LeetCode/kt/string-compression-iii.kt

class Solution {
    private data class State(val compressed: MutableList<Pair<Int, Char>>, val count: Int, val character: Char) {
        private fun add() {
            if (count <= 0) {
                return
            }

            compressed.add(count to character)
        }

        fun update(c: Char): State {
            if (c != character || count >= 9) {
                add()
                return State(compressed, 1, c)
            }

            return State(compressed, 1 + count, c)
        }

        fun finalize(): State {
            add()
            return State(compressed, 0, '_')
        }
    }

    fun compressedString(word: String): String =
        word.fold(State(mutableListOf(), 0, word[0])) { acc, it ->
            acc.update(it)
        }.finalize().compressed.joinToString(separator = "") {
            "${it.first}${it.second}"
        }
}