From 2f871fa83ac561bd7ca97bdc58ac5395b6daa6e1 Mon Sep 17 00:00:00 2001
From: Smaug123 <patrick+github@patrickstevens.co.uk>
Date: Fri, 15 Dec 2023 10:35:35 +0000
Subject: [PATCH] Day 15

---
 .../AdventOfCode2023.FSharp.Lib/Day15.fs      | 79 ++++++++++++++++++-
 AdventOfCode2023.FSharp/Test/TestDay15.fs     |  8 +-
 .../Test/samples/day15.txt                    |  1 +
 3 files changed, 82 insertions(+), 6 deletions(-)

diff --git a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day15.fs b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day15.fs
index d7a9d7b..0aba77d 100644
--- a/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day15.fs
+++ b/AdventOfCode2023.FSharp/AdventOfCode2023.FSharp.Lib/Day15.fs
@@ -1,11 +1,86 @@
 namespace AdventOfCode2023
 
 open System
+open System.Globalization
 
 [<RequireQualifiedAccess>]
 module Day15 =
+
+    let hash (s : ReadOnlySpan<char>) : int =
+        let mutable v = 0
+
+        for c in s do
+            v <- v + int (byte c)
+            v <- (17 * v) % 256
+
+        v
+
     let part1 (s : string) =
-        0
+        let s = s.AsSpan().TrimEnd ()
+        use chunks = StringSplitEnumerator.make' ',' s
+        let mutable answer = 0
+
+        for chunk in chunks do
+            answer <- answer + hash chunk
+
+        answer
+
+    let removeFirst<'a> (toRemove : 'a -> bool) (arr : ResizeArray<'a>) : unit =
+        let mutable i = 0
+
+        while i < arr.Count do
+            if toRemove arr.[i] then
+                for j = i to arr.Count - 2 do
+                    arr.[j] <- arr.[j + 1]
+
+                arr.RemoveAt (arr.Count - 1)
+                i <- arr.Count
+
+            i <- i + 1
+
+    let replace (withKey : 'a -> 'key) (key : 'key) (value : 'a) (arr : ResizeArray<'a>) : unit =
+        let mutable i = 0
+
+        while i < arr.Count do
+            if withKey arr.[i] = key then
+                arr.[i] <- value
+                i <- arr.Count
+
+            i <- i + 1
+
+        if i < arr.Count + 1 then
+            // no replacement was made
+            arr.Add value
+
+    let inline focusingPower (boxNumber : int) (arr : ResizeArray<_ * _>) : int =
+        let mutable answer = 0
+
+        for i = 0 to arr.Count - 1 do
+            answer <- answer + (boxNumber + 1) * (i + 1) * snd arr.[i]
+
+        answer
 
     let part2 (s : string) =
-        0
+        let s = s.AsSpan().TrimEnd ()
+        use chunks = StringSplitEnumerator.make' ',' s
+        let lenses = Array.init 256 (fun _ -> ResizeArray<string * _> ())
+
+        for chunk in chunks do
+            if chunk.[chunk.Length - 1] = '-' then
+                let label = chunk.Slice(0, chunk.Length - 1).ToString ()
+                removeFirst (fun (label2, _focalLength) -> label2 = label) lenses.[hash (label.AsSpan ())]
+            else
+                let equalsPos = chunk.IndexOf '='
+
+                let focalLength =
+                    Int32.Parse (chunk.Slice (equalsPos + 1), NumberStyles.None, CultureInfo.InvariantCulture)
+
+                let label = chunk.Slice(0, equalsPos).ToString ()
+                replace fst label (label, focalLength) lenses.[hash (label.AsSpan ())]
+
+        let mutable answer = 0
+
+        for i = 0 to 255 do
+            answer <- answer + focusingPower i lenses.[i]
+
+        answer
diff --git a/AdventOfCode2023.FSharp/Test/TestDay15.fs b/AdventOfCode2023.FSharp/Test/TestDay15.fs
index 0cfb73e..48e9758 100644
--- a/AdventOfCode2023.FSharp/Test/TestDay15.fs
+++ b/AdventOfCode2023.FSharp/Test/TestDay15.fs
@@ -15,11 +15,11 @@ module TestDay15 =
 
     [<Test>]
     let part1Sample () =
-        sample |> Day15.part1 |> shouldEqual -1
+        sample |> Day15.part1 |> shouldEqual 1320
 
     [<Test>]
     let part2Sample () =
-        sample |> Day15.part2 |> shouldEqual -1
+        sample |> Day15.part2 |> shouldEqual 145
 
     [<Test>]
     let part1Actual () =
@@ -32,7 +32,7 @@ module TestDay15 =
                 Assert.Inconclusive ()
                 failwith "unreachable"
 
-        Day15.part1 s |> shouldEqual -1
+        Day15.part1 s |> shouldEqual 521434
 
     [<Test>]
     let part2Actual () =
@@ -45,4 +45,4 @@ module TestDay15 =
                 Assert.Inconclusive ()
                 failwith "unreachable"
 
-        Day15.part2 s |> shouldEqual -1
+        Day15.part2 s |> shouldEqual 248279
diff --git a/AdventOfCode2023.FSharp/Test/samples/day15.txt b/AdventOfCode2023.FSharp/Test/samples/day15.txt
index e69de29..4f58f74 100644
--- a/AdventOfCode2023.FSharp/Test/samples/day15.txt
+++ b/AdventOfCode2023.FSharp/Test/samples/day15.txt
@@ -0,0 +1 @@
+rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7