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

Author: SimonBaars

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)));
+ }
+} 

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)));
- }
 }