package prodfib

class FibonacciPairs : Iterable<Pair<Long, Long>> {
    private class FibonacciPairsIterator : Iterator<Pair<Long, Long>> {
        private var a: Long = 0
        private var b: Long = 1

        /**
         * Returns `true` if the iteration has more elements.
         */
        override fun hasNext(): Boolean = true

        /**
         * Returns the next element in the iteration.
         */
        override fun next(): Pair<Long, Long> {
            val current = a to b

            a = current.second
            b = current.first + current.second

            return current
        }
    }

    /**
     * Returns an iterator over the elements of this object.
     */
    override fun iterator(): Iterator<Pair<Long, Long>> = FibonacciPairsIterator()
}

fun productFib(prod: Long): LongArray =
    FibonacciPairs()
        .asSequence()
        .dropWhile { (a, b) -> a * b < prod }
        .first()
        .let { (a, b) ->
            longArrayOf(
                a, b, if (a * b == prod) {
                    1
                } else {
                    0
                }
            )
        }

fun main() {
    (0 until 10).zip(FibonacciPairs()).forEach { (i, pair) ->
        println("$i → $pair")
    }

    val tests = mapOf(
        4895L to longArrayOf(55, 89, 1),
        5895L to longArrayOf(89, 144, 0),
        714L to longArrayOf(21, 34, 1),
        800L to longArrayOf(34, 55, 0),
    )
    tests.forEach { (prod, expected) ->
        check(productFib(prod).contentEquals(expected))
    }
}