An iterator that automatically wraps back to the `firstIndex` after the `endIndex` is reached.

InfiniteIterator.swift

import Foundation

public struct InfiniteIterator<Base: Collection>: IteratorProtocol {
    
    private let collection: Base
    private var index: Base.Index
    
    public init(collection: Base) {
        self.collection = collection
        self.index = collection.startIndex
    }
    
    public mutating func next() -> Base.Iterator.Element? {
        guard !collection.isEmpty else {
            return nil
        }
        
        let result = collection[index]
        collection.formIndex(after: &index)
        
        if index == collection.endIndex {
            index = collection.startIndex
        }
        
        return result
    }
    
}

let letters = ["a", "b", "c"]
var iterator = InfiniteIterator(collection: letters)

iterator.next() // a
iterator.next() // b
iterator.next() // c
iterator.next() // a
iterator.next() // b