2024-13 * **

src/aoc_2024/day_13.gleam

@@ -0,0 +1,83 @@
+import extra
+import gleam/bool
+import gleam/int
+import gleam/io
+import gleam/list
+import gleam/string
+import grid.{type XY}
+
+pub type Machine {
+  Machine(a: XY, b: XY, prize: XY)
+}
+
+pub fn parse(input: String) -> List(Machine) {
+  input
+  |> string.split("\n\n")
+  |> list.map(parse_machine)
+}
+
+fn parse_machine(input: String) -> Machine {
+  case string.split(input, "\n") {
+    ["Button A: X+" <> but_a, "Button B: X+" <> but_b, "Prize: X=" <> prize] -> {
+      case
+        string.split(but_a, ", Y+"),
+        string.split(but_b, ", Y+"),
+        string.split(prize, ", Y=")
+      {
+        [ax, ay], [bx, by], [px, py] ->
+          Machine(
+            a: #(extra.yolo_int(ax), extra.yolo_int(ay)),
+            b: #(extra.yolo_int(bx), extra.yolo_int(by)),
+            prize: #(extra.yolo_int(px), extra.yolo_int(py)),
+          )
+        _, _, _ -> panic as "Bad input 2?"
+      }
+    }
+    _ -> panic as "Bad input?"
+  }
+}
+
+pub fn pt_1(input: List(Machine)) {
+  input
+  |> list.map(solve_machine)
+  |> int.sum
+}
+
+fn solve_machine(machine: Machine) -> Int {
+  // button A = X+i, Y+j
+  // button B = X+l, Y+m
+  // prize = (K,N)
+  // ---------------------
+  // ia + jb = k
+  // la + mb = n
+  // ---------------------
+  // b = (kl-in)/(jl-im)
+  // a = (n-mb)/l
+  // ---------------------
+  // cost = 3 * a + b
+  // ---------------------
+  // we need to check for integer divisibility and short-circuit to 0
+  let #(i, l) = machine.a
+  let #(j, m) = machine.b
+  let #(k, n) = machine.prize
+  use <- bool.guard(when: { k * l - i * n } % { j * l - i * m } != 0, return: 0)
+  let b = { k * l - i * n } / { j * l - i * m }
+  use <- bool.guard(when: { n - m * b } % l != 0, return: 0)
+  let a = { n - m * b } / l
+  3 * a + b
+}
+
+pub fn pt_2(input: List(Machine)) {
+  input
+  |> list.map(fn(machine) {
+    Machine(
+      ..machine,
+      prize: grid.xy_add(machine.prize, #(
+        10_000_000_000_000,
+        10_000_000_000_000,
+      )),
+    )
+  })
+  |> list.map(solve_machine)
+  |> int.sum
+}