Merge pull request #563 from shabirjan/KaratsubaMultiplication-Swift4

Karatsuba multiplication Swift4

Author: kelvinlauKL

Karatsuba Multiplication/KaratsubaMultiplication.playground/Contents.swift

@@ -1,7 +1,10 @@
 //: 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
@@ -17,9 +20,9 @@ func ^^ (radix: Int, power: Int) -> Int {
 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 {
@@ -29,32 +32,32 @@ func multiply(_ num1: Int, by num2: Int, base: Int = 10) -> Int {
     }
     product[i + num2Array.count] += carry
   }
-
+  
   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
 }

Karatsuba Multiplication/KaratsubaMultiplication.swift

@@ -22,24 +22,24 @@ func ^^ (radix: Int, power: Int) -> Int {
 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 num1 * 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
 }