day 11

src/main/kotlin/me/gergo/Aoc11.kt

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+package me.gergo
+
+import java.io.File
+import java.util.*
+
+private const val Rounds = 10000
+
+/**
+ * Note: this only has Part Two, as I did Part One with a naiive solution that probably takes all the memory in the world, multiplying horribly large
+ * BigDecimals :) The trick in this one is to remember but delay the computation for as long as possible. As we're only really interested in whether
+ * the result at the end of a long chain of operations is divisible by N, we can modulo N per every operation - but we must do that for each monkey
+ * individually, which means we must remember the computation chain (Items.operations in this implementation)!
+ */
+fun main() {
+    val monkeys = File("src/main/resources/input11.txt").readText().split("\n\n")
+        .map { it.split("\n") }
+        .map(::parseMonkey)
+
+
+    for (i in 1..Rounds) {
+        for (monkey in monkeys) {
+            monkey.inspectAndThrowAll(monkeys)
+        }
+    }
+
+    for (monkey in monkeys) {
+        println("Monkey ${monkey.name} inspected items ${monkey.inspections} times")
+    }
+
+    val result = monkeys.map(Monkey::inspections).sortedDescending().take(2).reduce(Long::times)
+    println("Level of monkey business: $result")
+}
+
+private class Monkey(val name: Int, val items: Queue<Item>, val operation: Operation, val divisor: Int, val throwToTrue: Int, val throwToFalse: Int) {
+    var inspections = 0L
+
+    fun inspectAndThrowAll(monkeys: List<Monkey>) {
+        while (true) {
+            val item = items.poll() ?: return
+            item.operations.add(operation)
+            inspections++
+            val throwTo = if (item.divisibleBy(divisor)) throwToTrue else throwToFalse
+            monkeys[throwTo].catch(item)
+        }
+    }
+
+    fun catch(item: Item) {
+        items.add(item)
+    }
+}
+
+private data class Item(val value: Int, val operations: MutableList<Operation>) {
+    fun divisibleBy(divisor: Int): Boolean {
+        var current = value
+        for (operation in operations) {
+            current = when (operation) {
+                is Mul -> (current * operation.multiplier) % divisor
+                is Add -> (current + operation.add) % divisor
+                Square -> (current * current) % divisor
+            }
+        }
+        return current % divisor == 0
+    }
+}
+
+private sealed interface Operation
+private data class Mul(val multiplier: Int) : Operation
+private data class Add(val add: Int) : Operation
+private object Square : Operation
+
+private fun parseMonkey(lines: List<String>): Monkey {
+    val name = lines[0].replace(Regex("\\D+"), "").toInt()
+    val items = lines[1].substring("  Starting items: ".length).split(", ").map { Item(it.toInt(), mutableListOf()) }
+    val operation = parseOperation(lines[2].substring("  Operation: new = ".length))
+    val divisor = lines[3].substring("  Test: divisible by ".length).toInt()
+    val throwToTrue = lines[4].substring("    If true: throw to monkey ".length).toInt()
+    val throwToFalse = lines[5].substring("    If false: throw to monkey ".length).toInt()
+    return Monkey(
+        name,
+        items.toCollection(LinkedList()),
+        operation,
+        divisor,
+        throwToTrue,
+        throwToFalse
+    )
+}
+
+private fun parseOperation(value: String): Operation {
+    val mr = Regex("(\\w+) ([*+]) (\\w+)").matchEntire(value)!!
+    assert(mr.groupValues[1] == "old")
+    val op = mr.groupValues[2]
+    val opB = mr.groupValues[3]
+    return if (op == "*") {
+        if (opB == "old") {
+            Square
+        } else {
+            Mul(opB.toInt())
+        }
+    } else {
+        Add(opB.toInt())
+    }
+}

src/main/resources/input11.txt

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+Monkey 0:
+  Starting items: 89, 84, 88, 78, 70
+  Operation: new = old * 5
+  Test: divisible by 7
+    If true: throw to monkey 6
+    If false: throw to monkey 7
+
+Monkey 1:
+  Starting items: 76, 62, 61, 54, 69, 60, 85
+  Operation: new = old + 1
+  Test: divisible by 17
+    If true: throw to monkey 0
+    If false: throw to monkey 6
+
+Monkey 2:
+  Starting items: 83, 89, 53
+  Operation: new = old + 8
+  Test: divisible by 11
+    If true: throw to monkey 5
+    If false: throw to monkey 3
+
+Monkey 3:
+  Starting items: 95, 94, 85, 57
+  Operation: new = old + 4
+  Test: divisible by 13
+    If true: throw to monkey 0
+    If false: throw to monkey 1
+
+Monkey 4:
+  Starting items: 82, 98
+  Operation: new = old + 7
+  Test: divisible by 19
+    If true: throw to monkey 5
+    If false: throw to monkey 2
+
+Monkey 5:
+  Starting items: 69
+  Operation: new = old + 2
+  Test: divisible by 2
+    If true: throw to monkey 1
+    If false: throw to monkey 3
+
+Monkey 6:
+  Starting items: 82, 70, 58, 87, 59, 99, 92, 65
+  Operation: new = old * 11
+  Test: divisible by 5
+    If true: throw to monkey 7
+    If false: throw to monkey 4
+
+Monkey 7:
+  Starting items: 91, 53, 96, 98, 68, 82
+  Operation: new = old * old
+  Test: divisible by 3
+    If true: throw to monkey 4
+    If false: throw to monkey 2