Grouping the elements of a sequence by a key function. (This may not be the fastest implementation out there.)

GroupBy.swift

func groupBy<C: CollectionType, K: Hashable>
    (xs: C, key: C.Generator.Element -> K) -> [K:[C.Generator.Element]]
{
    var gs: [K:[C.Generator.Element]] = [:]
    for x in xs {
        let k = key(x)
        var ys = gs[k] ?? []
        ys.append(x)
        gs.updateValue(ys, forKey: k)
    }
    return gs
}

func isEven<T: protocol<IntegerArithmeticType, IntegerLiteralConvertible>>(n: T) -> Bool { return n % 2 == 0 }
func isOdd <T: protocol<IntegerArithmeticType, IntegerLiteralConvertible>>(n: T) -> Bool { return n % 2 != 0 }

groupBy([0,1,0,3,4,2,2,0], isEven)  //=> [false: [1, 3], true: [0, 0, 4, 2, 2, 0]]
groupBy([0,1,0,3,4,2,2,0]) {$0 % 3} //=> [2: [2, 2], 0: [0, 0, 3, 0], 1: [1, 4]]

I couldn't find a good way to perform updates in-place to the arrays already contained in the dictionary. Maybe something like this might work:

let k = key(x)
var ys = gs.updateValue([], forKey: k) ?? [] // move array out of the dictionary, or create new
ys.append(x)
gs.updateValue(ys, forKey: k) // move back in (and ignore the placeholder)