Returns a sequence that consists of all possible pair of original list elements, does nothing with potential duplicates

pairs-sequence.kt

/**
 * Returns a sequence that consists of all possible pair of original list elements, does nothing with potential duplicates
 * @param skipSamePairs indicates whether it should produce pairs from the same element at both first and second positions
 */
fun <T> List<T>.allPairs(skipSamePairs: Boolean = true): Sequence<Pair<T, T>> = PairsSequence(this, skipSamePairs)

private class PairsSequence<T>(val source: List<T>, val skipSamePairs: Boolean) : Sequence<Pair<T, T>> {
    override fun iterator(): Iterator<Pair<T, T>> = PairsIterator(source, skipSamePairs)
}

private class PairsIterator<T>(val source: List<T>, val skipSamePairs: Boolean) : AbstractIterator<Pair<T, T>>() {
    private var index = 0

    override fun computeNext() {
        if (source.isEmpty()) {
            done()
            return
        }

        index++

        val i1 = index / source.size
        val i2 = index % source.size

        if (i1 >= source.lastIndex) {
            done()
            return
        }

        if (skipSamePairs && i1 == i2) {
            return computeNext()
        }

        setNext(Pair(source[i1], source[i2]))
    }
}