Visitor.kt

import org.assertj.core.api.Assertions.assertThat
import org.junit.jupiter.api.Test

interface ReportVisitable {
    fun <R> accept(visitor: ReportVisitor<R>): R
}

class FixedPriceContract(val costPerYear: Long) : ReportVisitable {
    override fun <R> accept(visitor: ReportVisitor<R>): R = visitor.visit(this)
}

class TimeAndMaterialsContract(val costPerHour: Long, val hours: Long) : ReportVisitable {
    override fun <R> accept(visitor: ReportVisitor<R>): R = visitor.visit(this)
}

class SupportContract(val costPerMonth: Long) : ReportVisitable {
    override fun <R> accept(visitor: ReportVisitor<R>): R = visitor.visit(this)
}

interface ReportVisitor<out R> {

    fun visit(contract: FixedPriceContract): R
    fun visit(contract: TimeAndMaterialsContract): R
    fun visit(contract: SupportContract): R
}

class MonthlyCostReportVisitor : ReportVisitor<Long> {

    override fun visit(contract: FixedPriceContract): Long =
        contract.costPerYear / 12

    override fun visit(contract: TimeAndMaterialsContract): Long =
        contract.costPerHour * contract.hours

    override fun visit(contract: SupportContract): Long =
        contract.costPerMonth
}

class YearlyReportVisitor : ReportVisitor<Long> {

    override fun visit(contract: FixedPriceContract): Long =
        contract.costPerYear

    override fun visit(contract: TimeAndMaterialsContract): Long =
        contract.costPerHour * contract.hours

    override fun visit(contract: SupportContract): Long =
        contract.costPerMonth * 12
}

class VisitorTest {

    @Test
    fun Visitor() {
        val projectAlpha = FixedPriceContract(costPerYear = 10000)
        val projectBeta = SupportContract(costPerMonth = 500)
        val projectGamma = TimeAndMaterialsContract(hours = 150, costPerHour = 10)
        val projectKappa = TimeAndMaterialsContract(hours = 50, costPerHour = 50)

        val projects = arrayOf(projectAlpha, projectBeta, projectGamma, projectKappa)

        val monthlyCostReportVisitor = MonthlyCostReportVisitor()

        val monthlyCost = projects.map { it.accept(monthlyCostReportVisitor) }
            .sum()
        println("Monthly cost: $monthlyCost")
        assertThat(monthlyCost).isEqualTo(5333)

        val yearlyReportVisitor = YearlyReportVisitor()
        val yearlyCost = projects.map { it.accept(yearlyReportVisitor) }
            .sum()
        println("Yearly cost: $yearlyCost")
        assertThat(yearlyCost).isEqualTo(20000)
    }
}