Solutions for Days 1 and 2 of 2019.

src/advent2019/Puzzle1.java

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+package advent2019;
+
+import com.google.common.io.CharStreams;
+import java.io.InputStreamReader;
+import java.io.Reader;
+import java.util.List;
+
+/**
+ * @author Éamonn McManus
+ */
+public class Puzzle1 {
+  public static void main(String[] args) throws Exception {
+    try (Reader r = new InputStreamReader(Puzzle1.class.getResourceAsStream("puzzle1.txt"))) {
+      List<Long> numbers = CharStreams.readLines(r).stream().map(Long::valueOf).toList();
+      long part1 = numbers.stream().mapToLong(n -> n / 3 - 2).sum();
+      System.out.printf("Part 1 sum is %d\n", part1);
+      long part2 = 0;
+      for (long n : numbers) {
+        for (long m = n; (m = m / 3 - 2) > 0; ) {
+          part2 += m;
+        }
+      }
+      System.out.printf("Part 2 sum is %d\n", part2);
+    }
+  }
+}

src/advent2019/Puzzle2.java

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+package advent2019;
+
+import static com.google.common.collect.ImmutableList.toImmutableList;
+import static java.lang.Math.addExact;
+import static java.lang.Math.multiplyExact;
+
+import com.google.common.base.Splitter;
+import com.google.common.collect.ImmutableList;
+import com.google.common.io.CharStreams;
+import java.io.InputStreamReader;
+import java.io.Reader;
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * @author Éamonn McManus
+ */
+public class Puzzle2 {
+  public static void main(String[] args) throws Exception {
+    try (Reader r = new InputStreamReader(Puzzle2.class.getResourceAsStream("puzzle2.txt"))) {
+      String line = CharStreams.toString(r).trim();
+      ImmutableList<Integer> originalMemory =
+          Splitter.on(',').splitToStream(line).map(Integer::valueOf).collect(toImmutableList());
+
+      // Part 1
+      System.out.printf("Part 1 result is %d\n", run(originalMemory, 12, 2));
+
+      // Part 2
+      outer:
+      for (int i = 0; i < 100; i++) {
+        for (int j = 0; j < 100; j++) {
+          if (run(originalMemory, i, j) == 19690720) {
+            System.out.printf("Part 2 result is %d,%d => %d\n", i, j, i * 100 + j);
+            break outer;
+          }
+        }
+      }
+    }
+  }
+
+  private static int run(ImmutableList<Integer> originalMemory, int input1, int input2) {
+    List<Integer> memory = new ArrayList<>(originalMemory);
+    memory.set(1, input1);
+    memory.set(2, input2);
+    for (int pc = 0; memory.get(pc) != 99; pc += 4) {
+      int lhs = memory.get(memory.get(pc + 1));
+      int rhs = memory.get(memory.get(pc + 2));
+      int result =
+          switch (memory.get(pc)) {
+            case 1 -> addExact(lhs, rhs);
+            case 2 -> multiplyExact(lhs, rhs);
+            default -> throw new AssertionError(memory.get(pc));
+          };
+      memory.set(memory.get(pc + 3), result);
+    }
+    return memory.get(0);
+  }
+}