Generic frequencies method for SequenceTypes

frequencies.swift

//: # Generic frequencies for SequenceType
//:
//: Based on a tweet from Airspeed Velocity @airspeedswift
//: describing a custom subscript operation that takes 
//: an initial value if the key is not found.

import Foundation

extension Dictionary {
  subscript(key: Key, or initial: Value) -> Value {
    get {
      return self[key] ?? initial
    }
    set {
      self[key] = newValue
    }
  }
}

extension SequenceType where Generator.Element: Hashable {
  typealias Key = Generator.Element
  func frequencies() -> [Key:Int] {
    var output: [Key: Int] = [:]
    for value in self {
      output[value, or: 0] += 1
    }
    return output
  }
}

let message = "Now is the time for all good people to come to the aid of their country."

let wordCount = message.componentsSeparatedByString(" ").frequencies()

(1...10).frequencies()

[1,2,4,5,2,1,1,1].frequencies()

message.characters.frequencies()

let sorted = wordCount.sort { $0.1 > $1.1 }
sorted