Day 09 part 2

src/main/java/at/kocmana/aoc23/dec09/MirageMaintenance.java → src/main/java/at/kocmana/aoc23/dec09/MirageMaintenancePartOne.java

@@ -5,7 +5,7 @@
 import java.util.ArrayList;
 import java.util.List;
 
-public class MirageMaintenance {
+public class MirageMaintenancePartOne {
 
   public static void main(String[] args) {
     var puzzleInput = ResourceToString.from("dec09", "MirageMaintenance.txt");
@@ -15,7 +15,7 @@ public static void main(String[] args) {
 
   static int play(String input) {
     return parseInput(input).stream()
-        .map(MirageMaintenance::inferNextInLine)
+        .map(MirageMaintenancePartOne::inferNextInLine)
         .reduce(0, (i, j) -> i + j);
   }
 
@@ -39,7 +39,7 @@ static int inferNextInLine(List<Integer> list) {
 
   static List<List<Integer>> parseInput(String input) {
     return input.lines()
-        .map(MirageMaintenance::parseLine)
+        .map(MirageMaintenancePartOne::parseLine)
         .toList();
   }
 

src/main/java/at/kocmana/aoc23/dec09/MirageMaintenancePartTwo.java

@@ -0,0 +1,61 @@
+package at.kocmana.aoc23.dec09;
+
+import at.kocmana.aoc23.common.Parser;
+import at.kocmana.aoc23.common.ResourceToString;
+import java.util.ArrayList;
+import java.util.List;
+
+public class MirageMaintenancePartTwo {
+
+  public static void main(String[] args) {
+    var puzzleInput = ResourceToString.from("dec09", "MirageMaintenance.txt");
+    var result = play(puzzleInput);
+    System.out.printf("Result: %d%n", result);
+  }
+
+  static int play(String input) {
+    return parseInput(input).stream()
+        .map(MirageMaintenancePartTwo::inferPreviousInLine)
+        .reduce(0, (i, j) -> i + j);
+  }
+
+  static int inferPreviousInLine(List<Integer> list) {
+    var currentList = list;
+    var reductions = new ArrayList<List<Integer>>();
+    //reduce downward
+    while (canBeFurtherReduced(currentList)) {
+      var reducedList = reduce(currentList);
+      reductions.add(reducedList);
+      currentList = reducedList;
+    }
+    //infer upward
+    for (int i = 0; i < reductions.size() - 1; i++) {
+      var last = reductions.get(reductions.size() - i - 1);
+      var secondToLast = reductions.get(reductions.size() - i - 2);
+      secondToLast.addFirst(secondToLast.getFirst() - last.getFirst());
+    }
+    return list.getFirst() - reductions.getFirst().getFirst();
+  }
+
+  static List<List<Integer>> parseInput(String input) {
+    return input.lines()
+        .map(MirageMaintenancePartTwo::parseLine)
+        .toList();
+  }
+
+  static List<Integer> parseLine(String input) {
+    return Parser.toNumberList(input);
+  }
+
+  static List<Integer> reduce(List<Integer> list) {
+    List<Integer> reducedList = new ArrayList<>(list.size() - 1);
+    for (int i = 0; i < list.size() - 1; i++) {
+      reducedList.add(list.get(i + 1) - list.get(i));
+    }
+    return reducedList;
+  }
+
+  static boolean canBeFurtherReduced(List<Integer> list) {
+    return !list.stream().allMatch(i -> i.equals(0));
+  }
+}

src/test/java/at/kocmana/aoc23/dec09/MirageMaintenanceTest.java → src/test/java/at/kocmana/aoc23/dec09/MirageMaintenancePartOneTest.java

@@ -5,15 +5,15 @@
 import java.util.List;
 import org.junit.jupiter.api.Test;
 
-class MirageMaintenanceTest {
+class MirageMaintenancePartOneTest {
 
   @Test
   void canParseLine() {
     //given
     var input = "0 3 6 9 12 15";
 
     //when
-    var actualResult = MirageMaintenance.parseLine(input);
+    var actualResult = MirageMaintenancePartOne.parseLine(input);
 
     //then
     assertThat(actualResult).hasSize(6)
@@ -26,7 +26,7 @@ void canReduce() {
     var valuesToReduce = List.of(0, 3, 6, 9, 12, 15);
 
     //when
-    var actualResult = MirageMaintenance.reduce(valuesToReduce);
+    var actualResult = MirageMaintenancePartOne.reduce(valuesToReduce);
 
     //then
     assertThat(actualResult).hasSize(valuesToReduce.size() - 1)
@@ -39,7 +39,7 @@ void canAssessIfFurtherReductionIsPossiblePositive() {
     var valuesToAssess = List.of(3, 6, 9, 12);
 
     //when
-    var actualResult = MirageMaintenance.canBeFurtherReduced(valuesToAssess);
+    var actualResult = MirageMaintenancePartOne.canBeFurtherReduced(valuesToAssess);
 
     //then
     assertThat(actualResult).isTrue();
@@ -51,7 +51,7 @@ void canAssessIfFurtherReductionIsPossibleNegative() {
     var valuesToAssess = List.of(0, 0, 0, 0);
 
     //when
-    var actualResult = MirageMaintenance.canBeFurtherReduced(valuesToAssess);
+    var actualResult = MirageMaintenancePartOne.canBeFurtherReduced(valuesToAssess);
 
     //then
     assertThat(actualResult).isFalse();
@@ -63,7 +63,7 @@ void canInferNextInLine() {
     var lineToInferValueFrom = List.of(10, 13, 16, 21, 30, 45);
 
     //when
-    var actualResult = MirageMaintenance.inferNextInLine(lineToInferValueFrom);
+    var actualResult = MirageMaintenancePartOne.inferNextInLine(lineToInferValueFrom);
 
     //then
     assertThat(actualResult).isEqualTo(68);
@@ -79,7 +79,7 @@ void canIntegrate() {
         """;
 
     //when
-    var actualResult = MirageMaintenance.play(testInput);
+    var actualResult = MirageMaintenancePartOne.play(testInput);
 
     //then
     assertThat(actualResult).isEqualTo(114);

src/test/java/at/kocmana/aoc23/dec09/MirageMaintenancePartTwoTest.java

@@ -0,0 +1,21 @@
+package at.kocmana.aoc23.dec09;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+class MirageMaintenancePartTwoTest {
+
+  @Test
+  void canInferPreviousInLine() {
+    //given
+    var input = List.of(10, 13, 16, 21, 30, 45);
+
+    //when
+    var actualResult = MirageMaintenancePartTwo.inferPreviousInLine(input);
+
+    //then
+    assertThat(actualResult).isEqualTo(5);
+  }
+}