Shuffle host array at client init

Fixes #56.

Author: Clément Le Provost

Source/Client.swift

@@ -100,18 +100,20 @@ import Foundation
         self.apiKey = apiKey
 
         // Initialize hosts to their default values.
-        readHosts = [
-            "\(appID)-dsn.algolia.net",
+        //
+        // NOTE: The host list comes in two parts:
+        //
+        // 1. The fault-tolerant, load-balanced DNS host.
+        // 2. The non-fault-tolerant hosts. Those hosts must be randomized to ensure proper load balancing in case
+        //    of the first host's failure.
+        //
+        let fallbackHosts = [
             "\(appID)-1.algolianet.com",
             "\(appID)-2.algolianet.com",
             "\(appID)-3.algolianet.com"
-        ]
-        writeHosts = [
-            "\(appID).algolia.net",
-            "\(appID)-1.algolianet.com",
-            "\(appID)-2.algolianet.com",
-            "\(appID)-3.algolianet.com"
-        ]
+        ].shuffle()
+        readHosts = [ "\(appID)-dsn.algolia.net" ] + fallbackHosts
+        writeHosts = [ "\(appID).algolia.net" ] + fallbackHosts
 
         // WARNING: Those headers cannot be changed for the lifetime of the session.
         let version = NSBundle(forClass: self.dynamicType).infoDictionary!["CFBundleShortVersionString"] as! String

Source/Helpers.swift

@@ -29,4 +29,29 @@ extension String {
         let customAllowedSet = NSCharacterSet(charactersInString: "!*'();:@&=+$,/?%#[]").invertedSet
         return stringByAddingPercentEncodingWithAllowedCharacters(customAllowedSet)!
     }
-}
+}
+
+// Collection shuffling taken from <http://stackoverflow.com/questions/24026510/how-do-i-shuffle-an-array-in-swift>.
+
+extension CollectionType {
+    /// Return a copy of `self` with its elements shuffled.
+    func shuffle() -> [Generator.Element] {
+        var list = Array(self)
+        list.shuffleInPlace()
+        return list
+    }
+}
+
+extension MutableCollectionType where Index == Int {
+    /// Shuffle the elements of `self` in-place.
+    mutating func shuffleInPlace() {
+        // Empty and single-element collections don't shuffle.
+        if count < 2 { return }
+        
+        for i in 0..<count - 1 {
+            let j = Int(arc4random_uniform(UInt32(count - i))) + i
+            guard i != j else { continue }
+            swap(&self[i], &self[j])
+        }
+    }
+}