Solution: Workout manager

import java.io.File

interface NextRepetitionsRepository {
    fun read(): Repetitions?
    fun write(repetitions: Repetitions)
}

class FileNextRepetitionsRepository: NextRepetitionsRepository {
    private val store = File("next_repetitions.txt")

    override fun read(): Repetitions? {
        if (!store.exists()) return null
        val content = store.readLines()
        val firstRound = content.getOrNull(0)?.toIntOrNull() 
            ?: return null
        val secondRound = content.getOrNull(1)?.toIntOrNull() 
            ?: return null
        return Repetitions(
            firstRound = firstRound,
            secondRound = secondRound,
        )
    }
    
    override fun write(repetitions: Repetitions) {
        if (!store.exists()) store.createNewFile()
        store.writeText("${repetitions.firstRound}\n"+
            "${repetitions.secondRound}")
    }
}

data class Repetitions(val firstRound: Int, val secondRound: Int)

class WorkoutManager {
    fun nextRepetitions(
        firstRoundDone: Int,
        secondRoundDone: Int,
        thirdRoundDone: Int,
        repetitions: Repetitions,
    ): Repetitions = when {
        firstRoundDone < repetitions.firstRound -> {
            // Could not do enough push-ups in the first round
            repetitions.copy(
                firstRound = repetitions.firstRound - 1, 
                secondRound = repetitions.secondRound - 1
            )
        }
        secondRoundDone < repetitions.secondRound -> {
            // Could not do enough push-ups in the second round
            repetitions.copy(
                secondRound = repetitions.secondRound - 1
            )
        }
        thirdRoundDone > repetitions.firstRound -> {
            // The third round is above the first round, 
            // so we increase the repetitions for both rounds
            repetitions.copy(
                firstRound = repetitions.firstRound + 1,
                secondRound = repetitions.secondRound + 1,
            )
        }
        thirdRoundDone > repetitions.secondRound -> {
            // The third round is below the first round,
            // but above the second round, so we increase
            // the repetitions for the second round
            repetitions.copy(
                secondRound = repetitions.secondRound + 1,
            )
        }
        else -> repetitions
    }
}

class WorkoutInteractor(
    val manager: WorkoutManager,
    val nextRepetitionsRepository: NextRepetitionsRepository
) {
    fun start() {
        println("Hello, I am your push-ups workout assistant!")
        val repetitions = nextRepetitionsRepository.read() 
            ?: Repetitions(5, 5)
        println("Now do ${repetitions.firstRound} push-ups")
        val firstRoundDone = getValidIntInput(
              "How many push-ups did you do?"
        )
        println("Now rest for 1 minute")
        Thread.sleep(1000 * 60)
        println("Now do ${repetitions.secondRound} push-ups")
        val secondRoundDone = getValidIntInput(
              "How many push-ups did you do?"
        )
        println("Now rest for 1 minute")
        Thread.sleep(1000 * 60)
        println("Now do as many push-ups as you can!")
        val thirdRoundDone = getValidIntInput(
            "How many push-ups did you do?"
        )
        val next = manager.nextRepetitions(
            firstRoundDone = firstRoundDone,
            secondRoundDone = secondRoundDone,
            thirdRoundDone = thirdRoundDone,
            repetitions = repetitions,
        )
        nextRepetitionsRepository.write(next)
        println("Your next repetitions will be: "+
            "${next.firstRound} and ${next.secondRound}")
    }
    
    private fun getValidIntInput(prompt: String): Int {
        while (true) {
            println(prompt)
            val input = readlnOrNull()?.toIntOrNull()
            if (input != null && input >= 0) {
                return input
            }
            println("Please enter a valid number.")
        }
    }
}

fun main() {
    val interactor = WorkoutInteractor(
        manager = WorkoutManager(),
        nextRepetitionsRepository = 
            FileNextRepetitionsRepository(),
    )
    interactor.start()
}