vampire

nmcb/src/main/scala/examples/VampireNumber.scala

@@ -9,20 +9,34 @@ object VampireNumber:
     def possibleFangs(length: Int): Iterator[String] =
       digits.combinations(length).flatMap(_.permutations).distinct
 
-  def isVampireNumber(n: Int): Boolean =
-    val digits: String = n.toString
+  def isVampireNumber(number: Int): Boolean =
+    val digits: String = number.toString
     if digits.length % 2 != 0 then
       false
     else
-      val fangLength = digits.length / 2
-      digits.possibleFangs(fangLength).exists: left =>
-        digits.diff(left).possibleFangs(fangLength).exists: right =>
-          n == left.toInt * right.toInt
+      val fangWidth = digits.length / 2
+      digits.possibleFangs(fangWidth).exists: leftDigits =>
+        val leftFang = leftDigits.toInt
+        if leftFang != 0 then
+          val rightFang = number / leftFang
+          if leftFang % 10 == 0 && rightFang % 10 == 0 then
+            false
+          else
+            if rightFang * leftFang == number then
+              val rightDigits = rightFang.toString
+              digits.sorted == (leftDigits + rightDigits).sorted
+            else false
+        else
+          false
+
+  def sieve(max: Int): Seq[Int] =
+    (0 to max).filter(isVampireNumber)
+
 
   def main(args: Array[String]): Unit =
     val max: Int         = Try(args(0).toInt).getOrElse(1000000)
     val start: Long      = currentTimeMillis
-    val result: Seq[Int] = (0 to max).filter(isVampireNumber)
+    val result: Seq[Int] = sieve(max)
 
     println(s"vampire numbers up to $max:")
     println(result.mkString(", "))

nmcb/src/test/scala/examples/TestVampireNumber.scala

@@ -0,0 +1,17 @@
+package examples
+
+import org.scalatest.funsuite.AnyFunSuite
+
+class TestVampireNumber extends AnyFunSuite:
+
+  /** [OEIS-A014575](https://oeis.org/A014575) */
+  test("VampireNumber") {
+    println(VampireNumber.sieve(135837))
+    assertResult(
+      Vector(
+        1260, 1395, 1435, 1530, 1827, 2187, 6880, 102510, 104260, 105210, 105264, 105750, 108135,
+        110758, 115672, 116725, 117067, 118440, 120600, 123354, 124483, 125248, 125433, 125460,
+        125500, 126027, 126846, 129640, 129775, 131242, 132430, 133245, 134725, 135828, 135837
+      )
+    )(VampireNumber.sieve(135837))
+  }