Add Day7

src/main/kotlin/Day07.kt

@@ -0,0 +1,44 @@
+import utils.AoCDay
+import utils.splitAndMapToLong
+
+class Day07 : AoCDay() {
+
+    val equations = input.map {
+        val parts = it.split(":")
+        parts[0].toLong() to parts[1].splitAndMapToLong()
+    }
+
+    override fun part1(): Any {
+        return equations.filter { solve(it.second, it.first) }.sumOf { it.first }
+    }
+
+    override fun part2(): Any {
+        return equations.filter { solve(it.second, it.first, true) }.sumOf { it.first }
+    }
+
+    fun solve(numbers: List<Long>, target: Long, partB: Boolean = false): Boolean {
+        if (numbers.size == 1) {
+            return numbers.first() == target
+        }
+        if (numbers.first() > target) {
+            return false
+        }
+        val unused = numbers.drop(2)
+        val addition = mutableListOf(numbers[0] + numbers[1])
+        val multiplication = mutableListOf(numbers[0] * numbers[1])
+        addition.addAll(unused)
+        multiplication.addAll(unused)
+        val result = solve(addition, target, partB) || solve(multiplication, target, partB)
+
+        if (!partB) {
+            return result
+        }
+
+        val concat = mutableListOf("${numbers[0]}${numbers[1]}".toLong())
+        return result || solve(concat + unused, target, true)
+    }
+}
+
+fun main() {
+    Day07().execute()
+}