Solution 2015-20 (Infinite Elves and Infinite Houses)

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year2015/day20/InfiniteElvesAndInfiniteHouses.kt

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+package de.ronny_h.aoc.year2015.day20
+
+import de.ronny_h.aoc.AdventOfCode
+
+fun main() = InfiniteElvesAndInfiniteHouses().run(776160, 786240)
+
+class InfiniteElvesAndInfiniteHouses : AdventOfCode<Int>(2015, 20) {
+
+    override fun part1(input: List<String>): Int {
+        val s = input.single().toInt() / 10
+        var houseNumber = s / 5
+        var sum = 0
+        println("Searching for the hous with at least $s presents, starting at $houseNumber")
+        while (sum < s) {
+            houseNumber++
+            sum = numberOfPresentsForHouse(houseNumber)
+            if (houseNumber % 10000 == 0) {
+                println("at house number $houseNumber: $sum presents")
+            }
+        }
+        return houseNumber
+    }
+
+    override fun part2(input: List<String>): Int {
+        val s = input.single().toInt()
+        var houseNumber = s / 50
+        var sum = 0
+        println("Searching for the hous with at least $s presents, starting at $houseNumber")
+        while (sum < s) {
+            houseNumber++
+            sum = numberOfPresentsForHouseWithMax50Visits(houseNumber) * 11
+            if (houseNumber % 10000 == 0) {
+                println("at house number $houseNumber: $sum presents")
+            }
+        }
+        return houseNumber
+    }
+}
+
+// this is an implementation of the Divisor function:
+// https://en.wikipedia.org/wiki/Divisor_function
+fun numberOfPresentsForHouse(number: Int) = (1..number)
+    .sumOf { if (number % it == 0) it else 0 }
+
+fun numberOfPresentsForHouseWithMax50Visits(number: Int) = (1..number)
+    .sumOf { if (number % it == 0 && number / it <= 50) it else 0 }

src/test/kotlin/de/ronny_h/aoc/year2015/day20/InfiniteElvesAndInfiniteHousesTest.kt

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+package de.ronny_h.aoc.year2015.day20
+
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.data.forAll
+import io.kotest.data.row
+import io.kotest.matchers.shouldBe
+
+class InfiniteElvesAndInfiniteHousesTest : StringSpec({
+    "the divisor function for some small numbers" {
+        forAll(
+            row(1, 1),
+            row(2, 3),
+            row(3, 4),
+            row(4, 7),
+            row(5, 6),
+            row(6, 12),
+            row(7, 8),
+            row(8, 15),
+            row(9, 13),
+        ) { number, presents ->
+            numberOfPresentsForHouse(number) shouldBe presents
+        }
+    }
+
+    "part 1: the lowest house number to get at least 120 presents" {
+        InfiniteElvesAndInfiniteHouses().part1(listOf("120")) shouldBe 6
+    }
+
+    "part 2: the lowest house number to get at least 120 presents with 11 presents each" {
+        InfiniteElvesAndInfiniteHouses().part2(listOf("77")) shouldBe 4
+    }
+})