Day 9 (#10)

AdventOfCode2022/AdventOfCode2022.fsproj

@@ -17,6 +17,7 @@
       <Compile Include="Day6.fs" />
       <Compile Include="Day7.fs" />
       <Compile Include="Day8.fs" />
+      <Compile Include="Day9.fs" />
     </ItemGroup>
 
 </Project>

AdventOfCode2022/Day9.fs

@@ -0,0 +1,97 @@
+namespace AdventOfCode2022
+
+open System.Collections.Generic
+open System
+
+type Direction =
+    | Up
+    | Down
+    | Left
+    | Right
+
+[<RequireQualifiedAccess>]
+module Day9 =
+
+    let parse (lines : StringSplitEnumerator) : (Direction * int) IReadOnlyList =
+        use mutable enum = lines
+        let output = ResizeArray ()
+
+        for line in enum do
+            let line = line.TrimEnd ()
+
+            if not (line.IsWhiteSpace ()) then
+                let dir =
+                    match Char.ToUpperInvariant line.[0] with
+                    | 'U' -> Direction.Up
+                    | 'D' -> Direction.Down
+                    | 'L' -> Direction.Left
+                    | 'R' -> Direction.Right
+                    | _ -> failwith "Unexpected direction"
+
+                let distance = Int32.Parse (line.Slice 2)
+
+                output.Add (dir, distance)
+
+        output :> _
+
+    type Position = int * int
+
+    let bringTailTogether (head : Position) (tail : Position) : Position =
+        if abs (fst head - fst tail) <= 1 && abs (snd head - snd tail) <= 1 then
+            tail
+        elif fst head = fst tail then
+            fst head, snd head + (if snd head < snd tail then 1 else -1)
+        elif snd head = snd tail then
+            fst head + (if fst head < fst tail then 1 else -1), snd head
+        else
+            let fstCoord =
+                if abs (fst head - fst tail + 1) <= 1 then
+                    fst tail - 1
+                else
+                    fst tail + 1
+
+            let sndCoord =
+                if abs (snd head - snd tail + 1) <= 1 then
+                    snd tail - 1
+                else
+                    snd tail + 1
+
+            (fstCoord, sndCoord)
+
+    let newHead (pos : Position) (direction : Direction) : int * int =
+        match direction with
+        | Direction.Up -> fst pos, snd pos + 1
+        | Direction.Down -> fst pos, snd pos - 1
+        | Direction.Left -> fst pos - 1, snd pos
+        | Direction.Right -> fst pos + 1, snd pos
+
+    let go (count : int) (directions : (Direction * int) seq) : int =
+        let knots = Array.create count (0, 0)
+        let tailVisits = HashSet ()
+        tailVisits.Add (0, 0) |> ignore
+
+        for direction, distance in directions do
+            for _ in 1..distance do
+                let newHead = newHead knots.[0] direction
+                knots.[0] <- newHead
+
+                for knot in 1 .. knots.Length - 2 do
+                    knots.[knot] <- bringTailTogether knots.[knot - 1] knots.[knot]
+
+                let newTail = bringTailTogether knots.[knots.Length - 2] knots.[knots.Length - 1]
+
+                if newTail <> knots.[knots.Length - 1] then
+                    knots.[knots.Length - 1] <- newTail
+                    tailVisits.Add newTail |> ignore
+
+        tailVisits.Count
+
+    let part1 (lines : StringSplitEnumerator) : int =
+        let directions = parse lines
+
+        go 2 directions
+
+    let part2 (lines : StringSplitEnumerator) : int =
+        let directions = parse lines
+
+        go 10 directions