Day 13

src/day13.scala

@@ -0,0 +1,46 @@
+package aoc
+package day13
+
+val dayNumber = "13"
+
+@main def part1: Unit =
+  println(part1(loadInput(dayNumber)))
+
+@main def part2: Unit =
+  println(part2(loadInput(dayNumber)))
+
+def part1(input: String): String =
+  val machines = parseInput(input)
+  machines.flatMap(_.tokenCost).sum.toString
+
+def part2(input: String): String =
+  val correction = 10000000000000L
+  val machines = parseInput(input)
+    .map(m => m.copy(x = m.x + correction, y = m.y + correction))
+  machines.flatMap(_.tokenCost).sum.toString
+
+case class Machine(ax: Long, ay: Long, bx: Long, by: Long, x: Long, y: Long):
+  def tokenCost: Option[Long] =
+    // Calculations come from solving the given equations
+    // a * ax + b * bx == x
+    // a * ay + b * by == y
+    val b = (x * ay - y * ax) / (bx * ay - by * ax).toDouble
+    val a = (x - b * bx) / ax.toDouble
+    Option.when(a.isWhole && b.isWhole)(a.toLong * 3 + b.toLong)
+
+def parseInput(input: String): List[Machine] =
+  object l:
+    def unapply(s: String): Option[Long] = s.toLongOption
+  input
+    .split("\n\n")
+    .map(_.split("\n").toList)
+    .flatMap {
+      case List(
+            s"Button A: X+${l(ax)}, Y+${l(ay)}",
+            s"Button B: X+${l(bx)}, Y+${l(by)}",
+            s"Prize: X=${l(x)}, Y=${l(y)}"
+          ) =>
+        Some(Machine(ax, ay, bx, by, x, y))
+      case _ => None
+    }
+    .toList