Day 09 part 1

README.adoc

@@ -31,7 +31,7 @@ Solutions for the https://adventofcode.com/2023/[advent of code] of 2023.
 |06|👍|👍
 |07| |
 |08|👍|👍*
-|09| |
+|09|👍|
 |10| |
 |11| |
 |12| |

src/main/java/at/kocmana/aoc23/dec09/MirageMaintenance.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 MirageMaintenance {
+
+  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(MirageMaintenance::inferNextInLine)
+        .reduce(0, (i, j) -> i + j);
+  }
+
+  static int inferNextInLine(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.add(last.getLast() + secondToLast.getLast());
+    }
+    return list.getLast() + reductions.getFirst().getLast();
+  }
+
+  static List<List<Integer>> parseInput(String input) {
+    return input.lines()
+        .map(MirageMaintenance::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));
+  }
+}