equals

listOf("A", "A", "B", "C", "C", "C", "C", "A", "A")
// to
listOf("A" to 2, "B" to 1, "C" to 4, "A" to 2)

fun <T> Collection<T>.runLengths() = buildList {
    val last = this@runLengths.withIndex().fold(null) { prev: IndexedValue<T>?, cur ->
        when {
            prev == null -> cur
            prev.value == cur.value -> prev
            else -> {
                add(prev.value to cur.index - prev.index)
                cur
            }
        }
    }
    last?.let { add(it.value to this@runLengths.size - it.index) }
}

fun <T> Collection<T>.runLengths() = takeIf { isNotEmpty() }?.let {
    buildList {
        it.fold(it.first() to 0) { (prev, count), cur ->
            if (cur == prev) (prev to count + 1) else {
                add(prev to count)
                cur to 1
            }
        }.run(::add)
    }
}.orEmpty()