Day 7!

day07.kts

@@ -0,0 +1,45 @@
+#!/usr/bin/env kotlin
+import java.util.Scanner
+
+val scanner = Scanner(System.`in`)
+
+var total = 0L
+var totalTwo = 0L
+val operations = listOf("*", "+")
+while (scanner.hasNext()) {
+    val line = scanner.nextLine()
+    val (targetString, rest) = line.split(":", limit = 2).map { it.trim() }
+    val target = targetString.toLong()
+    val values = rest.split(Regex("""\s+""")).map { it.toLong() }
+    if (hitsTarget(target, values, operations)) {
+        total += target
+        totalTwo += target
+    } else if (hitsTarget(target, values, listOf("||") + operations)) {
+        totalTwo += target
+    }
+}
+println("Total result: $total")
+println("Total result (part 2): $totalTwo")
+
+fun hitsTarget(target: Long, values: List<Long>, operations: List<String>): Boolean {
+    if (values.size == 1) {
+        return target == values[0]
+    }
+    val (operandOne, operandTwo) = values.take(2)
+    if (operandOne > target) {
+        return false
+    }
+    val rest = values.drop(2)
+    for (op in operations) {
+        val result = when (op) {
+            "+" -> operandOne + operandTwo
+            "*" -> operandOne * operandTwo
+            "||" -> "$operandOne$operandTwo".toLong()
+            else -> throw IllegalArgumentException("Unknown operation: $op")
+        }
+        if (hitsTarget(target, listOf(result) + rest, operations)) {
+            return true
+        }
+    }
+    return false
+}