Refactor Day 5 2016 and add MD5 utility class. Extracted common MD5 hashing functionality into a reusable utility class and simplified Day 5 implementation.

SimonBaars · SimonBaars · commit 002bc9103889 · 2025-02-10T14:23:29.000+01:00
diff --git a/src/main/java/com/sbaars/adventofcode/common/MD5.java b/src/main/java/com/sbaars/adventofcode/common/MD5.java
@@ -0,0 +1,38 @@
+package com.sbaars.adventofcode.common;
+
+import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+
+public class MD5 {
+    private static final MessageDigest md;
+    
+    static {
+        try {
+            md = MessageDigest.getInstance("MD5");
+        } catch (NoSuchAlgorithmException e) {
+            throw new RuntimeException("MD5 algorithm not available", e);
+        }
+    }
+    
+    public static byte[] hash(String input) {
+        return md.digest(input.getBytes(StandardCharsets.UTF_8));
+    }
+    
+    public static boolean hasLeadingZeros(byte[] hash, int numZeros) {
+        int fullBytes = numZeros / 2;
+        int remainingNibbles = numZeros % 2;
+        
+        // Check full bytes
+        for (int i = 0; i < fullBytes; i++) {
+            if (hash[i] != 0) return false;
+        }
+        
+        // Check remaining nibble if needed
+        return remainingNibbles == 0 || (hash[fullBytes] & 0xF0) == 0;
+    }
+    
+    public static char toHexChar(int nibble) {
+        return (char) (nibble < 10 ? ('0' + nibble) : ('a' + (nibble - 10)));
+    }
+} 
diff --git a/src/main/java/com/sbaars/adventofcode/year16/days/Day5.java b/src/main/java/com/sbaars/adventofcode/year16/days/Day5.java
@@ -1,10 +1,8 @@
 package com.sbaars.adventofcode.year16.days;
 
 import com.sbaars.adventofcode.year16.Day2016;
+import com.sbaars.adventofcode.common.MD5;
 
-import java.nio.charset.StandardCharsets;
-import java.security.MessageDigest;
-import java.security.NoSuchAlgorithmException;
 import java.util.Arrays;
 
 public class Day5 extends Day2016 {
@@ -23,22 +21,12 @@ public Object part1() {
     StringBuilder password = new StringBuilder();
     int index = 0;
 
-    try {
-      MessageDigest md = MessageDigest.getInstance("MD5");
-      while (password.length() < 8) {
-        byte[] digest = md.digest((doorId + index).getBytes(StandardCharsets.UTF_8));
-
-        // Check if first 20 bits (5 hex zeroes) are zero
-        // digest[0] == 0 && digest[1] == 0 && (digest[2] & 0xF0) == 0
-        if (digest[0] == 0 && digest[1] == 0 && (digest[2] & 0xF0) == 0) {
-          // The 6th hex character is the low nibble of digest[2]
-          int nib5 = digest[2] & 0x0F;
-          password.append(toHexChar(nib5));
-        }
-        index++;
+    while (password.length() < 8) {
+      byte[] hash = MD5.hash(doorId + index);
+      if (MD5.hasLeadingZeros(hash, 5)) {
+        password.append(MD5.toHexChar(hash[2] & 0x0F));
       }
-    } catch (NoSuchAlgorithmException e) {
-      throw new RuntimeException(e);
+      index++;
     }
     return password.toString();
   }
@@ -51,30 +39,17 @@ public Object part2() {
     int filled = 0;
     int index = 0;
 
-    try {
-      MessageDigest md = MessageDigest.getInstance("MD5");
-      while (filled < 8) {
-        byte[] digest = md.digest((doorId + index).getBytes(StandardCharsets.UTF_8));
-        // Same check as above
-        if (digest[0] == 0 && digest[1] == 0 && (digest[2] & 0xF0) == 0) {
-          // 6th hex character is nib5 (low nibble of digest[2]), 7th is nib6 (high nibble of digest[3])
-          int nib5 = digest[2] & 0x0F;            // position
-          int nib6 = (digest[3] & 0xF0) >>> 4;    // actual character
-
-          if (nib5 < 8 && password[nib5] == '_') {
-            password[nib5] = toHexChar(nib6);
-            filled++;
-          }
+    while (filled < 8) {
+      byte[] hash = MD5.hash(doorId + index);
+      if (MD5.hasLeadingZeros(hash, 5)) {
+        int pos = hash[2] & 0x0F;
+        if (pos < 8 && password[pos] == '_') {
+          password[pos] = MD5.toHexChar((hash[3] & 0xF0) >>> 4);
+          filled++;
         }
-        index++;
       }
-    } catch (NoSuchAlgorithmException e) {
-      throw new RuntimeException(e);
+      index++;
     }
     return new String(password);
   }
-
-  private static char toHexChar(int nibble) {
-    return (char) (nibble < 10 ? ('0' + nibble) : ('a' + (nibble - 10)));
-  }
 }
