Grouping collections

Collection.swift

public extension Collection {
    
    func keyedBy<T>(_ keyer: (Iterator.Element) -> T?) -> Dictionary<T, Iterator.Element> {
        var d = Dictionary<T, Iterator.Element>()
        for item in self {
            if let key = keyer(item) {
                d[key] = item
            }
        }
        return d
    }
    
    func keyedBy<T>(_ keyer: (Iterator.Element) -> Array<T>) -> Dictionary<T, Iterator.Element> {
        var d = Dictionary<T, Iterator.Element>()
        for item in self {
            let keys = keyer(item)
            for key in keys {
                d[key] = item
            }
        }
        return d
    }
    
    func groupedBy<T>(_ keyer: (Iterator.Element) -> T?) -> Dictionary<T, Array<Iterator.Element>> {
        var d = Dictionary<T, Array<Iterator.Element>>()
        for item in self {
            if let key = keyer(item) {
                var items = d[key] ?? []
                items.append(item)
                d[key] = items
            }
        }
        return d
    }
    
    func intersperse(_ separator: Iterator.Element) -> Array<Iterator.Element> {
        return intersperse { separator }
    }
    
    func intersperse(_ item: () -> Iterator.Element) -> Array<Iterator.Element> {
        var everything = Array<Iterator.Element>()
        for element in self {
            everything.append(element)
            everything.append(item())
        }
        everything.removeLast()
        return everything
    }
    
    func divide(_ isInFirst: (Iterator.Element) -> Bool) -> (Array<Iterator.Element>, Array<Iterator.Element>) {
        var first = Array<Iterator.Element>()
        var second = Array<Iterator.Element>()
        for item in self {
            if isInFirst(item) {
                first.append(item)
            } else {
                second.append(item)
            }
        }
        return (first, second)
    }
    
}

public extension Collection where Iterator.Element: Hashable {
    
    func unique() -> Array<Iterator.Element> {
        var uniqued = Array<Iterator.Element>()
        var soFar = Set<Iterator.Element>()
        for item in self {
            if soFar.contains(item) == false {
                uniqued.append(item)
                soFar.insert(item)
            }
        }
        return uniqued
    }
}