feat(aoc-2023): day 08 part 1 and part 2

Author: markkovari

2023/src/main/scala/com/markkovari/adventofcode/day08.scala

@@ -0,0 +1,73 @@
+package com.markkovari.adventofcode.day08
+
+type State = String
+
+type Transition = (State, Instr) => State
+
+enum Instr:
+  case GoLeft, GoRight
+
+object Instr:
+  def parse(inp: String): LazyList[Instr] =
+    inp
+      .map {
+        case 'L' => Instr.GoLeft
+        case 'R' => Instr.GoRight
+      }
+      .to(LazyList) #::: Instr.parse(inp)
+
+def parseNetwork(inp: List[String]): Map[String, Vector[String]] =
+  inp.map { case s"$a = ($b, $c)" =>
+    (a -> Vector(b, c))
+  }.toMap
+
+def countStepsUntil(
+    state: State,
+    instrs: LazyList[Instr],
+    trans: Transition,
+    count: Int,
+    pred: State => Boolean
+): Int =
+  if pred(state) then count
+  else
+    countStepsUntil(
+      trans(state, instrs.head),
+      instrs.tail,
+      trans,
+      count + 1,
+      pred
+    )
+
+def transitions(network: Map[String, Vector[String]]): Transition =
+  (n, d) =>
+    d match
+      case Instr.GoLeft  => network(n)(0)
+      case Instr.GoRight => network(n)(1)
+
+def part1(input: String): Int =
+  val inpL = input.split("\n\n")
+  val instructions = Instr.parse(inpL.head)
+  val network = parseNetwork(inpL.tail.head.split("\n").toList)
+  val trans = transitions(network)
+
+  countStepsUntil("AAA", instructions, trans, 0, _ == "ZZZ")
+
+def part2(input: String): Long =
+  val inpL = input.split("\n\n")
+  val instructions = Instr.parse(inpL.head)
+  val network = parseNetwork(inpL.tail.head.split("\n").toList)
+  val trans = transitions(network)
+
+  val starts: Set[State] = network.keySet.filter(_.endsWith("A"))
+
+  def leastCommonMultiple(a: Long, b: Long): Long =
+    a * b / greatestCommonDivisor(a, b)
+
+  def greatestCommonDivisor(a: Long, b: Long): Long =
+    if b == 0 then a else greatestCommonDivisor(b, a % b)
+
+  starts
+    .map(state =>
+      countStepsUntil(state, instructions, trans, 0, _.endsWith("Z")).toLong
+    )
+    .reduce(leastCommonMultiple)

2023/src/test/resources/8/example_1

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+RL
+
+AAA = (BBB, CCC)
+BBB = (DDD, EEE)
+CCC = (ZZZ, GGG)
+DDD = (DDD, DDD)
+EEE = (EEE, EEE)
+GGG = (GGG, GGG)
+ZZZ = (ZZZ, ZZZ)

2023/src/test/resources/8/example_2

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+LLR
+
+AAA = (BBB, BBB)
+BBB = (AAA, ZZZ)
+ZZZ = (ZZZ, ZZZ)

2023/src/test/resources/8/example_3

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+LR
+
+11A = (11B, XXX)
+11B = (XXX, 11Z)
+11Z = (11B, XXX)
+22A = (22B, XXX)
+22B = (22C, 22C)
+22C = (22Z, 22Z)
+22Z = (22B, 22B)
+XXX = (XXX, XXX)