The result of what happens when a developer couple's curiosity sparks while playing Spot it.

Spotit.kt

val group = listOf('a', 'b', 'c', 'd', 'e','f','g','h','i')

fun main() {
    val potentialCards = getPotentialCards(4, group)
    println(potentialCards)
    println(potentialCards.size)

    val builtCards = selectCards(potentialCards)
    println(builtCards)
    println(builtCards.size)
}

private fun selectCards(cards: Set<Set<Char>>): Set<Set<Char>> {
    val diff = (cards.firstOrNull()?.size ?: 0) - 1
    val savedCards = mutableSetOf<Set<Char>>()

    cards.forEach { card ->
        if (savedCards.all { saved ->  (card - saved).size == diff}) {
            savedCards.add(card)
        }
    }

    return savedCards
}

private fun getPotentialCards(slots: Int, options: List<Char>): Set<Set<Char>> {
    if (slots == 0) {
        return setOf(setOf())
    }

    return options.map { option ->
        getPotentialCards(slots - 1, options.filterNot { it == option }).map { subset ->
            setOf(option) + subset
        }
    }.flatten().toSet()
}