A simple method for grouping elements with given key calculating closure in Swift 3

Grouping.swift

extension Array {
  func group<Key: Hashable>(by: (Element) -> Key) -> [Key: [Element]] {
    var values = [Key: [Element]]()
    let keys = self.map(by)
    let pairs = zip(keys, self)
    
    pairs.forEach { (key, value) in
      var valuesOfKeys = values[key] ?? []
      valuesOfKeys.append(value)
      values[key] = valuesOfKeys
    }
    return values
  }
}