package moe.kageru.sekwences

class PeekableSequence<T>(source: Sequence<T>) : Sequence<T> {
    private val iterator: PeekableIterator<T> = PeekableIterator(source.iterator())

    override fun iterator(): Iterator<T> = iterator

    fun peek(): T? = iterator.peek()
}

internal class PeekableIterator<T>(private val source: Iterator<T>) : Iterator<T> {
    private var peeked: T? = null
    override fun hasNext(): Boolean {
        return source.hasNext() || peeked != null
    }

    override fun next(): T {
        return if (peeked == null) {
            source.next()
        } else {
            val ret = peeked
            peeked = null
            ret!!
        }
    }

    fun peek(): T? {
        return when {
            peeked != null -> peeked
            hasNext() -> {
                peeked = next()
                peeked!!
            }
            else -> null
        }
    }
}