diff --git a/scan/scan_ulong.c b/scan/scan_ulong.c
index 385f98e..e11f564 100644
--- a/scan/scan_ulong.c
+++ b/scan/scan_ulong.c
@@ -5,9 +5,13 @@ unsigned int scan_ulong(const char* src,unsigned long int* dest) {
   register unsigned long int l=0;
   register unsigned char c;
   while ((c=*tmp-'0')<10) {
-    unsigned long int m=l;
-    l=l*10+c;
-    if ((l>>3) < m) break;
+    unsigned long int n;
+    /* division is very slow on most architectures */
+    n=l<<3; if ((n>>3)!=l) break;
+    if (n+(l<<1) < n) break;
+    n+=l<<1;
+    if (n+c < n) break;
+    l=n+c;
     ++tmp;
   }
   *dest=l;
diff --git a/scan/scan_ulonglong.c b/scan/scan_ulonglong.c
index 77e87d1..6d53c58 100644
--- a/scan/scan_ulonglong.c
+++ b/scan/scan_ulonglong.c
@@ -5,9 +5,13 @@ unsigned int scan_ulonglong(const char *src,unsigned long long *dest) {
   register unsigned long long l=0;
   register unsigned char c;
   while ((c=*tmp-'0')<10) {
-    unsigned long long m=l;
-    l=l*10+c;
-    if ((l>>3) < m) break;
+    unsigned long long n;
+    /* division is very slow on most architectures */
+    n=l<<3; if ((n>>3)!=l) break;
+    if (n+(l<<1) < n) break;
+    n+=l<<1;
+    if (n+c < n) break;
+    l=n+c;
     ++tmp;
   }
   *dest=l;
diff --git a/test/scan.c b/test/scan.c
new file mode 100644
index 0000000..10c4614
--- /dev/null
+++ b/test/scan.c
@@ -0,0 +1,16 @@
+#include <assert.h>
+#include <limits.h>
+#include "scan.h"
+
+int main() {
+  unsigned long l;
+  char buf[100];
+  assert(scan_ulong("12345",&l)==5 && l==12345);
+  assert(scan_ulong("-12345",&l)==0);
+  assert(scan_ulong("4294967295",&l)==10 && l==4294967295ul);
+  if (sizeof(unsigned long)==4) {
+    assert(scan_ulong("4294967296",&l)==9 && l==429496729);
+    assert(scan_ulong("42949672950",&l)==10 && l==4294967295ul);
+    assert(scan_ulong("5294967295",&l)==9 && l==529496729);
+  }
+}