Day 3.

2024/src/main/kotlin/Utils.kt

@@ -1,4 +1,8 @@
-fun readInput(name: String) = object {}.javaClass.getResourceAsStream(name)
-    ?.bufferedReader()
+fun readInput(name: String) = bufferedReader(name)
     ?.lineSequence()
     ?: error("Cannot read input")
+
+fun readInputString(name: String): String = bufferedReader(name)?.readText() ?: error("Cannot read input")
+
+private fun bufferedReader(name: String) = object {}.javaClass.getResourceAsStream(name)
+    ?.bufferedReader()

2024/src/main/kotlin/day3.kt

@@ -1,3 +1,26 @@
 fun main() {
+    val input = readInputString("day3.txt")
+    val regexPart1 = "mul\\((\\d+),(\\d+)\\)".toRegex()
 
+    val part1 = regexPart1.findAll(input).fold(0) { acc, result ->
+        val (a, b) = result.destructured
+        acc + a.toInt() * b.toInt()
+    }
+
+    println("Part 1: $part1")
+
+    val regexPart2 = "do\\(\\)|don't\\(\\)|mul\\((\\d+),(\\d+)\\)".toRegex()
+    var enabled = true
+    var part2 = 0
+    for (command in regexPart2.findAll(input)) {
+        when (command.value) {
+            "do()" -> enabled = true
+            "don't()" -> enabled = false
+            else -> if (enabled) {
+                val (a, b) = command.destructured
+                part2 += a.toInt() * b.toInt()
+            }
+        }
+    }
+    println("Part 2: $part2")
 }