2023 day 9

Package.resolved

@@ -15,7 +15,7 @@
       "location" : "[email protected]:davedelong/ExtendedSwift.git",
       "state" : {
         "branch" : "main",
-        "revision" : "069c78bc9e6f8d54afcb88cc577908c8902912dd"
+        "revision" : "ee59b1b4aaaf8ddfc4d3fd03a28f80b7e7c67e0f"
       }
     },
     {

Sources/AOC2023/Day 9/2023-Day9.swift

@@ -7,23 +7,44 @@
 //
 
 struct Day9: Day {
-    typealias Part1 = String
-    typealias Part2 = String
+    typealias Part1 = Int
+    typealias Part2 = Int
 
     static var rawInput: String? { nil }
 
     func run() async throws -> (Part1, Part2) {
-        let p1 = try await part1()
-        let p2 = try await part2()
+
+        let sequences = input().lines.map(\.integers)
+
+        var p1 = 0
+        var p2 = 0
+
+        for sequence in sequences {
+            var derivitives = [sequence]
+            var current = sequence
+            while derivitives.last!.contains(where: { $0 != 0 }) {
+                let diff = current.adjacentPairs().map { $1 - $0 }
+                derivitives.append(diff)
+                current = diff
+            }
+
+            // we've derived the zero sequence
+            var p1Extrapolation = 0
+            var p2Extrapolation = 0
+
+            for index in derivitives.indices.reversed() {
+                let nextP1 = derivitives[index].last! + p1Extrapolation
+                let nextP2 = derivitives[index].first! - p2Extrapolation
+
+                p1Extrapolation = nextP1
+                p2Extrapolation = nextP2
+            }
+
+            p1 += p1Extrapolation
+            p2 += p2Extrapolation
+        }
+
         return (p1, p2)
     }
 
-    func part1() async throws -> Part1 {
-        return #function
-    }
-
-    func part2() async throws -> Part2 {
-        return #function
-    }
-
 }