Edenbridge Coders March 5th

ListTest.kt

import arrow.core.None
import arrow.core.Option
import arrow.core.Some
import org.junit.Test

class ListTest {
    @Test
    fun `a new list should be empty`() {
        assert(length(createNewList()) == 0)
    }

    @Test
    fun `adding an item to an empty list should return a list of length 1`() {
        assert(length(add(1, createNewList())) == 1)
    }

    @Test
    fun `adding two items to an empty list should return a list of length 2`() {
        assert(length(add(3, add(1, createNewList()))) == 2)
    }

    @Test
    fun `adding three items to an empty list should return a list of length 3`() {
        assert(length(add(5, add(3, add(1, createNewList())))) == 3)
    }

    @Test
    fun `given a list of 3 items should be able to get the first item`() {
        val list = add(5, add(3, add(1, createNewList())))
        assert(get(0, list) == Some(5))
    }

    @Test
    fun `given a list of 3 items should be able to get the second item`() {
        val list = add(5, add(3, add(1, createNewList())))
        assert(get(1, list) == Some(3))
    }

    @Test
    fun `given an empty list should return none when getting first item`() {
        val list = createNewList()
        assert(get(0, list) == None)
    }

    @Test
    fun `should return none when fetching an item at an index less than zero`() {
        val list = add(1, createNewList())
        assert(get(-1, list) == None)
    }

    @Test
    fun `given a list of one item should be able to remove the last`() {
        val list = add(1, createNewList())
        assert(remove(0, list) == createNewList())
    }

    @Test
    fun `given a list of two items should be able to remove the last`() {
        val list = add(3, add(1, createNewList()))
        assert(remove(1, list) == add(3, createNewList()))
    }

    @Test
    fun `given a list of three items should be able to remove the last`() {
        val list = add(5, add(3, add(1, createNewList())))
        assert(remove(2, list) == add(5, add(3, createNewList())))
    }

    @Test
    fun `given a list of three items should be able to remove the second item`() {
        val list = add(5, add(3, add(1, createNewList())))
        assert(remove(1, list) == add(5, add(1, createNewList())))
    }
}

fun remove(i: Int, list: List): List {
    return when (list) {
        List.Nil -> List.Nil
        is List.Cons ->
            if (i == 0) list.tail
            else add(list.value, remove(i - 1, list.tail))
    }
}

fun get(i: Int, list: List): Option<Int> {
    return when (list) {
        List.Nil -> None
        is List.Cons ->
            if (i == 0) Some(list.value)
            else get(i - 1, list.tail)
    }
}

sealed class List {
    object Nil : List()
    data class Cons(val value: Int, val tail: List) : List()
}

fun add(i: Int, list: List): List {
    return List.Cons(i, list)
}

fun length(list: List): Int {
    return when (list) {
        List.Nil -> 0
        is List.Cons -> 1 + length(list.tail)
    }
}

fun createNewList(): List {
    return List.Nil
}