InterspersedSequence.swift

//
//  main.swift
//  InterspesedSequence
//
//  Created by Luciano Almeida on 19/01/19.
//  Copyright © 2019 Luciano Almeida. All rights reserved.
//


/// A lazy view for sequence that creates a new sequece where for each iteration of the base sequence the value is inserted.
struct InterspersedSequence<Base: Sequence> {
    var base: Base
    var value: Element
    
    struct Iterator {
        var _base: Base.Iterator
        var _value: InterspersedSequence.Element
        private var _intersperse: Bool = false
        private var _nextValue: Element?
        
        init(_base: Base.Iterator, _value: InterspersedSequence.Element) {
            self._base = _base
            self._value = _value
            self._nextValue = self._base.next()
        }
    }
}

extension InterspersedSequence.Iterator: IteratorProtocol {
    typealias Element = InterspersedSequence<Base>.Element
    mutating func next() -> Element? {
        defer { _intersperse.toggle() }
        if _intersperse && _nextValue != nil {
            return _value
        } else {
            defer { _nextValue = _base.next() }
            return _nextValue
        }
    }
}

extension InterspersedSequence: Sequence {
    typealias Element = Base.Element

    func makeIterator() -> InterspersedSequence<Base>.Iterator {
        return InterspersedSequence.Iterator(_base: base.makeIterator(), _value: value)
    }
}

extension Sequence {
    func interspersed(with value: Element) -> InterspersedSequence<Self> {
        return InterspersedSequence(base: self, value: value)
    }
}

let r = [1, 2, 3].interspersed(with: 4).map { $0 }

print(r) // [1, 4, 2, 4, 3]