From bec09f2a341a270fae1ba38871eb4bdc796510e5 Mon Sep 17 00:00:00 2001
From: Ummon <greg.burri@gmail.com>
Date: Mon, 18 Dec 2017 08:16:48 +0100
Subject: [PATCH] Get ride off 'mutable' variables.

---
 AdventOfCode2017/AdventOfCode2017.fsproj |  2 +-
 AdventOfCode2017/Day16.fs                | 36 +++++++++++++-----------
 2 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/AdventOfCode2017/AdventOfCode2017.fsproj b/AdventOfCode2017/AdventOfCode2017.fsproj
index 2fbe84d..57bd951 100644
--- a/AdventOfCode2017/AdventOfCode2017.fsproj
+++ b/AdventOfCode2017/AdventOfCode2017.fsproj
@@ -37,7 +37,7 @@
     <PlatformTarget>AnyCPU</PlatformTarget>
     <DocumentationFile>bin\$(Configuration)\$(AssemblyName).XML</DocumentationFile>
     <Prefer32Bit>true</Prefer32Bit>
-    <StartArguments>17</StartArguments>
+    <StartArguments>16</StartArguments>
   </PropertyGroup>
   <PropertyGroup>
     <MinimumVisualStudioVersion Condition="'$(MinimumVisualStudioVersion)' == ''">11</MinimumVisualStudioVersion>
diff --git a/AdventOfCode2017/Day16.fs b/AdventOfCode2017/Day16.fs
index 8a6d7a6..1a124d7 100644
--- a/AdventOfCode2017/Day16.fs
+++ b/AdventOfCode2017/Day16.fs
@@ -24,16 +24,13 @@ let parseInput (input : string) : DanceMove list =
 
 let dance (size : int) (nb : int) (moves : DanceMove list) : string =
     let initialState = Array.init size (fun i -> char i + 'a')
-    let danceFloor = Array.copy initialState
 
-    let find c = danceFloor |> Array.findIndex ((=) c)
-    let swap p1 p2 =
-        let tmp = danceFloor.[p1]
-        danceFloor.[p1] <- danceFloor.[p2]
-        danceFloor.[p2] <- tmp
-
-    let mutable n = nb
-    while n <> 0 do
+    let applyMoves (danceFloor : char[]) =
+        let find c = danceFloor |> Array.findIndex ((=) c)
+        let swap p1 p2 =
+            let tmp = danceFloor.[p1]
+            danceFloor.[p1] <- danceFloor.[p2]
+            danceFloor.[p2] <- tmp
         for move in moves do
             match move with
             | Spin s ->
@@ -47,11 +44,16 @@ let dance (size : int) (nb : int) (moves : DanceMove list) : string =
             | Partner (a, b) ->
                 swap (find a) (find b)
 
-        n <- n - 1
-        if danceFloor |=| initialState then
-            n <- nb % (nb - n)
-
-    String danceFloor
-
-
-
+    let cycle =
+        ((0, initialState), Seq.initInfinite id)
+        ||> Seq.scan (
+            fun (_, previous) i ->
+                let current = previous.[*]
+                applyMoves current
+                i + 1, current
+        )
+        |> Seq.takeWhile (fun (i, state) -> i = 0 || not (state |=| initialState))
+        |> Seq.map snd
+        |> Array.ofSeq
+
+    cycle.[nb % cycle.Length] |> String
\ No newline at end of file
-- 
2.45.2