Day 7

2024/src/main/kotlin/day7.kt

@@ -0,0 +1,34 @@
+data class Equation(val test: Long, val numbers: List<Long>) {
+    fun canSolve(): Boolean = trySolve(numbers[0], numbers.drop(1))
+
+    private fun trySolve(already: Long, rest: List<Long>): Boolean = when {
+        rest.isEmpty() -> already == test
+        already > test -> false
+        else -> trySolve(already + rest.first(), rest.drop(1)) ||
+                trySolve(already * rest.first(), rest.drop(1))
+    }
+
+
+    fun canSolve2(): Boolean = trySolve2(numbers[0], numbers.drop(1))
+
+    private fun trySolve2(already: Long, rest: List<Long>): Boolean = when {
+        rest.isEmpty() -> already == test
+        already > test -> false
+        else -> trySolve2(already + rest.first(), rest.drop(1)) ||
+                trySolve2(already * rest.first(), rest.drop(1)) ||
+                trySolve2("$already${rest.first()}".toLong(), rest.drop(1))
+    }
+}
+
+fun main() {
+    val equations = readInput("day7.txt").map { line ->
+        val (test, equation) = line.split(": ", limit = 2)
+        Equation(test.toLong(), equation.split(" ").map { it.toLong() })
+    }.toList()
+
+    val part1 = equations.filter { it.canSolve() }.sumOf { it.test }
+    println("Part 1: $part1")
+
+    val part2 = equations.filter { it.canSolve2() }.sumOf { it.test }
+    println("Part 2: $part2")
+}