Day 9

Alright then. Not quite the difficulty I was expecting from a weekend problem this year.

src/09.cs

@@ -0,0 +1,91 @@
+namespace aoc2023;
+
+internal class Day09 : Day
+{
+    private List<List<long>> sequences = new();
+    internal override void Parse()
+    {
+        var lines = Util.Parsing.ReadAllLines($"{GetDay()}");
+        foreach (var line in lines)
+        {
+            sequences.Add(new List<long>(line.Split().Select(long.Parse)));
+        }
+    }
+
+    private List<long> GetDiffs(List<long> list)
+    {
+        List<long> ret = new(list.Count);
+        for (int i = 1; i < list.Count; i++)
+        {
+            ret.Add(list[i] - list[i-1]);
+        }
+
+        return ret;
+    }
+
+    private List<List<long>> GetDiffLists(List<long> list)
+    {
+        List<List<long>> diffLists = new(list.Count);
+        List<long> diffs = GetDiffs(list);
+        diffLists.Add(diffs);
+        while (diffs.Any(x => x != 0))
+        {
+            diffs = GetDiffs(diffs);
+            diffLists.Add(diffs);
+        }
+
+        return diffLists;
+    }
+
+    private long GetNextNum(List<long> list)
+    {
+        List<List<long>> diffLists = GetDiffLists(list);
+
+        long nextVal = 0;
+        for (int i = diffLists.Count - 1; i > 0; i--)
+        {
+            nextVal = diffLists[i].Last() + diffLists[i - 1].Last();
+            diffLists[i - 1].Add(nextVal);
+        }
+
+        return nextVal + list.Last();
+    }
+
+    private long GetPrevNum(List<long> list)
+    {
+        List<List<long>> diffLists = GetDiffLists(list);
+
+        long nextVal = 0;
+        for (int i = diffLists.Count - 1; i > 0; i--)
+        {
+            nextVal = diffLists[i - 1].First() - diffLists[i].First();
+            diffLists[i - 1].Insert(0, nextVal);
+        }
+
+        return list.First() - nextVal;
+    }
+
+    internal override string Part1()
+    {
+        long total = 0;
+        foreach (var seq in sequences)
+        {
+            long nextnum = GetNextNum(seq);
+            total += nextnum;
+        }
+
+        return $"Sum of all next values: <+white>{total}";
+    }
+
+    internal override string Part2()
+    {
+        long total = 0;
+        foreach (var seq in sequences)
+        {
+            long prevnum = GetPrevNum(seq);
+            total += prevnum;
+        }
+
+        return $"Sum of all previous values: <+white>{total}";
+    }
+}