Data update

Task/Descending-primes/Scala/descending-primes.scala

@@ -0,0 +1,42 @@
+def isPrime(n: Long): Boolean = {
+  @annotation.tailrec
+  def hasDivisor(f: Long): Boolean =
+    if (f * f > n) false
+    else if (n % f == 0 || n % (f + 2) == 0) true
+    else hasDivisor(f + 6)
+
+  if (n < 2) false
+  else if (n == 2 || n == 3) true
+  else if (n % 2 == 0 || n % 3 == 0) false
+  else {
+    !hasDivisor(5)
+  }
+}
+
+def descendingPrimes(): Seq[Int] = {
+  val digits = Seq(9, 8, 7, 6, 5, 4, 3, 2, 1)
+
+  val (_, primes) = digits.foldLeft((Seq(0), Seq.empty[Int])) { case ((candidates, primes), digit) =>
+    val newCandidates = candidates.map(_ * 10 + digit)
+    val newPrimes = primes ++ newCandidates.filter(isPrime)
+    (candidates ++ newCandidates, newPrimes)
+  }
+
+  primes.sorted
+}
+
+@main def main(): Unit = {
+  def test(): Unit = {
+    val primes = descendingPrimes()
+
+    val maxDigits = primes.map(_.toString.length).max
+    val columnsPerLine = 8
+    val groupedPrimes = primes.grouped(columnsPerLine)
+    groupedPrimes.foreach { group =>
+      val formattedGroup = group.map(p => String.format(s"%${maxDigits}d", Int.box(p))).mkString("  ")
+      println(formattedGroup)
+    }
+  }
+
+  test()
+}