Updated number to be prime; and Swift 4 update

Author: Bill Barbour

Rabin-Karp/Rabin-Karp.playground/Contents.swift

@@ -8,7 +8,7 @@ print("Hello, Swift 4!")
 import UIKit
 
 struct Constants {
-    static let hashMultiplier = 69069
+    static let hashMultiplier = 69061
 }
 
 precedencegroup PowerPrecedence { higherThan: MultiplicationPrecedence }
@@ -30,8 +30,8 @@ extension Character {
 // Find first position of pattern in the text using Rabin Karp algorithm
 public func search(text: String, pattern: String) -> Int {
     // convert to array of ints
-    let patternArray = pattern.flatMap { $0.asInt }
-    let textArray = text.flatMap { $0.asInt }
+    let patternArray = pattern.compactMap { $0.asInt }
+    let textArray = text.compactMap { $0.asInt }
 
     if textArray.count < patternArray.count {
         return -1