Day 07 completed.

src/main/kotlin/day07/day07.kt

@@ -0,0 +1,74 @@
+// Advent of Code 2024, Day 07.
+// By Sebastian Raaphorst, 2024.
+
+package day07
+
+import common.day
+import common.readInput
+import java.math.BigInteger
+
+data class Equation(val total: BigInteger, val numbers: List<BigInteger>) {
+    /**
+     * The numbers list is evaluated left-to-right, and entries can be joined with + or *,
+     * or if allowConcat is true, they can also be concatenated.
+     */
+    private fun canBeMade(allowConcat: Boolean): Boolean {
+        fun aux(currTotal: BigInteger = numbers.first(),
+            remainingNumbers: List<BigInteger> = numbers.drop(1)): Boolean {
+            // currTotal can only ever increase, so if we already passed total, there is no
+            // reason to continue.
+            if (currTotal > total) return false
+
+            // If we have used up all the numbers and have achieved total, success.
+            if (remainingNumbers.isEmpty()) return currTotal == total
+
+            // Calculate all of the possible extensions.
+            val nextNumber = remainingNumbers.first()
+            val nextRemainingNumbers = remainingNumbers.drop(1)
+            return aux(currTotal + nextNumber, nextRemainingNumbers) ||
+                    aux(currTotal * nextNumber, nextRemainingNumbers) ||
+                    (allowConcat && aux("$currTotal$nextNumber".toBigInteger(), nextRemainingNumbers))
+        }
+        return aux()
+    }
+
+    fun canBeMadeWithPlusTimes() =
+        canBeMade(false)
+
+    fun canBeMadeWithPlusTimesConcat() =
+        canBeMade(true)
+
+    companion object {
+        fun parseLine(input: String): Equation {
+            val (total, remaining) = input.split(":")
+            val numbers = remaining
+                .trim()
+                .split(Regex("""\s+"""))
+                .map(String::toBigInteger)
+            return Equation(total.toBigInteger(), numbers)
+        }
+    }
+}
+
+fun parse(input: String): List<Equation> =
+    input.trim().lines().map(Equation::parseLine)
+
+fun answer1(equations: List<Equation>): BigInteger =
+    equations.filter(Equation::canBeMadeWithPlusTimes).sumOf(Equation::total)
+
+fun answer2(equations: List<Equation>): BigInteger =
+    equations.filter(Equation::canBeMadeWithPlusTimesConcat).sumOf(Equation::total)
+
+
+fun main() {
+    val input = readInput({}::class.day()).trim()
+    val equations = parse(input)
+
+    println("--- Day 7: Bridge Repair ---")
+
+    // Part 1: 2437272016585
+    println("Part 1: ${answer1(equations)}")
+
+    // Part 2: 162987117690649
+    println("Part 2: ${answer2(equations)}")
+}