Day 9

AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day9.fs

@@ -1,37 +1,55 @@
 namespace AdventOfCode2023
 
 open System
-open System.Collections.Generic
 
 [<RequireQualifiedAccess>]
 module Day9 =
 
-    let parse (s : string) =
-        use mutable lines = StringSplitEnumerator.make '\n' s
+    let extrapolate (isStart : bool) (arr : ResizeArray<int64>) =
+        let mutable answer = 0L
+        let pos = if isStart then -1L else int64 arr.Count
 
-        lines.MoveNext () |> ignore
+        for i = 0 to arr.Count - 1 do
+            let mutable product = Rational.ofInt arr.[i]
 
-        let stepsLine = lines.Current.TrimEnd ()
-        let steps = Array.zeroCreate stepsLine.Length
+            for j = 0 to arr.Count - 1 do
+                if j <> i then
+                    product <- product * Rational.make (pos - int64 j) (int64 i - int64 j)
 
-        for i = 0 to stepsLine.Length - 1 do
-            steps.[i] <- (stepsLine.[i] = 'R')
+            answer <- answer + Rational.assertIntegral product
 
-        let dict = Dictionary ()
-
-        while lines.MoveNext () do
-            if not lines.Current.IsEmpty then
-                use mutable line = StringSplitEnumerator.make' ' ' lines.Current
-                line.MoveNext () |> ignore
-        {
-            Steps = steps
-            Nodes = dict
-        }
+        answer
 
     let part1 (s : string) =
-        let data = parse s
+        use s = StringSplitEnumerator.make '\n' s
+        let mutable answer = 0L
+        let arr = ResizeArray ()
+
+        for line in s do
+            arr.Clear ()
+            use line = StringSplitEnumerator.make' ' ' line
+
+            for number in line do
+                let number = Int64.Parse number
+                arr.Add number
+
+            answer <- answer + extrapolate false arr
+
         answer
 
     let part2 (s : string) =
-        let data = parse s
-        0
+        use s = StringSplitEnumerator.make '\n' s
+        let mutable answer = 0L
+        let arr = ResizeArray ()
+
+        for line in s do
+            arr.Clear ()
+            use line = StringSplitEnumerator.make' ' ' line
+
+            for number in line do
+                let number = Int64.Parse number
+                arr.Add number
+
+            answer <- answer + extrapolate true arr
+
+        answer