EndlessIterator.swift

// Do you like my endless iterator?

struct EndlessIterator {

    var baseIterator: String.Iterator?
    var start: String

    init(_ start: String) {
        self.start = start
    }

    mutating func next() -> Character {
        var item = baseIterator?.next()
        if item == nil {
            baseIterator = start.makeIterator()
            item = baseIterator?.next()
        }
        guard let result = item else {
            fatalError("Endless iterator ended")
        }
        return result
    }
}

func loopingForever<S: Sequence>(
    over sequence: S
) -> AnySequence<S.Element> {
    let repeating = AnyIterator { sequence }
    let result = repeating.lazy.flatMap { $0 }
    return AnySequence(result)
}

let example = loopingForever(over: "abc")
print(Array(example.prefix(7)))
// ["a", "b", "c", "a", "b", "c", "a"]

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