ilyannn

// TODO: insert guards to use open-source version of random() if necessary

func random(x: Int) -> Int {
    return Int(arc4random_uniform(UInt32(endBoundary)))
}

extension CollectionType {
    
    /// create a lazy succession of randomly ordered indices
    ///

ilyannn

Instead of

assert(!array.isEmpty, "Array guaranteed to be non-empty because...")
let lastItem = array.last!

guard !array.isEmpty 
    else { fatalError("Array guaranteed to be non-empty because...") }

// ... etc ...

ilyannn

def fold(s, p = 0): 
    for i in [i+1 for (i, c), n in zip(enumerate(s), s[1:]+' ') if c != n]: p = print(s[p], i-p, sep="", end="") or i