minutest-after-TDD.kt

class PartitionTests : JUnit5Minutests {

    override val tests = rootContext<Unit> {
        test("empty input and predicates") {
            val list = emptyList<Int>()
            val predicates = emptyList<(Int) -> Boolean>()
            val result = list.partition(predicates)
            assertEquals(emptyList<List<List<Int>>>(), result)
        }
        test("empty input some predicates") {
            val list = emptyList<Int>()
            val predicates = listOf(::isNegative, ::isZero, ::isPositive)
            val result = list.partition(predicates)
            assertEquals(listOf(emptyList<Int>(), emptyList(), emptyList()), result)
        }
        test("items but no predicates") {
            val list = listOf(-1, 0, 1, 2, 3)
            val predicates = emptyList<(Int) -> Boolean>()
            val result = list.partition(predicates)
            assertEquals(emptyList<List<Int>>(), result)
        }
        test("not empty") {
            val list = listOf(-1, 0, 1, 2, 3)
            val predicates = listOf(::isNegative, ::isZero, ::isPositive)
            val result = list.partition(predicates)
            assertEquals(listOf(listOf(-1), listOf(0), listOf(1, 2, 3)), result)
        }
        test("a predicate doesn't match any item") {
            val list = listOf(-1, 1, 2, 3)
            val predicates = listOf(::isNegative, ::isZero, ::isPositive)
            val result = list.partition(predicates)
            assertEquals(listOf(listOf(-1), listOf(), listOf(1, 2, 3)), result)
        }
        test("an item doesn't match any predicate") {
            val list = listOf(-1, 0, 1, 2, 3)
            val predicates = listOf(::isNegative, ::isPositive)
            val result = list.partition(predicates)
            assertEquals(listOf(listOf(-1), listOf(1, 2, 3)), result)
        }
        test("input order is preserved") {
            val list = listOf(3, 2, 1, 0, -1)
            val predicates = listOf(::isNegative, ::isZero, ::isPositive)
            val result = list.partition(predicates)
            assertEquals(listOf(listOf(-1), listOf(0), listOf(3, 2, 1)), result)
        }
    }
}

fun isNegative(x: Int) = x < 0
fun isZero(x: Int) = x == 0
fun isPositive(x: Int) = x > 0

fun <T> Iterable<T>.partition(predicates: List<(T) -> Boolean>): List<List<T>> {
    val groups = groupBy { item -> predicates.firstMatch(item) }
    return predicates.map { predicate -> groups.getOrDefault(predicate, emptyList()) }
}

private fun <T> List<(T) -> Boolean>.firstMatch(item: T) = find { predicate -> predicate(item) }