1 files changed, 52 insertions, 0 deletions
diff --git a/libmp/port/mpsub.c b/libmp/port/mpsub.c
new file mode 100644
index 00000000..3fe6ca09
--- /dev/null
+++ b/libmp/port/mpsub.c
@@ -0,0 +1,52 @@
+#include "os.h"
+#include <mp.h>
+#include "dat.h"
+
+// diff = abs(b1) - abs(b2), i.e., subtract the magnitudes
+void
+mpmagsub(mpint *b1, mpint *b2, mpint *diff)
+{
+	int n, m, sign;
+	mpint *t;
+
+	// get the sizes right
+	if(mpmagcmp(b1, b2) < 0){
+		sign = -1;
+		t = b1;
+		b1 = b2;
+		b2 = t;
+	} else
+		sign = 1;
+	n = b1->top;
+	m = b2->top;
+	if(m == 0){
+		mpassign(b1, diff);
+		diff->sign = sign;
+		return;
+	}
+	mpbits(diff, n*Dbits);
+
+	mpvecsub(b1->p, n, b2->p, m, diff->p);
+	diff->sign = sign;
+	diff->top = n;
+	mpnorm(diff);
+}
+
+// diff = b1 - b2
+void
+mpsub(mpint *b1, mpint *b2, mpint *diff)
+{
+	int sign;
+
+	if(b1->sign != b2->sign){
+		sign = b1->sign;
+		mpmagadd(b1, b2, diff);
+		diff->sign = sign;
+		return;
+	}
+
+	sign = b1->sign;
+	mpmagsub(b1, b2, diff);
+	if(diff->top != 0)
+		diff->sign *= sign;
+}