From 44d3c20b25b93b73374388a41b9282cb8e557cc2 Mon Sep 17 00:00:00 2001
From: Nat Lasseter <user@4574.co.uk>
Date: Sun, 10 Dec 2023 17:50:56 +0000
Subject: Day 09 part 1

---
 day09/day09.java | 77 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)
 create mode 100644 day09/day09.java

(limited to 'day09/day09.java')

diff --git a/day09/day09.java b/day09/day09.java
new file mode 100644
index 0000000..4609508
--- /dev/null
+++ b/day09/day09.java
@@ -0,0 +1,77 @@
+import java.io.IOException;
+import java.io.RandomAccessFile;
+import java.util.Arrays;
+import java.util.ArrayList;
+
+public class day09 {
+  public static void main(String args[]) throws IOException {
+    RandomAccessFile input = new RandomAccessFile("input", "r");
+    System.out.println("Day 09 Part 1: " + part1(input));
+    input.seek(0);
+    System.out.println("Day 09 Part 2: " + part2(input));
+    input.close();
+  }
+
+  public static String part1(RandomAccessFile input) throws IOException {
+    String line;
+    int sum = 0;
+    while ((line = input.readLine()) != null) {
+      String[] linea = line.split(" ");
+      int[] nums = new int[linea.length];
+      for (int i = 0; i < linea.length; i++) {
+        nums[i] = Integer.parseInt(linea[i]);
+      }
+
+      sum += predictNext(nums);
+    }
+    return Integer.toString(sum);
+  }
+
+  public static String part2(RandomAccessFile input) throws IOException {
+    return "WIP";
+  }
+
+  private static int predictNext(int[] line) {
+    ArrayList<ArrayList<Integer>> differences = new ArrayList<>();
+    ArrayList<Integer> lastDiffs = new ArrayList<>();
+
+    for (int i: line) {
+      lastDiffs.add(i);
+    }
+
+    differences.add(lastDiffs);
+
+    while (!allZero(lastDiffs)) {
+      lastDiffs = getDifferences(lastDiffs);
+      differences.add(lastDiffs);
+    }
+
+    for (int i = differences.size() - 2; i >= 0; i--) {
+      ArrayList<Integer> lastrow = differences.get(i+1);
+      ArrayList<Integer> row = differences.get(i);
+      row.add(row.get(row.size() - 1) + lastrow.get(lastrow.size() - 1));
+    }
+
+    ArrayList<Integer> first = differences.get(0);
+    return first.get(first.size() - 1);
+  }
+
+  private static boolean allZero(ArrayList<Integer> list) {
+    for(int i: list) {
+      if (i != 0) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  private static ArrayList<Integer> getDifferences(ArrayList<Integer> last) {
+    ArrayList<Integer> next = new ArrayList<>();
+
+    for (int i = 1; i < last.size(); i++) {
+      next.add(last.get(i) - last.get(i-1));
+    }
+
+    return next;
+  }
+}
-- 
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