Data commit

Task/Combinations-and-permutations/Swift/combinations-and-permutations.swift

@@ -0,0 +1,55 @@
+import BigInt
+
+func permutations(n: Int, k: Int) -> BigInt {
+  let l = n - k + 1
+
+  guard l <= n else {
+    return 1
+  }
+
+  return (l...n).reduce(BigInt(1), { $0 * BigInt($1) })
+}
+
+func combinations(n: Int, k: Int) -> BigInt {
+  let fact = {() -> BigInt in
+    guard k > 1 else {
+      return 1
+    }
+
+    return (2...k).map({ BigInt($0) }).reduce(1, *)
+  }()
+
+  return permutations(n: n, k: k) / fact
+}
+
+print("Sample of permutations from 1 to 12")
+
+for i in 1...12 {
+  print("\(i) P \(i / 3) = \(permutations(n: i, k: i / 3))")
+}
+
+print("\nSample of combinations from 10 to 60")
+
+for i in stride(from: 10, through: 60, by: 10) {
+  print("\(i) C \(i / 3) = \(combinations(n: i, k: i / 3))")
+}
+
+print("\nSample of permutations from 5 to 15,000")
+
+for i in [5, 50, 500, 1000, 5000, 15000] {
+  let k = i / 3
+  let res = permutations(n: i, k: k).description
+  let extra = res.count > 40 ? "... (\(res.count - 40) more digits)" : ""
+
+  print("\(i) P \(k) = \(res.prefix(40))\(extra)")
+}
+
+print("\nSample of combinations from 100 to 1000")
+
+for i in stride(from: 100, through: 1000, by: 100) {
+  let k = i / 3
+  let res = combinations(n: i, k: k).description
+  let extra = res.count > 40 ? "... (\(res.count - 40) more digits)" : ""
+
+  print("\(i) C \(k) = \(res.prefix(40))\(extra)")
+}