Transforming a Flow to a Flow in Kotlin

Question

Consider I have a cold source of UTF-8 bytes (e. g.: reading a file on disk, or the body of an HTTP response), in a form of a Flow<Byte>. How do I convert the above source to a flow of strings?

In other words, I want the following behaviour:

        /*
         * A multi-line string, not terminated with a newline character.
         */
        val string = """
            first line
            第二行
            третья строка
        """.trimIndent()

        assertNotEquals('\n', string.last())
        assertEquals(2, string.asSequence().count { it == '\n' })

        val source: Flow<Byte> = string.toByteArray().asSequence().asFlow()

        val transformed: Flow<String> = TODO()

        val target = runBlocking {
            transformed.toList(mutableListOf()).toTypedArray()
        }

        assertArrayEquals(
            arrayOf("first line", "第二行", "третья строка"),
            target
        )

As an extra restriction, this is a Kotlin/JS project, so java.io APIs can't be used.

Bass · Accepted Answer · 2023-02-13T14:55:24.433

Eventually, I came up with the following solution:

fun Flow<Byte>.decodeToString(): Flow<String> =
    flow {
        val buffer: MutableList<Byte> = arrayListOf()

        collect { value ->
            when (value) {
                /*
                 * Ignore.
                 */
                '\r'.code.toByte() -> Unit

                '\n'.code.toByte() -> {
                    emit(buffer)
                    buffer.clear()
                }

                else -> buffer.add(value)
            }
        }

        if (buffer.isNotEmpty()) {
            emit(buffer)
        }
    }
        .map(Collection<Byte>::toByteArray)
        .map(ByteArray::decodeToString)

The ArrayList<Byte> above can be replaced with either okio.Buffer from okio or kotlinx.io.core.BytePacketBuilder from kotlinx-io, e.g.:

import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.flow
import okio.Buffer

fun Flow<Byte>.decodeToString(): Flow<String> =
    flow {
        val buffer = Buffer()

        collect { value ->
            when (value) {
                /*
                 * Ignore.
                 */
                '\r'.code.toByte() -> Unit

                '\n'.code.toByte() -> {
                    emit(buffer.readUtf8())
                    buffer.clear()
                }

                else -> buffer.writeByte(value.toInt())
            }
        }

        if (buffer.size > 0) {
            emit(buffer.readUtf8())
        }
    }

Transforming a Flow to a Flow in Kotlin

1 Answers1