Group elements in a sequence. Note that this implementation predates Swift 4's `grouping` operation

GroupBy.swift

extension Sequence {
    typealias Element = Iterator.Element
    
    func groupBy<T: Hashable>(grouper: (Element) -> (T)) -> Dictionary<T, [Element]> {
        var groups = [T : [Element]]()
        
        for element in self {
            let groupKey = grouper(element)
            var group = groups[groupKey] ?? [Element]()
            
            group.append(element)
            groups[groupKey] = group
        }
        
        return groups
    }
}

let grouped = (0...5).groupBy { x in
    return x % 2 == 0 ? "Even" : "Odd"
}

print(grouped) // ["Odd": [1, 3, 5], "Even": [0, 2, 4]]