Interview.kt

import org.junit.jupiter.api.Test
import kotlin.test.assertEquals

class Interview {
    val input = "aaaabbbcca"
    val expectedOutput = listOf('a' to 4, 'b' to 3, 'c' to 2, 'a' to 1)

    @Test
    fun `interview - fold`() {

        val output = input.fold(listOf<Pair<Char, Int>>()) { acc, c -> acc.inc(c) }

        assertEquals(expectedOutput, output)
    }

    fun List<Pair<Char, Int>>.inc(c: Char): List<Pair<Char, Int>> {
        val prev = lastOrNull().takeIf { it?.first == c }
        return when {
            size == 0 -> this + (c to 1)
            prev == null -> this + (c to 1)
            else -> this.dropLast(1) + (c to prev.second + 1)
        }
    }

    @Test
    fun `interview - fold plus`() {

        val output = input.fold(listOf<Pair<Char, Int>>()) { acc, c -> acc + c }

        assertEquals(expectedOutput, output)
    }

    operator fun List<Pair<Char, Int>>.plus(c: Char): List<Pair<Char, Int>> =
        inc(c)

    @Test
    fun `interview tailrec`() {

        val output = listOf<Pair<Char, Int>>().count(input)

        assertEquals(expectedOutput, output)
    }

    tailrec fun List<Pair<Char, Int>>.count(remaining: String): List<Pair<Char, Int>> =
        when (val c = remaining.firstOrNull()) {
            null -> this
            else -> inc(c).count(remaining.drop(1))
        }

    @Test
    fun `interview mutable`() {

        val output = countMutable(input)

        assertEquals(expectedOutput, output)
    }

    fun countMutable(input: String): List<Pair<Char, Int>> {
        var accumulator = emptyList<Pair<Char, Int>>()
        for (c in input) {
            accumulator = accumulator.inc(c)
        }
        return accumulator
    }
    fun countMutable2(input: String): List<Pair<Char, Int>> {
        var accumulator = emptyList<Pair<Char, Int>>()
        for (c in input) {
            accumulator += c
        }
        return accumulator
    }
}