;
+	}
+
+	// b and res may be the same so remember the old top
+	otop = b->top;
+
+	// shift
+	mpbits(res, otop*Dbits + shift);	// overkill
+	res->top = DIGITS(otop*Dbits + shift);
+	d = shift/Dbits;
+	l = shift - d*Dbits;
+	r = Dbits - l;
+
+	if(l == 0){
+		for(i = otop-1; i >= 0; i--)
+			res->p[i+d] = b->p[i];
+	} else {
+		last = 0;
+		for(i = otop-1; i >= 0; i--) {
+			this = b->p[i];
+			res->p[i+d+1] = (last<<l) | (this>>r);
+			last = this;
+		}
+		res->p[d] = last<<l;
+	}
+	for(i = 0; i < d; i++)
+		res->p[i] = 0;
+
+	// normalize
+	while(res->top > 0 && res->p[res->top-1] == 0)
+		res->top--;
+}