Make Day 7 faster

aoc/src/dev/ctsk/aoc/days/Day07.scala

@@ -8,33 +8,47 @@ object Day07 extends Solver(7):
     while m * 10 <= b do m *= 10
     a * (m * 10) + b
 
+  def deconcat(a: Long, b: Long): Option[Long] =
+    val b_len = Math.log10(b).floor.toLong + 1
+    if (a % Math.pow(10, b_len).toLong == b)
+    then Some(a / Math.pow(10, b_len).toLong)
+    else None
+
   class Searcher(target: Long, nums: Vector[Long], elephants: Boolean):
     def search(acc: Long = nums(0), pos: Int = 1): Boolean =
       if (pos >= nums.length) return acc == target
-      if (acc > target) return false
       search(acc + nums(pos), pos + 1)
-        || search(acc * nums(pos), pos + 1)
-        || (elephants && search(concat(acc, nums(pos)), pos + 1))
+      || search(acc * nums(pos), pos + 1)
+      || (elephants && search(concat(acc, nums(pos)), pos + 1))
+
+  class Tester(nums: Vector[Long], elephants: Boolean):
+    def test(target: Long, pos: Int = nums.length - 1): Boolean =
+      if pos == 0 then return target == nums(0)
+      if target <= 0 then return false
+      test(target - nums(pos), pos - 1)
+      || (target % nums(pos) == 0 && test(target / nums(pos), pos - 1))
+      || (elephants && deconcat(target, nums(pos)).exists(test(_, pos - 1)))
+
+  def check(target: Long, nums: Vector[Long], thirdElephant: Boolean): Boolean =
+    if nums.contains(0)
+    then Searcher(target, nums, thirdElephant).search()
+    else Tester(nums, thirdElephant).test(target)
+
   def run(input: os.ReadablePath): (Timings, Solution) =
     val REGEX = """(\d+): (.*)""".r
     val (pre_time, in) = timed {
-      os.read.lines(input).map {
-        case REGEX(value, rest) => (value.toLong, longs(rest))
-      }.toVector
+      os.read
+        .lines(input)
+        .map { case REGEX(value, rest) => (value.toLong, longs(rest)) }
+        .toVector
     }
 
     val (p1_time, p1_solution) = timed {
-      in
-        .filter((target, nums) => Searcher(target, nums, false).search())
-        .map(_.head)
-        .sum
+      in.filter(p => check(p._1, p._2, false)).map(_.head).sum
     }
 
     val (p2_time, p2_solution) = timed {
-      in
-        .filter((target, nums) => Searcher(target, nums, true).search())
-        .map(_.head)
-        .sum
+      in.filter(p => check(p._1, p._2, true)).map(_.head).sum
     }
 
     (