2023-09 part 2

tetsuo13 · Jan 15, 2024 · a6bfa39 · a6bfa39
1 parent ea39f5b
commit a6bfa39
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Showing 3 changed files with 16 additions and 8 deletions.
diff --git a/README.md b/README.md
@@ -21,4 +21,4 @@ AdventOfCode --year 2023 --day 1
 | [Scratchcards](https://adventofcode.com/2023/day/4) | [Solution](./src/AdventOfCode/Calendar/2023/Day04/Solution.cs) | 2023-04 | Completed |
 | [Wait For It](https://adventofcode.com/2023/day/6) | [Solution](./src/AdventOfCode/Calendar/2023/Day06/Solution.cs) | 2023-06 | Completed |
 | [Haunted Wasteland](https://adventofcode.com/2023/day/8) | [Solution](./src/AdventOfCode/Calendar/2023/Day08/Solution.cs) | 2023-08 | Completed |
-| [Mirage Maintenance](https://adventofcode.com/2023/day/9) | [Solution](./src/AdventOfCode/Calendar/2023/Day09/Solution.cs) | 2023-09 | Part 1 |
+| [Mirage Maintenance](https://adventofcode.com/2023/day/9) | [Solution](./src/AdventOfCode/Calendar/2023/Day09/Solution.cs) | 2023-09 | Completed |
diff --git a/src/AdventOfCode/Calendar/2023/Day09/Solution.cs b/src/AdventOfCode/Calendar/2023/Day09/Solution.cs
@@ -9,27 +9,34 @@ public override async Task<object> Run(RunMode runMode)
 
         return runMode switch
         {
-            RunMode.PartOne => SumExtrapolatedValues(histories),
-            RunMode.PartTwo => 0,
+            RunMode.PartOne => SumExtrapolatedValues(histories, false),
+            RunMode.PartTwo => SumExtrapolatedValues(histories, true),
             _ => throw new ArgumentOutOfRangeException(nameof(runMode))
         };
     }
 
-    private static int SumExtrapolatedValues(IEnumerable<IEnumerable<int>> histories)
+    private static int SumExtrapolatedValues(IEnumerable<IEnumerable<int>> histories, bool backwards)
     {
         return histories
-            .Select(sequence => sequence.Last() + GetNextInSequence(sequence))
+            .Select(sequence =>
+            {
+                return (backwards ? sequence.First() : sequence.Last()) +
+                    GetNextInSequence(sequence, backwards);
+            })
             .Sum();
     }
 
-    private static int GetNextInSequence(IEnumerable<int> sequence)
+    private static int GetNextInSequence(IEnumerable<int> sequence, bool backwards)
     {
         if (sequence.All(x => x == 0))
         {
             return 0;
         }
 
-        var nextSequence = sequence.Skip(1).Zip(sequence, (curr, prev) => curr - prev);
-        return nextSequence.Last() + GetNextInSequence(nextSequence);
+        var nextSequence = sequence.Skip(1)
+            .Zip(sequence, (curr, prev) => backwards ? prev - curr : curr - prev);
+
+        return (backwards ? nextSequence.First() : nextSequence.Last()) +
+            GetNextInSequence(nextSequence, backwards);
     }
 }
diff --git a/tests/AdventOfCode.Tests/Calendar/2023/Day09/SolutionTests.cs b/tests/AdventOfCode.Tests/Calendar/2023/Day09/SolutionTests.cs
@@ -7,6 +7,7 @@ public class SolutionTests
 {
     [Theory]
     [InlineData(RunMode.PartOne, 114)]
+    [InlineData(RunMode.PartTwo, 2)]
     public async Task Example(RunMode runMode, int expected)
     {
         string[] input =