add solution for day 24 part 2 based on a cool pattern matching solution in the Gleam discord

Author: devries

README.md

@@ -42,5 +42,5 @@ information.
 - [Day 21](https://adventofcode.com/2024/day/21): [⭐ ⭐ solution](src/day21/solution.gleam)
 - [Day 22](https://adventofcode.com/2024/day/22): [⭐ ⭐ solution](src/day22/solution.gleam)
 - [Day 23](https://adventofcode.com/2024/day/23): [⭐ ⭐ solution](src/day23/solution.gleam)
-- [Day 24](https://adventofcode.com/2024/day/24): [⭐    solution](src/day24/solution.gleam) - Note: I just decided to solve part B by hand with a few editor macros.
+- [Day 24](https://adventofcode.com/2024/day/24): [⭐ ⭐ solution](src/day24/solution.gleam) - Note: Part 2 was implemented based on [this code](https://topaz.github.io/paste/#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) from super makioka sisters. I solved it by using editor macros and rearranging my input.
 - [Day 25](https://adventofcode.com/2024/day/25): [⭐ ⭐ solution](src/day25/solution.gleam)

src/day24/solution.gleam

@@ -1,3 +1,4 @@
+import gleam/bool
 import gleam/dict.{type Dict}
 import gleam/int
 import gleam/io
@@ -46,8 +47,52 @@ pub fn solve_p1(lines: List(String)) -> Result(String, String) {
 // Part 2
 // Easiest way to do this was to do some sorting and use editor macros to pull
 // together the relevant half adder and full adders into groups.
-pub fn solve_p2(_lines: List(String)) -> Result(String, String) {
-  Ok("solved by hand in inputs/day24-resolved.txt")
+// I then went looking at results and tried to implement a solution based on
+// this explanation: https://old.reddit.com/r/adventofcode/comments/1hla5ql/2024_day_24_part_2_a_guide_on_the_idea_behind_the/
+// and from a really great pattern matching example in the Gleam discord by
+// super makioka sisters. This is essentially a copy of their example which
+// I implemented in order to better understand it.
+pub fn solve_p2(lines: List(String)) -> Result(String, String) {
+  use #(_, gates) <- result.map(parse(lines))
+
+  // Take the gates and create a dict by output
+  gates
+  |> list.map(fn(g) { #(g.output, g) })
+  |> dict.from_list
+  |> dict.filter(fn(k, v) {
+    // filter the dictionary for gates that do not match a half adder followed by
+    // a bunch of full adders.
+    case k, v {
+      // Outputs should be from Xor gates, except the last one
+      "z45", Or(_, _, _) -> False
+      "z" <> _, Xor(_, _, _) -> False
+      "z" <> _, _ -> True
+
+      // Xor gates for inputs should connect to AND and XOR gates, but not OR gates
+      _, Xor("x" <> _, "y" <> _, _) | _, Xor("y" <> _, "x" <> _, _) -> {
+        find_gates_with_input(gates, k)
+        |> has_or_gates
+      }
+
+      // Any other Xor gate is not valid
+      _, Xor(_, _, _) -> True
+
+      // And gates should have Or gates after them, except for the
+      // output of the half-adder.
+      // This assumes the output of the And from the first half adder
+      // is not swapped.
+      _, And("x00", _, _) | _, And(_, "x00", _) -> False
+      _, And(_, _, _) -> {
+        find_gates_with_input(gates, k)
+        |> has_or_gates
+        |> bool.negate
+      }
+      _, _ -> False
+    }
+  })
+  |> dict.keys
+  |> list.sort(string.compare)
+  |> string.join(",")
 }
 
 type Gate {
@@ -151,3 +196,22 @@ fn get_number(wires: Dict(String, Int), prefix: String) -> Int {
   |> list.sort(fn(tupa, tupb) { string.compare(tupb.0, tupa.0) })
   |> list.fold(0, fn(value, tup) { { value * 2 } + tup.1 })
 }
+
+fn find_gates_with_input(gates: List(Gate), input: String) -> List(Gate) {
+  gates
+  |> list.filter(fn(g) {
+    case g.input1, g.input2 {
+      w, _ if w == input -> True
+      _, w if w == input -> True
+      _, _ -> False
+    }
+  })
+}
+
+fn has_or_gates(gates: List(Gate)) -> Bool {
+  case gates {
+    [Or(_, _, _), ..] -> True
+    [_, ..rest] -> has_or_gates(rest)
+    [] -> False
+  }
+}