1 files changed, 34 insertions, 0 deletions
diff --git a/libmp/port/mpvecsub.c b/libmp/port/mpvecsub.c
new file mode 100644
index 00000000..db93b65b
--- /dev/null
+++ b/libmp/port/mpvecsub.c
@@ -0,0 +1,34 @@
+#include "os.h"
+#include <mp.h>
+#include "dat.h"
+
+// prereq: a >= b, alen >= blen, diff has at least alen digits
+void
+mpvecsub(mpdigit *a, int alen, mpdigit *b, int blen, mpdigit *diff)
+{
+	int i, borrow;
+	mpdigit x, y;
+
+	borrow = 0;
+	for(i = 0; i < blen; i++){
+		x = *a++;
+		y = *b++;
+		y += borrow;
+		if(y < borrow)
+			borrow = 1;
+		else
+			borrow = 0;
+		if(x < y)
+			borrow++;
+		*diff++ = x - y;
+	}
+	for(; i < alen; i++){
+		x = *a++;
+		y = x - borrow;
+		if(y > x)
+			borrow = 1;
+		else
+			borrow = 0;
+		*diff++ = y;
+	}
+}