Solution for Day 22 of 2024.

src/advent2024/Puzzle22.java

@@ -0,0 +1,110 @@
+package advent2024;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.collect.ImmutableSet.toImmutableSet;
+import static com.google.common.collect.MoreCollectors.onlyElement;
+
+import com.google.common.base.Splitter;
+import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.ImmutableSet;
+import com.google.common.graph.ImmutableValueGraph;
+import com.google.common.graph.MutableValueGraph;
+import com.google.common.graph.ValueGraphBuilder;
+import com.google.common.io.CharStreams;
+import java.io.InputStreamReader;
+import java.io.Reader;
+import java.io.StringReader;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.LinkedHashMap;
+import java.util.LinkedHashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.Callable;
+
+/**
+ * @author Éamonn McManus
+ */
+public class Puzzle22 {
+  private static final String SAMPLE1 =
+      """
+      1
+      10
+      100
+      2024
+      """;
+  private static final String SAMPLE2 =
+      """
+      1
+      2
+      3
+      2024
+      """;
+
+  private static final Map<String, Callable<Reader>> INPUT_PRODUCERS =
+      ImmutableMap.of(
+          "sample 1",
+          () -> new StringReader(SAMPLE1),
+          "sample 2",
+          () -> new StringReader(SAMPLE2),
+          "problem",
+          () -> new InputStreamReader(Puzzle22.class.getResourceAsStream("puzzle22.txt")));
+
+  public static void main(String[] args) throws Exception {
+    for (var entry : INPUT_PRODUCERS.entrySet()) {
+      String name = entry.getKey();
+      try (Reader r = entry.getValue().call()) {
+        List<String> lines = CharStreams.readLines(r);
+        List<Long> secrets = lines.stream().map(Long::valueOf).toList();
+        long sum = secrets.stream().map(s -> nthNextSecret(2000, s)).reduce(0L, Math::addExact);
+        System.out.printf("For %s, sum of 2000th secret numbers is %d\n", name, sum);
+        System.out.printf("For %s, max bananas is %d\n", name, part2(secrets));
+      }
+    }
+  }
+
+  private static long part2(List<Long> secrets) {
+    Map<List<Integer>, Long> totals = new LinkedHashMap<>();
+    for (long s : secrets) {
+      long s0 = s;
+      Map<List<Integer>, Integer> results = new LinkedHashMap<>();
+      List<Integer> window = new ArrayList<>();
+      int lastPrice = 0;
+      for (int i = 0; i < 2000; i++) {
+        int price = (int) s % 10;
+        if (i > 0) {
+          int diff = price - lastPrice;
+          window.add(diff);
+          if (window.size() > 4) {
+            window.removeFirst();
+          }
+          if (window.size() == 4) {
+            results.putIfAbsent(new ArrayList<>(window), price);
+          }
+        }
+        lastPrice = price;
+        s = nextSecret(s);
+      }
+      results.forEach(
+          (diffList, price) -> totals.put(diffList, totals.getOrDefault(diffList, 0L) + price));
+    }
+    return Collections.max(totals.values());
+  }
+
+  private static long nthNextSecret(int n, long s) {
+    for (int i = 0; i < n; i++) {
+      s = nextSecret(s);
+    }
+    return s;
+  }
+
+  private static long nextSecret(long s) {
+    long mask = (1 << 24) - 1;
+    s = ((s << 6) ^ s) & mask;
+    s = (s >> 5) ^ s;
+    s = ((s << 11) ^ s) & mask;
+    return s;
+  }
+}