multiDistinctBy function for Kotlin: Like stdlib distinctBy but for multiple fields

MultiDistinctBy.kt

fun <T, K> Iterable<T>.multiDistinctBy(vararg selectors: (T) -> K): List<T> {
    require(selectors.isNotEmpty())
    val set = HashSet<Int>()
    val distinct = ArrayList<T>()

    for (element in this) {
        val key = selectors.fold(0) { sum, selector ->
            sum.plus(selector(element)?.hashCode() ?: 0) }

        if (set.add(key))
            distinct.add(element)
    }
    return distinct
}

MultiDistinctByTest.kt

import org.junit.Assert.assertEquals
import org.junit.Test

class MultiDistinctByTest {

    data class Item(val code: String, val name: String?, val value: Int)

    @Test
    fun multiDistinctBy_byTwoStringSelectors_shouldReturnOnlyDistinctValues() {
        val someItems = listOf(
                Item("1", "Item 1", 34),
                Item("1", "Item 1", 36),
                Item("2", "Item 1", 38),
                Item("2", "Item 2", 40),
                Item("2", "Item 3", 42),
                Item("2", "", 44),
                Item("2", "", 46)
        )

        val expected = listOf(
                Item("1", "Item 1", 34),
                Item("2", "Item 1", 38),
                Item("2", "Item 2", 40),
                Item("2", "Item 3", 42),
                Item("2", "", 44)
        )

        val distinct = someItems.multiDistinctBy({it.code}, {it.name})

        assertEquals(expected, distinct)
    }

    @Test
    fun multiDistinctBy_withNullValue_shouldIncludeNullValue() {
        val someItems = listOf(
                Item("1", "Item 1", 34),
                Item("1", "Item 1", 36),
                Item("2", null, 38),
                Item("2", "Item 2", 40)
        )

        val expected = listOf(
                Item("1", "Item 1", 34),
                Item("2", null, 38),
                Item("2", "Item 2", 40)
        )

        val distinct = someItems.multiDistinctBy({it.code}, {it.name})

        assertEquals(expected, distinct)
    }

    @Test
    fun multiDistinctBy_withTwoNullValuesInSelectorField_shouldReturnOnlyOneNullValue() {
        val someItems = listOf(
                Item("1", "Item 1", 34),
                Item("1", "Item 1", 36),
                Item("2", null, 38),
                Item("2", null, 40)
        )

        val expected = listOf(
                Item("1", "Item 1", 34),
                Item("2", null, 38)
        )

        val distinct = someItems.multiDistinctBy({it.code}, {it.name})

        assertEquals(expected, distinct)
    }

    @Test
    fun multiDistinctBy_withStringAndNumeric_shouldReturnDistinctByStringAndNumericFields() {
        val someItems = listOf(
                Item("1", "Item 1", 34),
                Item("1", "Item 1", 34),
                Item("2", null, 38),
                Item("2", null, 40)
        )

        val expected = listOf(
                Item("1", "Item 1", 34),
                Item("2", null, 38),
                Item("2", null, 40)
        )

        val distinct = someItems.multiDistinctBy({it.code}, {it.value})

        assertEquals(expected, distinct)
    }

    @Test
    fun multiDistinctBy_allFieldsAreDifferent_shouldReturnOriginalList() {
        val someItems = listOf(
                Item("1", "Item 1", 34),
                Item("1", "Item 2", 34),
                Item("2", null, 38),
                Item("2", null, 40)
        )

        val distinct = someItems.multiDistinctBy({it.code}, {it.name}, {it.value})

        assertEquals(someItems, distinct)
    }

}

@danielocampo2 a "blast from the past" xD very nice
Don't feel obligated however to fix this. I am now using distinctBy { listof(...) }, it appears to be working for the time being.