LeetCode/kt/closest-prime-numbers-in-range.kt

class Solution {
    private fun isPrime(n: Int): Boolean =
        when {
            n < 2 -> false
            n % 2 == 0 -> n == 2
            else ->
                (3..n).step(2)
                    .asSequence()
                    .takeWhile { it * it <= n }
                    .all { div -> n % div != 0 }
        }

    private data class Acc(val lastPrime: Int, val p: Int, val q: Int, val diff: Int) {
        constructor() : this(-1, -1, -1, 1_000_000) {}

        val result: IntArray =
            when {
                p == -1 -> intArrayOf(-1, -1)
                else -> intArrayOf(p, q)
            }

        private fun check(r: Int): Triple<Int, Int, Int>? =
            when {
                lastPrime == -1 -> null
                r - lastPrime >= diff -> null
                else -> Triple(lastPrime, r, r - lastPrime)
            }

        fun update(r: Int): Acc =
            (check(r) ?: Triple(p, q, diff)).let { (p, q, diff) ->
                copy(lastPrime = r, p = p, q = q, diff = diff)
            }
    }

    fun closestPrimes(
        left: Int,
        right: Int,
    ): IntArray =
        (left..right)
            .asSequence()
            .filter { p -> isPrime(p) }
            .fold(Acc()) { acc, p -> acc.update(p) }
            .result
}