diff --git a/13.md b/13.md
index 0900d2d7..cf5b1acd 100644
--- a/13.md
+++ b/13.md
@@ -48,37 +48,30 @@ Validating
 Here is some reference C code for calculating the difficulty (aka number of leading zero bits) in a nostr event id:
 
 ```c
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
+int zero_bits(unsigned char b)
+{
+        int n = 0;
 
-int countLeadingZeroes(const char *hex) {
-    int count = 0;
+        if (b == 0)
+                return 8;
 
-    for (int i = 0; i < strlen(hex); i++) {
-        int nibble = (int)strtol((char[]){hex[i], '\0'}, NULL, 16);
-        if (nibble == 0) {
-            count += 4;
-        } else {
-            count += __builtin_clz(nibble) - 28;
-            break;
-        }
-    }
+        while (b >>= 1)
+                n++;
 
-    return count;
+        return 7-n;
 }
 
-int main(int argc, char *argv[]) {
-    if (argc != 2) {
-        fprintf(stderr, "Usage: %s <hex_string>\n", argv[0]);
-        return 1;
-    }
-
-    const char *hex_string = argv[1];
-    int result = countLeadingZeroes(hex_string);
-    printf("Leading zeroes in hex string %s: %d\n", hex_string, result);
-
-    return 0;
+/* find the number of leading zero bits in a hash */
+int count_leading_zero_bits(unsigned char *hash)
+{
+        int bits, total, i;
+        for (i = 0, total = 0; i < 32; i++) {
+                bits = zero_bits(hash[i]);
+                total += bits;
+                if (bits != 8)
+                        break;
+        }
+        return total;
 }
 ```