sum-array.kt

import kotlin.reflect.KFunction1
import kotlin.test.assertEquals

fun main() {
    assertSums(::makeSum)
    assertSums(::makeSum2)
    assertSums(::makeSum3)
}

fun assertSums(kFunction1: KFunction1<List<Int>, List<Int>>) {
    assertEquals(
        actual = kFunction1(listOf(1, 2, 3, 4)),
        expected = listOf(1, 3, 6, 10)
    )
    assertEquals(
        actual = kFunction1(emptyList()),
        expected = emptyList()
    )
}

fun makeSum(list: List<Int>): List<Int> {
    if (list.isEmpty()) return emptyList()

    val summed = IntArray(list.size)
    summed[0] = list[0]
    for (i in 1 until list.size) {
        summed[i] = list[i] + summed[i - 1]
    }
    return summed.asList()
}

fun makeSum2(list: List<Int>): List<Int> {
    return indexedSequence(list.firstOrNull()) { index, prev ->
        prev + list[index]
    }
        .take(list.size)
        .toList()
}

inline fun <T : Any> indexedSequence(
    seed: T?,
    crossinline nextFunction: (Int, T) -> T?
): Sequence<T> {
    var index = 0
    return generateSequence(seed) { prev ->
        index++
        nextFunction(index, prev)
    }
}

fun makeSum3(list: List<Int>): List<Int> {
    if (list.isEmpty()) return emptyList()

    val size = list.size
    val summed = IntArray(size)

    tailrec fun sumIt(index: Int) {
        if (index == size) return
        summed[index] = summed[index - 1] + list[index]
        sumIt(index + 1)
    }

    summed[0] = list[0]
    sumIt(1)
    return summed.toList()
}