Every combination of every element of two sequences in Swift - ugly brute force version

combos1.swift

// after https://twitter.com/cocoaphony/status/490708834415947776

func combinations
    <First: SequenceType, Second: CollectionType>
    (first: First, second: Second)
    -> GeneratorOf<(First.Generator.Element, Second.Generator.Element)> {
        var first_gen = first.generate()
        var second_gen = second.generate()
        var current_first = first_gen.next()
        return GeneratorOf {
            // check if there's more of first to consume
            if let this_first = current_first {
                // if so, is there more of this go-around
                // of second to consume?
                if let this_second = second_gen.next() {
                    return (this_first, this_second)
                }
                    // if second used up, reset it
                else {
                    // go back to the beginning of second
                    second_gen = second.generate()
                    // and take the first element of it
                    let next_second = second_gen.next()
                    // was there a first element?
                    if let this_second = next_second {
                        // move on to the next element of first
                        current_first = first_gen.next()
                        // is there such an element?
                        if let this_first = current_first {
                            return (this_first, this_second)
                        }
                            // if not, we've reached the end of
                            // the first sequence
                        else {
                            // so we've finished
                            return nil
                        }
                    }
                        // if not, second is empty
                    else {
                        // so we need to guard against
                        // infinite looping in that case
                        return nil
                    }
                }
            }
                // if not, we've reached the end of
                // the first sequence
            else {
                // so we've finished
                return nil
            }
        }
}

let combos = map(combinations("ABC", "123")) { [$0] + [$1] }

for c in combos {
    println(c)
}


===>
A1
A2
A3
B1
B2
B3
C1
C2
C3