Solve 2025 day 3 part 2

Author: sim642

src/main/scala/eu/sim642/adventofcode2025/Day3.scala

@@ -1,27 +1,49 @@
 package eu.sim642.adventofcode2025
 
+import scala.collection.mutable
+
 object Day3 {
 
   type Bank = String
 
-  def maxJoltage(bank: Bank): Int = {
-    // TODO: doesn't keep order
-    //bank.combinations(2).map(_.toInt).max
-    // TODO: can be made linearly?
-    (for {
-      i <- bank.indices
-      j <- (i + 1) until bank.length
-    } yield s"${bank(i)}${bank(j)}".toInt).max
+  def maxJoltageDigits(bank: Bank, digits: Int): Long = {
+    val memo = mutable.Map.empty[(Int, Int), Long]
+
+    def helper(i: Int, digits: Int): Long = {
+      memo.getOrElseUpdate((i, digits), {
+        if (i >= 0 && digits >= 1) {
+          helper(i - 1, digits) max (helper(i - 1, digits - 1) * 10 + bank(i).asDigit)
+        } else
+          0
+      })
+    }
+
+    helper(bank.length - 1, digits)
+  }
+
+  trait Part {
+    val digits: Int
+
+    def maxJoltage(bank: Bank): Long = maxJoltageDigits(bank, digits)
+
+    def totalJoltage(banks: Seq[Bank]): Long = banks.map(maxJoltage).sum
   }
 
-  def totalJoltage(banks: Seq[Bank]): Int = banks.map(maxJoltage).sum
+  object Part1 extends Part {
+    override val digits: Int = 2
+  }
+
+  object Part2 extends Part {
+    override val digits: Int = 12
+  }
 
   def parseBanks(input: String): Seq[Bank] = input.linesIterator.toSeq
 
   lazy val input: String = scala.io.Source.fromInputStream(getClass.getResourceAsStream("day3.txt")).mkString.trim
 
   def main(args: Array[String]): Unit = {
-    println(totalJoltage(parseBanks(input)))
+    println(Part1.totalJoltage(parseBanks(input)))
+    println(Part2.totalJoltage(parseBanks(input)))
 
     // part 1: 16769 - not right (combinations don't keep order)
   }

src/test/scala/eu/sim642/adventofcode2025/Day3Test.scala

@@ -12,10 +12,21 @@ class Day3Test extends AnyFunSuite {
       |818181911112111""".stripMargin
 
   test("Part 1 examples") {
-    assert(totalJoltage(parseBanks(exampleInput)) == 357)
+    assert(Part1.maxJoltage("98") == 98)
+    assert(Part1.maxJoltage("987") == 98)
+    assert(Part1.totalJoltage(parseBanks(exampleInput)) == 357)
   }
 
   test("Part 1 input answer") {
-    assert(totalJoltage(parseBanks(input)) == 17301)
+    assert(Part1.totalJoltage(parseBanks(input)) == 17301)
+  }
+
+  test("Part 2 examples") {
+    assert(Part2.maxJoltage("987654321111111") == 987654321111L)
+    assert(Part2.totalJoltage(parseBanks(exampleInput)) == 3121910778619L)
+  }
+
+  test("Part 2 input answer") {
+    assert(Part2.totalJoltage(parseBanks(input)) == 172162399742349L)
   }
 }