Kotlin: Convert large List to sublist of set partition size

NOTE: For Kotlin 1.2 and newer, please see the chunked and windowed functions that are now in the standard library. There is no need for a custom solution.

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Here is an implementation of a lazy batching extension function which will take a collection, or anything that can become a Sequence and return a Sequence of List each of that size, with the last one being that size or smaller.

Example usage to iterate a list as batches:

myList.asSequence().batch(5).forEach { group ->
   // receive a Sequence of size 5 (or less for final)
}

Example to convert batches of List to Set:

myList.asSequence().batch(5).map { it.toSet() }

See the first test case below for showing the output given specific input.

Code for the function Sequence<T>.batch(groupSize):

public fun <T> Sequence<T>.batch(n: Int): Sequence<List<T>> {
    return BatchingSequence(this, n)
}

private class BatchingSequence<T>(val source: Sequence<T>, val batchSize: Int) : Sequence<List<T>> {
    override fun iterator(): Iterator<List<T>> = object : AbstractIterator<List<T>>() {
        val iterate = if (batchSize > 0) source.iterator() else emptyList<T>().iterator()
        override fun computeNext() {
            if (iterate.hasNext()) setNext(iterate.asSequence().take(batchSize).toList())
            else done() 
        }
    }
}

Unit tests proving it works:

class TestGroupingStream {

    @Test fun testConvertToListOfGroupsWithoutConsumingGroup() {
        val listOfGroups = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10).asSequence().batch(2).toList()
        assertEquals(5, listOfGroups.size)
        assertEquals(listOf(1,2), listOfGroups[0].toList())
        assertEquals(listOf(3,4), listOfGroups[1].toList())
        assertEquals(listOf(5,6), listOfGroups[2].toList())
        assertEquals(listOf(7,8), listOfGroups[3].toList())
        assertEquals(listOf(9,10), listOfGroups[4].toList())
    }

    @Test fun testSpecificCase() {
        val originalStream = listOf(1,2,3,4,5,6,7,8,9,10)

        val results = originalStream.asSequence().batch(3).map { group ->
            group.toList()
        }.toList()

        assertEquals(listOf(1,2,3), results[0])
        assertEquals(listOf(4,5,6), results[1])
        assertEquals(listOf(7,8,9), results[2])
        assertEquals(listOf(10), results[3])
    }


    fun testStream(testList: List<Int>, batchSize: Int, expectedGroups: Int) {
        var groupSeenCount = 0
        var itemsSeen = ArrayList<Int>()

        testList.asSequence().batch(batchSize).forEach { groupStream ->
            groupSeenCount++
            groupStream.forEach { item ->
                itemsSeen.add(item)
            }
        }

        assertEquals(testList, itemsSeen)
        assertEquals(groupSeenCount, expectedGroups)
    }

    @Test fun groupsOfExactSize() {
        testStream(listOf(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15), 5, 3)
    }

    @Test fun groupsOfOddSize() {
        testStream(listOf(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18), 5, 4)
        testStream(listOf(1,2,3,4), 3, 2)
    }

    @Test fun groupsOfLessThanBatchSize() {
        testStream(listOf(1,2,3), 5, 1)
        testStream(listOf(1), 5, 1)
    }

    @Test fun groupsOfSize1() {
        testStream(listOf(1,2,3), 1, 3)
    }

    @Test fun groupsOfSize0() {
        val testList = listOf(1,2,3)

        val groupCountZero =   testList.asSequence().batch(0).toList().size
        assertEquals(0, groupCountZero)

        val groupCountNeg =  testList.asSequence().batch(-1).toList().size
        assertEquals(0, groupCountNeg)

    }

    @Test fun emptySource() {
        listOf<Int>().asSequence().batch(1).forEach { groupStream ->
            fail()
        }

    }
}

With Kotlin 1.3, according to your needs, you may choose one of the following ways to solve your problem.

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#1. Using chunked

fun main() {
    val list = listOf(2, 4, 3, 10, 8, 7, 9)
    val newList = list.chunked(2)
    //val newList = list.chunked(size = 2) // also works
    print(newList)
}

/*
prints:
[[2, 4], [3, 10], [8, 7], [9]]
*/

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#2. Using windowed

fun main() {
    val list = listOf(2, 4, 3, 10, 8, 7, 9)
    val newList = list.windowed(2, 2, true)
    //val newList = list.windowed(size = 2, step = 2, partialWindows = true) // also works
    println(newList)
}

/*
prints:
[[2, 4], [3, 10], [8, 7], [9]]
*/