Repeating sequence
Today I had the need to generate an infinite repeating sequence from a fixed ordered collection of elements. That is, from the items a, b, c, I needed an infinite sequence like a, b, c, a, b, c, a, b, c, ....
I though maybe I’d share it the off-chance that someone might need one:
public struct RepeatingSequence<T: Collection>: Sequence {
private let base: T
public init(baseCollection: T) {
self.base = baseCollection
}
public func makeIterator() -> AnyIterator<T.Element> {
var i = base.startIndex
return AnyIterator({ [base] () -> T.Element in
defer {
i = (base.index(after: i) == base.endIndex)
? base.startIndex : base.index(after: i)
}
return base[i]
})
}
}
extension Collection {
public func repeated() -> RepeatingSequence<Self> {
return RepeatingSequence(baseCollection: self)
}
}
You can use it like any sequence, but remember to limit it before you traverse the entire sequence:
["a", "b", "c"].repeated().prefix(10)
// "a", "b", "c", "a", "b", "c", "a", "b", "c", "a"
["a", "b", "c"].reversed().repeated().dropFirst(5).prefix(2)
// "a", "c"
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