GroupBy.swift

GroupBy.swift

extension Collection{
    
    /**
     * Groups elements of the original collection by the key returned by the given [keySelector] function
     * applied to each element and returns a map where each group key is associated with a list of corresponding elements.
     *
     * The returned map preserves the entry iteration order of the keys produced from the original collection.
    **/
    public func groupBy<Value>(by keySelector:(Element) -> Value?) -> [Value?:[Element]]{
        
        var groups = [Value?:[Element]]()
        
        for (_, element) in self.enumerated(){
            
            let key = keySelector(element)
            
            if groups[key] != nil{
                groups[key]!.append(element)
            }else{
                groups[key] = [element]
            }
        }
        
        return groups
    }
}