Solution Day 22, part two (Monkey Market)

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year24/day22/MonkeyMarket.kt

@@ -1,8 +1,9 @@
 package de.ronny_h.aoc.year24.day22
 
 import de.ronny_h.aoc.AdventOfCode
+import de.ronny_h.aoc.extensions.onesDigit
 
-fun main() = MonkeyMarket().run(17965282217, 0)
+fun main() = MonkeyMarket().run(17965282217, 2152)
 
 class MonkeyMarket : AdventOfCode<Long>(2024, 22) {
 
@@ -26,7 +27,51 @@ class MonkeyMarket : AdventOfCode<Long>(2024, 22) {
             secret
         }
 
+    data class Buyer(val prices: List<Int>, val priceChanges: List<Int>)
+
     override fun part2(input: List<String>): Long {
-        TODO("Not yet implemented")
+        val buyers = input
+            .map(String::toLong)
+            .map {
+                val prices = listPrices(it, 2000)
+                Buyer(prices, prices.changes())
+            }
+        println("${buyers.size} buyers")
+
+        val bananas = sumUpBananasForSequences(buyers)
+
+        val (sequence, maxBananas) = bananas.maxBy { it.value }
+        println("sequence with max value: $sequence")
+        val numberOfMax = bananas.filter { it.value == maxBananas }
+        println("sequences with the same value: $numberOfMax from ${bananas.size} sequences total")
+        return maxBananas
+    }
+
+    fun sumUpBananasForSequences(buyers: List<Buyer>): Map<List<Int>, Long> {
+        val bananas = mutableMapOf<List<Int>, Long>()
+        buyers.forEach {
+            var index = 4
+            val seenForThatBuyer = mutableSetOf<List<Int>>()
+            it.priceChanges.windowed(4) { ephemeralSequence ->
+                val sequence = ephemeralSequence.toList()
+                // the hiding spot is sold on the first occurrence of a sequence -> don't add up values for later ones
+                if (seenForThatBuyer.add(sequence)) {
+                    bananas[sequence] = bananas.getOrElse(sequence) { 0 } + it.prices[index]
+                }
+                index++
+            }
+        }
+        return bananas
+    }
+
+    fun listPrices(seed: Long, n: Int): List<Int> {
+        var secret = seed
+        return listOf(seed.onesDigit()) + List(n) {
+            secret = nextSecretNumber(secret)
+            secret.onesDigit()
+        }
     }
 }
+
+fun List<Int>.changes(): List<Int> = windowed(2)
+    .map { (a, b) -> b - a }

src/test/kotlin/de/ronny_h/aoc/year24/day22/MonkeyMarketTest.kt

@@ -1,17 +1,24 @@
 package de.ronny_h.aoc.year24.day22
 
 import de.ronny_h.aoc.extensions.asList
+import de.ronny_h.aoc.year24.day22.MonkeyMarket.Buyer
 import io.kotest.core.spec.style.StringSpec
 import io.kotest.matchers.shouldBe
 
 class MonkeyMarketTest : StringSpec({
-
-    val input = """
+    val input1 = """
         1
         10
         100
         2024
     """.asList()
+    val input2 = """
+        1
+        2
+        3
+        2024
+    """.asList()
+
     val market = MonkeyMarket()
 
     "next secret number of 123" {
@@ -39,7 +46,30 @@ class MonkeyMarketTest : StringSpec({
         )
     }
 
-    "part 1" {
-        market.part1(input) shouldBe 37327623
+    "part 1: The sum of the 2000th secret number generated by each buyer" {
+        market.part1(input1) shouldBe 37327623
+    }
+
+    "list the first 10 prices with a see of 123" {
+        market.listPrices(123, 9) shouldBe listOf(3, 0, 6, 5, 4, 4, 6, 4, 4, 2)
+    }
+
+    "the changes of consecutive prices" {
+        listOf(3, 0, 6, 5, 4, 4, 6, 4, 4, 2).changes() shouldBe listOf(-3, 6, -1, -1, 0, 2, -2, 0, -2)
+    }
+
+    "sumUpBananasForSequences() for the example buyer yields the right highest price" {
+        val buyer = Buyer(
+            prices =       listOf(3, 0, 6,  5,  4, 4, 6,  4, 4,  2),
+            priceChanges = listOf(  -3, 6, -1, -1, 0, 2, -2, 0, -2))
+        val entryWithHighestPrice = market
+            .sumUpBananasForSequences(listOf(buyer))
+            .maxBy { it.value }
+        entryWithHighestPrice.key shouldBe listOf(-1, -1, 0, 2)
+        entryWithHighestPrice.value shouldBe 6
+    }
+
+    "part 2: The most bananas one can get" {
+        market.part2(input2) shouldBe 23
     }
 })