2024 day 13 in nix

2024/13/example.txt

@@ -0,0 +1,15 @@
+Button A: X+94, Y+34
+Button B: X+22, Y+67
+Prize: X=8400, Y=5400
+
+Button A: X+26, Y+66
+Button B: X+67, Y+21
+Prize: X=12748, Y=12176
+
+Button A: X+17, Y+86
+Button B: X+84, Y+37
+Prize: X=7870, Y=6450
+
+Button A: X+69, Y+23
+Button B: X+27, Y+71
+Prize: X=18641, Y=10279

2024/13/solution.nix

@@ -0,0 +1,82 @@
+{lib, ...}: input: let
+  inherit (builtins) elemAt match filter foldl';
+  inherit (lib.strings) toIntBase10;
+  inherit (lib) mod;
+
+  acost = 3;
+  bcost = 1;
+  part2offset = 10000000000000;
+
+  machines = input
+    |> lib.trim
+    |> lib.splitString "\n\n"
+    |> map (text: let
+        parts = text |> lib.splitString "\n";
+        buttona = elemAt parts 0;
+        buttonb = elemAt parts 1;
+        prize = elemAt parts 2;
+      in {
+        a = buttonToVec buttona;
+        b = buttonToVec buttonb;
+        prize = prizeToVec prize;
+      }
+    )
+  ;
+
+  buttonToVec = text: text
+    |> match ''Button .: X\+([0-9]+), Y\+([0-9]+)''
+    |> map toIntBase10
+    |> (nums: {
+      x = elemAt nums 0;
+      y = elemAt nums 1;
+    })
+  ;
+  prizeToVec = text: text
+    |> match ''Prize: X=([0-9]+), Y=([0-9]+)''
+    |> map toIntBase10
+    |> (nums: {
+      x = elemAt nums 0;
+      y = elemAt nums 1;
+    })
+  ;
+
+  getPresses = {a, b, prize}:
+  let
+    # see https://www.desmos.com/calculator/qfzdjjffso
+    # ma = 1.0 * a.y / a.x;
+    # mb = 1.0 * b.y / b.x;
+    # c = prize.y - prize.x * mb;
+    # x = c / (ma - mb);
+
+    # rearranged to avoid precision errors
+    x = (b.x * prize.y - prize.x * b.y) * a.x / (b.x * a.y - a.x * b.y);
+
+    apresses = x / a.x;
+    bpresses = (prize.x - x) / b.x;
+  in {
+    a = apresses;
+    b = bpresses;
+    doable = a.x * apresses + b.x * bpresses == prize.x &&
+    a.y * apresses + b.y * bpresses == prize.y;
+  };
+
+in {
+  inherit machines buttonToVec prizeToVec getPresses;
+
+  part1result = machines
+    |> map getPresses
+    |> filter ({doable, ...}: doable)
+    |> foldl' (cost: {a,b,...}: cost + a * acost + b * bcost) 0
+  ;
+
+  part2result = machines
+    |> map (machine: machine // {prize = {
+      x = machine.prize.x + part2offset;
+      y = machine.prize.y + part2offset;
+    };})
+    |> map getPresses
+    |> filter ({doable, ...}: doable)
+    |> foldl' (cost: {a,b,...}: cost + a * acost + b * bcost) 0
+  ;
+
+}

2024/13/solution.test.nix

@@ -0,0 +1,52 @@
+{it, describe}: let
+  pkgs = import <nixpkgs> {};
+  solution = import ./solution.nix pkgs;
+  exampleIn = builtins.readFile ./example.txt;
+in [
+  (describe "part 1" [
+
+    (it "parses buttons" {
+      actual = (solution "").buttonToVec "Button A: X+94, Y+34";
+      expected = {
+        x = 94;
+        y = 34;
+      };
+    })
+
+    (it "parses prize" {
+      actual = (solution "").prizeToVec "Prize: X=8400, Y=5400";
+      expected = {
+        x = 8400;
+        y = 5400;
+      };
+    })
+
+    (it "gets the machines" {
+      actual = (solution ''
+        Button A: X+94, Y+34
+        Button B: X+22, Y+67
+        Prize: X=8400, Y=5400
+      '').machines;
+      expected = [{
+        a = {x = 94; y = 34;};
+        b = {x = 22; y = 67;};
+        prize = {x = 8400; y = 5400;};
+      }];
+    })
+
+    (it "gets the answer for a machine" {
+      actual = (solution "").getPresses {
+        a = {x = 94; y = 34;};
+        b = {x = 22; y = 67;};
+        prize = {x = 8400; y = 5400;};
+      };
+      expected = {a = 80; b = 40;};
+    })
+
+    (it "gets the cost" {
+      actual = (solution exampleIn).part1result;
+      expected = 480;
+    })
+
+  ])
+]

2024/flake.lock

@@ -3,8 +3,8 @@
     "aoc-inputs": {
       "flake": false,
       "locked": {
-        "lastModified": 1733919427,
-        "narHash": "sha256-9QvGJVeFT1BfuoGcFUWSr6iq4VHiaEd9SKUrl6k2tzw=",
+        "lastModified": 1734085033,
+        "narHash": "sha256-UlLhIP9NbAywjOGRkrULOa0e+AtY2Eb7Vgem/o/pN7Y=",
         "path": "/tmp/aoc-inputs",
         "type": "path"
       },

2024/flake.nix

@@ -25,7 +25,7 @@
     inherit (pkgs) lib;
 
 
-  in (lib.range 1 11
+  in (lib.range 1 13
   |> map (i: let id = lib.fixedWidthNumber 2 i; in {
     name = "day-${id}";
     value = let