1 files changed, 30 insertions, 9 deletions

diff --git a/src/zsub.c b/src/zsub.c
index 8616542..5096eb0 100644
--- a/src/zsub.c
+++ b/src/zsub.c
@@ -5,28 +5,49 @@
 void
 zsub_unsigned(z_t a, z_t b, z_t c)
 {
-	int magcmp = zcmpmag(b, c);
-	z_t s;
-	size_t i, n;
 	zahl_char_t carry[] = {0, 0};
+	zahl_char_t *s;
+	size_t i, n;
+	int magcmp;
+
+	if (zzero(b)) {
+		zabs(a, c);
+		zneg(a, a);
+		return;
+	} else if (zzero(c)) {
+		zabs(a, b);
+		return;
+	} else if (a == b || a == c) {
+		/* TODO This should not be necessary. */
+		z_t tb, tc;
+		zinit(tb);
+		zinit(tc);
+		zset(tb, b);
+		zset(tc, c);
+		zsub_unsigned(a, tb, tc);
+		zfree(tb);
+		zfree(tc);
+		return;
+	}
 
+	magcmp = zcmpmag(b, c);
 	if (magcmp <= 0) {
 		if (magcmp == 0) {
 			SET_SIGNUM(a, 0);
 			return;
 		}
 		SET(a, c);
-		*s = *b;
+		n = MIN(a->used, b->used);
+		s = b->chars;
 	} else {
 		SET(a, b);
-		*s = *c;
+		n = MIN(a->used, c->used);
+		s = c->chars;
 	}
 
-	n = MIN(a->used, s->used);
-
 	for (i = 0; i < n; i++) {
-		carry[~i & 1] = carry[i & 1] ? (a->chars[i] <= s->chars[i]) : (a->chars[i] < s->chars[i]);
-		a->chars[i] -= s->chars[i];
+		carry[~i & 1] = carry[i & 1] ? (a->chars[i] <= s[i]) : (a->chars[i] < s[i]);
+		a->chars[i] -= s[i];
 		a->chars[i] -= carry[i & 1];
 	}