CODEWITHSUNDEEP
kotlin
Ben Green
Ben Green

Reputation: 4121

Kotlin - How to split list of strings based on the total length of characters

I am writing a program in Kotlin that is posting messages to a rest endpoint.

val messages : List<String> = getMessageToSend();
webClient
    .post()
    .uri { builder -> builder.path("/message").build() }
    .bodyValue(PostMessageRequest(
        messages.joinToString("\n")
    ))
    .exchange()
    .block()

However, the rest endpoint has a limit on the maximum size of messages sent. I'm fairly new to Kotlin, but I was looking for a functional way to achieve this, and i'm struggling. I know how I would write this in Java, but i'm keen to do it right. I want to split the messages list into a list of lists, with each list limited to the maximum size allowed and only whole strings added, and then post them individually. I've had a look at methods like chunked, but that doesn't seem flexible enough to achieve what i'm trying to do.

For example, if my message was [this, is, an, example] and the limit was 10, i'd expect my list of lists to be [[this, is an], [example]]

Any suggestions would be massively appreciated.

Upvotes: 4

Views: 3074

Answers (3)

Roland
Roland

Reputation: 1

Really nice. For the fun of it I have written a more "Kotlin idiomatic" version of it. The only complexity here is to put out also the remaining accumulator content when getting to the end of the list.

    fun <T> List<T>.chunkedBy(maxSize: Int, size: T.() -> Int) =
        sequence {
            runningFoldIndexed(emptyList<T>()) { index, acc, t ->
                with(acc + t) { if (sumOf(size) > maxSize) listOf(t).also { yield(acc) } else this }
                    .also { if (index == [email protected] - 1) yield(it) }
            }
        }.toList()

Upvotes: 0

gidds
gidds

Reputation: 18547

This looks rather like a situation I've hit before.  To solve it, I wrote the following general-purpose extension function:

/**
 * Splits a collection into sublists not exceeding the given size.  This is a
 * generalisation of [List.chunked]; but where that limits the _number_ of items in
 * each sublist, this limits their total size, according to a given sizing function.
 *
 * @param maxSize None of the returned lists will have a total size greater than this
 *                (unless a single item does).
 * @param size Function giving the size of an item.
 */
inline fun <T> Iterable<T>.chunkedBy(maxSize: Int, size: T.() -> Int): List<List<T>> {
    val result = mutableListOf<List<T>>()
    var sublist = mutableListOf<T>()
    var sublistSize = 0L
    for (item in this) {
        val itemSize = item.size()
        if (sublistSize + itemSize > maxSize && sublist.isNotEmpty()) {
            result += sublist
            sublist = mutableListOf()
            sublistSize = 0
        }
        sublist.add(item)
        sublistSize += itemSize
    }
    if (sublist.isNotEmpty())
        result += sublist

    return result
}

The implementation's a bit hairy, but it's pretty straightforward to use.  In your case, I expect you'd do something like:

messages.chunkedBy(1024){ length + 1 }
        .map{ it.joinToString("\n") }

to give a list of strings, each no more than 1024 chars*. (The + 1 is of course to allow for the newline characters.)

I'm surprised something like this isn't in the stdlib, to be honest.

(* Unless any of the initial strings is longer.)

Upvotes: 6

deHaar
deHaar

Reputation: 18568

You can split a List into chunks of a given length by using chunked like this

fun main() {
    val messages = listOf(1, 2, 3, 4, 5, 6)
    val chunks = messages.chunked(3)
    println("$messages ==> $chunks")
}

This prints

[1, 2, 3, 4, 5, 6] ==> [[1, 2, 3], [4, 5, 6]]

Upvotes: 1

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