Solve 2024 day 19 part 2

Author: sim642

src/main/scala/eu/sim642/adventofcode2024/Day19.scala

@@ -1,5 +1,6 @@
 package eu.sim642.adventofcode2024
 
+import scala.collection.mutable
 import scala.util.matching.Regex
 
 object Day19 {
@@ -18,6 +19,28 @@ object Day19 {
     input.designs.count(regex.matches)
   }
 
+  def countWays(patterns: Seq[Towel], design: Towel): Long = {
+    val memo = mutable.Map.empty[Int, Long]
+
+    def helper(i: Int): Long = {
+      memo.getOrElseUpdate(i, {
+        if (i == design.size)
+          1
+        else {
+          (for {
+            pattern <- patterns
+            if design.startsWith(pattern, i)
+          } yield helper(i + pattern.size)).sum
+        }
+      })
+    }
+
+    helper(0)
+  }
+
+  def sumDesignWays(input: Input): Long =
+    input.designs.map(countWays(input.patterns, _)).sum
+
   def parsePatterns(s: String): Seq[Towel] = s.split(", ").toSeq
 
   def parseDesigns(s: String): Seq[Towel] = s.linesIterator.toSeq
@@ -30,5 +53,8 @@ object Day19 {
 
   def main(args: Array[String]): Unit = {
     println(countPossibleDesigns(parseInput(input)))
+    println(sumDesignWays(parseInput(input)))
+
+    // part 2: 1006410103498494 - too high (had i >= design.size - 1 in countWays)
   }
 }

src/test/scala/eu/sim642/adventofcode2024/Day19Test.scala

@@ -24,4 +24,12 @@ class Day19Test extends AnyFunSuite {
   test("Part 1 input answer") {
     assert(countPossibleDesigns(parseInput(input)) == 226)
   }
+
+  test("Part 2 examples") {
+    assert(sumDesignWays(parseInput(exampleInput)) == 16)
+  }
+
+  test("Part 2 input answer") {
+    assert(sumDesignWays(parseInput(input)) == 601201576113503L)
+  }
 }