Karatsuba Multiplication updated to Swift4

Author: shabirjan

Karatsuba Multiplication/KaratsubaMultiplication.playground/Contents.swift

@@ -1,60 +1,63 @@
 //: Playground - noun: a place where people can play
 
 import Foundation
-
+// last checked with Xcode 9.0b4
+#if swift(>=4.0)
+print("Hello, Swift 4")
+#endif
 precedencegroup ExponentiativePrecedence {
-  higherThan: MultiplicationPrecedence
-  lowerThan: BitwiseShiftPrecedence
-  associativity: left
+    higherThan: MultiplicationPrecedence
+    lowerThan: BitwiseShiftPrecedence
+    associativity: left
 }
 
 infix operator ^^: ExponentiativePrecedence
 func ^^ (radix: Int, power: Int) -> Int {
-  return Int(pow(Double(radix), Double(power)))
+    return Int(pow(Double(radix), Double(power)))
 }
 
 // Long Multiplication - O(n^2)
 func multiply(_ num1: Int, by num2: Int, base: Int = 10) -> Int {
-  let num1Array = String(num1).characters.reversed().map { Int(String($0))! }
-  let num2Array = String(num2).characters.reversed().map { Int(String($0))! }
-
-  var product = Array(repeating: 0, count: num1Array.count + num2Array.count)
-
-  for i in num1Array.indices {
-    var carry = 0
-    for j in num2Array.indices {
-      product[i + j] += carry + num1Array[i] * num2Array[j]
-      carry = product[i + j] / base
-      product[i + j] %= base
+    let num1Array = String(num1).characters.reversed().map { Int(String($0))! }
+    let num2Array = String(num2).characters.reversed().map { Int(String($0))! }
+    
+    var product = Array(repeating: 0, count: num1Array.count + num2Array.count)
+    
+    for i in num1Array.indices {
+        var carry = 0
+        for j in num2Array.indices {
+            product[i + j] += carry + num1Array[i] * num2Array[j]
+            carry = product[i + j] / base
+            product[i + j] %= base
+        }
+        product[i + num2Array.count] += carry
     }
-    product[i + num2Array.count] += carry
-  }
-
-  return Int(product.reversed().map { String($0) }.reduce("", +))!
+    
+    return Int(product.reversed().map { String($0) }.reduce("", +))!
 }
 
 // Karatsuba Multiplication - O(n^log2(3))
 func karatsuba(_ num1: Int, by num2: Int) -> Int {
-  let num1Array = String(num1).characters
-  let num2Array = String(num2).characters
-
-  guard num1Array.count > 1 && num2Array.count > 1 else {
-    return multiply(num1, by: num2)
-  }
-
-  let n = max(num1Array.count, num2Array.count)
-  let nBy2 = n / 2
-
-  let a = num1 / 10^^nBy2
-  let b = num1 % 10^^nBy2
-  let c = num2 / 10^^nBy2
-  let d = num2 % 10^^nBy2
-
-  let ac = karatsuba(a, by: c)
-  let bd = karatsuba(b, by: d)
-  let adPlusbc = karatsuba(a+b, by: c+d) - ac - bd
-
-  let product = ac * 10^^(2 * nBy2) + adPlusbc * 10^^nBy2 + bd
-
-  return product
+    let num1Array = String(num1).characters
+    let num2Array = String(num2).characters
+    
+    guard num1Array.count > 1 && num2Array.count > 1 else {
+        return multiply(num1, by: num2)
+    }
+    
+    let n = max(num1Array.count, num2Array.count)
+    let nBy2 = n / 2
+    
+    let a = num1 / 10^^nBy2
+    let b = num1 % 10^^nBy2
+    let c = num2 / 10^^nBy2
+    let d = num2 % 10^^nBy2
+    
+    let ac = karatsuba(a, by: c)
+    let bd = karatsuba(b, by: d)
+    let adPlusbc = karatsuba(a+b, by: c+d) - ac - bd
+    
+    let product = ac * 10^^(2 * nBy2) + adPlusbc * 10^^nBy2 + bd
+    
+    return product
 }