CalculatorSpec.kt

package com.redspace.NamedMonads

import org.junit.Assert.assertEquals
import org.junit.runner.RunWith

@RunWith(SpecRunner::class)
class CalculatorTestsUsingRunner {

    val givenCalculator = "calculator" sut given {
        Calculator()
    }

    val noop = "nothing" define {
        it: Calculator ->
    }

    fun press(key: Int) = "press $key" define {
        it: Calculator ->
        it.press(key)
    }

    fun press(key: Char) = "press '$key'" define {
        it: Calculator ->
        it.press(key)
    }

    fun reads(n: Int) = "reads $n" define {
        it: Calculator ->
        assertEquals(n, it.reads())
    }

    @Spec
    fun `Zero at start`() = givenCalculator whenever noop then reads(0)

    @Spec
    fun `First digit press`() = givenCalculator whenever press(1) then reads(1)

    @Spec
    fun `Second digit press`() = givenCalculator given press(1) whenever press(2) then reads(12)

    @Spec
    fun `Add press doesn't clear`() = givenCalculator given press(1) whenever press('+') then reads(1)

    @Spec
    fun `Add before equals`() = givenCalculator given press(1) given press('+') whenever press(2) then reads(2)

    @Spec
    fun `One digit add`() = givenCalculator given press(1) given press('+') given press(2) whenever press('=') then reads(3)

    @Spec
    fun `Two digit add`() = givenCalculator given press(1) given press(2) given press('+') given press(3) given press(4) whenever press('=') then reads(12 + 34)

}

class Calculator {

    var buffer = 0
    var display = 0
    var clearNextPress = false
    var operation = ' '

    fun press(i: Int) {
        if (clearNextPress) {
            display = 0
            clearNextPress = false
        }
        display = display * 10 + i
    }

    fun reads(): Int {
        return display
    }

    fun press(operation: Char) {
        when (operation) {
            '+' -> {
                buffer = display
                this.operation = operation
                clearNextPress = true
            }
            '=' -> display += buffer
        }
    }

}