Day 24: Never Tell Me The Odds (part 1)

README.md

@@ -28,4 +28,4 @@ Development occurs in language-specific directories:
 |[Day21.hs](hs/src/Day21.hs)|[Day21.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day21.kt)|[day21.py](py/aoc2023/day21.py)|[day21.rs](rs/src/day21.rs)|
 |[Day22.hs](hs/src/Day22.hs)|[Day22.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day22.kt)|[day22.py](py/aoc2023/day22.py)|[day22.rs](rs/src/day22.rs)|
 ||[Day23.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day23.kt)|||
-|[Day24.hs](hs/src/Day24.hs) ½||[day24.py](py/aoc2023/day24.py) ½||
+|[Day24.hs](hs/src/Day24.hs) ½|[Day24.kt](kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day24.kt) ½|[day24.py](py/aoc2023/day24.py) ½||

kt/aoc2023-exe/src/commonMain/kotlin/com/github/ephemient/aoc2023/exe/Day24Bench.kt

@@ -0,0 +1,23 @@
+package com.github.ephemient.aoc2023.exe
+
+import com.github.ephemient.aoc2023.Day24
+import kotlinx.benchmark.Benchmark
+import kotlinx.benchmark.Scope
+import kotlinx.benchmark.Setup
+import kotlinx.benchmark.State
+
+@State(Scope.Benchmark)
+class Day24Bench {
+    private lateinit var input: String
+
+    @Setup
+    fun setup() {
+        input = getDayInput(24)
+    }
+
+    @Benchmark
+    fun part1() = Day24(input).part1()
+
+    @Benchmark
+    fun part2() = Day24(input).part2()
+}

kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Day24.kt

@@ -0,0 +1,43 @@
+package com.github.ephemient.aoc2023
+
+class Day24(input: String) {
+    private val hailstones = input.lines().mapNotNull { line ->
+        val (pos, vel) = line.split('@', limit = 2).takeIf { it.size == 2 } ?: return@mapNotNull null
+        val (x, y, z) = pos.split(',', limit = 3).takeIf { it.size == 3 } ?: return@mapNotNull null
+        val (vx, vy, vz) = vel.split(',', limit = 3).takeIf { it.size == 3 } ?: return@mapNotNull null
+        Hailstone(
+            x = x.trim().toLongOrNull() ?: return@mapNotNull null,
+            y = y.trim().toLongOrNull() ?: return@mapNotNull null,
+            z = z.trim().toLongOrNull() ?: return@mapNotNull null,
+            vx = vx.trim().toLongOrNull() ?: return@mapNotNull null,
+            vy = vy.trim().toLongOrNull() ?: return@mapNotNull null,
+            vz = vz.trim().toLongOrNull() ?: return@mapNotNull null,
+        )
+    }
+
+    fun part1(lo: Double = 200000000000000.0, hi: Double = 400000000000000.0): Int {
+        val lines = mutableListOf<Line>()
+        return hailstones.sumOf {
+            val m0 = it.vy / it.vx.toDouble()
+            val b0 = it.y - m0 * it.x
+            val range0 = if (it.vx < 0) {
+                Double.NEGATIVE_INFINITY..it.x.toDouble()
+            } else {
+                it.x.toDouble()..Double.POSITIVE_INFINITY
+            }
+            lines.count { (m1, b1, range1) ->
+                if (m0 == m1) return@count false
+                val x = (b0 - b1) / (m1 - m0)
+                x in lo..hi && m0 * x + b0 in lo..hi && x in range0 && x in range1
+            }.also { lines.add(Line(m0, b0, range0)) }
+        }
+    }
+
+    fun part2(): Long {
+        TODO()
+    }
+
+    private data class Hailstone(val x: Long, val y: Long, val z: Long, val vx: Long, val vy: Long, val vz: Long)
+
+    private data class Line(val m: Double, val b: Double, val range: ClosedFloatingPointRange<Double>)
+}

kt/aoc2023-lib/src/commonMain/kotlin/com/github/ephemient/aoc2023/Days.kt

@@ -24,6 +24,7 @@ val days: List<Day> = listOf(
     Day(21, ::Day21, Day21::part1, Day21::part2),
     Day(22, ::Day22, Day22::part1, Day22::part2),
     Day(23, ::Day23, Day23::part1, Day23::part2, skipByDefault = !isJvm),
+    Day(24, ::Day24, Day24::part1),
 )
 
 data class Day(

kt/aoc2023-lib/src/commonTest/kotlin/com/github/ephemient/aoc2023/Day24Test.kt

@@ -0,0 +1,29 @@
+package com.github.ephemient.aoc2023
+
+import kotlin.test.Ignore
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class Day24Test {
+    @Test
+    fun part1() {
+        assertEquals(2, Day24(example).part1(lo = 7.0, hi = 27.0))
+    }
+
+    @Test
+    @Ignore
+    fun part2() {
+        assertEquals(47, Day24(example).part2())
+    }
+
+    companion object {
+        private val example =
+            """
+            |19, 13, 30 @ -2,  1, -2
+            |18, 19, 22 @ -1, -1, -2
+            |20, 25, 34 @ -2, -2, -4
+            |12, 31, 28 @ -1, -2, -1
+            |20, 19, 15 @  1, -5, -3
+            |""".trimMargin()
+    }
+}