Day 10 part 1

AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day10.fs

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+namespace AdventOfCode2023
+
+#if DEBUG
+#else
+#nowarn "9"
+#endif
+
+open System
+
+[<RequireQualifiedAccess>]
+module Day10 =
+
+    type Pipe =
+        | Empty = '.'
+        | NorthSouth = '|'
+        | EastWest = '-'
+        | NorthEast = 'L'
+        | NorthWest = '7'
+        | SouthEast = 'F'
+        | SouthWest = '7'
+        | Animal = 'S'
+
+    /// Returns first the line number, then the position within that line.
+    /// lineLength includes the newline, as does pos.
+    let inline toRowAndCol (lineLength : int) (lineCount : int) (pos : int) : struct (int * int) =
+        let lineNum = pos / lineLength
+        let withinLine = pos % lineLength
+        struct (lineNum, withinLine)
+
+    let inline ofRowAndCol (lineLength : int) (lineCount : int) (lineNum : int) (col : int) : int =
+        lineNum * lineLength + col
+
+    let inline nextPoint (s : ReadOnlySpan<char>) (lineLength : int) (lineCount : int) (currPos : int) (prevPos : int) =
+        let struct (currLineNum, currCol) = toRowAndCol lineLength lineCount currPos
+        let struct (prevLineNum, prevCol) = toRowAndCol lineLength lineCount prevPos
+        match s.[currPos] with
+        | '|' ->
+            if prevLineNum < currLineNum then
+                ofRowAndCol lineLength lineCount (currLineNum + 1) currCol
+            else
+                ofRowAndCol lineLength lineCount (currLineNum - 1) currCol
+        | '-' ->
+            if prevCol < currCol then
+                ofRowAndCol lineLength lineCount currLineNum (currCol + 1)
+            else
+                ofRowAndCol lineLength lineCount currLineNum (currCol - 1)
+        | 'L' ->
+            if prevLineNum = currLineNum then
+                ofRowAndCol lineLength lineCount (currLineNum - 1) currCol
+            else
+                ofRowAndCol lineLength lineCount currLineNum (currCol + 1)
+        | '7' ->
+            if prevLineNum = currLineNum then
+                ofRowAndCol lineLength lineCount (currLineNum + 1) currCol
+            else
+                ofRowAndCol lineLength lineCount currLineNum (currCol - 1)
+        | 'F' ->
+            if prevLineNum = currLineNum then
+                ofRowAndCol lineLength lineCount (currLineNum + 1) currCol
+            else
+                ofRowAndCol lineLength lineCount currLineNum (currCol + 1)
+        | 'J' ->
+            if prevLineNum = currLineNum then
+                ofRowAndCol lineLength lineCount (currLineNum - 1) currCol
+            else
+                ofRowAndCol lineLength lineCount currLineNum (currCol - 1)
+        | c -> failwithf "unrecognised: %c" c
+
+    let part1 (s : string) =
+        let s = s.AsSpan()
+        let lineCount = s.Count '\n'
+        let lineLength = (s.IndexOf '\n' + 1)
+        let startPos = s.IndexOf 'S'
+
+        let struct (startLine, startCol) = toRowAndCol lineLength lineCount startPos
+        let mutable distance = 1
+        let mutable prevPointA = startPos
+        let mutable prevPointB = startPos
+
+        let mutable pointA =
+            let pos = ofRowAndCol lineLength lineCount startLine (startCol - 1)
+            match s.[pos] with
+            | '-'
+            | 'L'
+            | 'F' -> pos
+            | _ ->
+
+            let pos = ofRowAndCol lineLength lineCount startLine (startCol + 1)
+            match s.[pos] with
+            | '-'
+            | 'J'
+            | '7' -> pos
+            | _ ->
+
+            ofRowAndCol lineLength lineCount (startLine + 1) startCol
+
+        let mutable pointB =
+            let pos = ofRowAndCol lineLength lineCount (startLine - 1) startCol
+            match s.[pos] with
+            | '|'
+            | '7'
+            | 'F' -> pos
+            | _ ->
+
+            let pos = ofRowAndCol lineLength lineCount (startLine + 1) startCol
+            match s.[pos] with
+            | '|'
+            | 'L'
+            | 'J' -> pos
+            | _ ->
+
+            let pos = ofRowAndCol lineLength lineCount startLine (startCol + 1)
+            match s.[pos] with
+            | '-'
+            | 'J'
+            | '7' -> pos
+            | _ -> ofRowAndCol lineLength lineCount startLine (startCol - 1)
+
+        while pointA <> pointB do
+            let currentA = pointA
+            pointA <- nextPoint s lineLength lineCount pointA prevPointA
+            prevPointA <- currentA
+
+            let currentB = pointB
+            pointB <- nextPoint s lineLength lineCount pointB prevPointB
+            prevPointB <- currentB
+
+            distance <- distance + 1
+
+        distance
+
+    let part2 (s : string) =
+        0