DynamoDB Pagination

PagedResults.kt

// uses value4k: https://github.com/fork-handles/forkhandles/tree/trunk/values4k
class PaginationToken private constructor(value: String) : StringValue(value) {
    companion object : NonBlankStringValueFactory<PaginationToken>(::PaginationToken)
}

data class PagedResult<T>(
    private val result: List<T> = emptyList(),
    val token: PaginationToken? = null
) : List<T> by result {
    companion object
}

/**
 * Eagerly fetches additional results from DynamoDB until:
 * - we retrieved more records than the configured limit
 * - there aren't any more records available (pagination token is null)
 */
fun <T> PagedResults(
    limit: Long,
    initialValue: PaginationToken? = null,
    nextValue: (PaginationToken?) -> PagedResult<T>
): PagedResult<T> {
    val runningTotal = AtomicInteger(0)
    return generateSequence(nextValue(initialValue)) {
        val total = runningTotal.addAndGet(it.size)
        when {
            total >= limit -> null
            it.token == null -> null
            else -> nextValue(it.token)
        }
    }.reduce { acc, b -> PagedResult(acc + b, b.token) }
}