A new Sequence type that returns a repeating sequence of values.

RepeatingSequence.swift

/// The RepeatingSequence accepts a collection and returns the values sequentially until the
/// final element is returned, at which point the sequence wraps around to the first element
/// of the collection and continues from there.
struct RepeatingSequence<T>: Sequence {
    
    /// The Collection that we base our sequence on. We use a Collection and not
    /// another Sequence because Sequences are not guaranteed to be repeatedly iterated.
    let data: AnyCollection<T>
    
    /// We can optionally specify a maximum number of iterations. This is necessary
    /// to create non-infinite Sequences.
    let maxCount: Int?
    
    func makeIterator() -> AnyIterator<T> {
        var index: AnyCollection.Index = data.startIndex
        var count: Int = 0
        
        return AnyIterator<T> {
            if let max = self.maxCount, count >= max {
                return nil
            }
            defer {
                index = self.data.index(after: index)
                if index == self.data.endIndex {
                    index = self.data.startIndex
                }
                count += 1
            }
            return self.data[index]
        }
    }
}