Data commit

Task/Lah-numbers/Swift/lah-numbers.swift

@@ -0,0 +1,55 @@
+import BigInt
+import Foundation
+
+@inlinable
+public func factorial<T: BinaryInteger>(_ n: T) -> T {
+  guard n != 0 else {
+    return 1
+  }
+
+  return stride(from: n, to: 0, by: -1).reduce(1, *)
+}
+
+@inlinable
+public func lah<T: BinaryInteger>(n: T, k: T) -> T {
+  if k == 1 {
+    return factorial(n)
+  } else if k == n {
+    return 1
+  } else if k > n {
+    return 0
+  } else if k < 1 || n < 1 {
+    return 0
+  } else {
+    let a = (factorial(n) * factorial(n - 1))
+    let b = (factorial(k) * factorial(k - 1))
+    let c = factorial(n - k)
+
+    return a / b / c
+  }
+}
+
+print("Unsigned Lah numbers: L(n, k):")
+print("n\\k", terminator: "")
+
+for i in 0...12 {
+  print(String(format: "%10d", i), terminator: " ")
+}
+
+print()
+
+for row in 0...12 {
+  print(String(format: "%-2d", row), terminator: "")
+
+  for i in 0...row {
+    lah(n: BigInt(row), k: BigInt(i)).description.withCString {str in
+      print(String(format: "%11s", str), terminator: "")
+    }
+  }
+
+  print()
+}
+
+let maxLah = (0...100).map({ lah(n: BigInt(100), k: BigInt($0)) }).max()!
+
+print("Maximum value from the L(100, *) row: \(maxLah)")