pairs

import java.lang.IllegalStateException

fun main() {
    val elementsA = listOf(0, 1, 2, 3, 4)
    val elementsB = listOf(5, 6, 7)
    for (i in 0..1000) {
        val pairs: MutableSet<Pair<Int, Int>> = combineToPairs(elementsA, elementsB)
        check(pairs, elementsA.size)
        println("$pairs\n")
    }
}

fun combineToPairs(listA: List<Int>, listB: List<Int>, combinations: Int = 10): MutableSet<Pair<Int, Int>> {
    val result = mutableSetOf<Pair<Int, Int>>() // LinkedHashSet
    val listACopy = listA.shuffled().toMutableList()
    var i = 0
    var j = 0
    while (result.size < combinations) {
        val added = result.add(listACopy[i++] to listB[j])
        if (!added) {
            // Rollback previously added elements
            //println("${"-".repeat(20)}ROLLBACK${"-".repeat(20)}")
            val timesListAAdded = result.size / listA.size
            val uniquePairs = result.take(listA.size * timesListAAdded)
            result.replaceWith(uniquePairs)
            listACopy.replaceWith(uniquePairs.takeLast(listA.size).map { it.first })
            j = listB.indexOf(result.last().second)
        }
        if (i == listACopy.size || !added) {
            val listTemp = listACopy.toMutableList()
            while (listTemp == listACopy || listTemp.first() == listACopy.last()) {
                listTemp.shuffle()
            }
            listACopy.replaceWith(listTemp)
            i = 0
        }
        j = ++j % listB.size
    }
    return result
}

fun <T> MutableCollection<T>.replaceWith(newElements: Collection<T>) {
    clear()
    addAll(newElements)
}

fun check(pairs: MutableSet<Pair<Int, Int>>, size: Int) {
    val halves = pairs.chunked(size)
    val firstNumbers = halves.map { half ->
        val nums = half.map { it.first }
        print(nums)
        LinkedHashSet(nums)
    }
    println(" <- first numbers of pairs")

    val isFirstNumbersCombinationsUnique = firstNumbers.map { it.toList() }.toSet().size == firstNumbers.size

    val actualSecondNumbers = pairs.map { it.second }
    val expectedSecondNumbers = listOf(5, 6, 7, 5, 6, 7, 5, 6, 7, 5)

    val isHalvesWithCorrectSize = firstNumbers.all { it.size == size }

    if (!isHalvesWithCorrectSize) {
        throw IllegalStateException("halves have duplicates of first numbers")
    }
    if (!isFirstNumbersCombinationsUnique){
        throw IllegalStateException("halves have the same combinations of first numbers")
    }
    if (actualSecondNumbers != expectedSecondNumbers) {
        throw IllegalStateException("seconds numbers are incorrect")
    }
}