package com.ustadmobile.libcache.downloader

import com.ustadmobile.libcache.EntryLockRequest
import com.ustadmobile.libcache.UstadCache
import com.ustadmobile.libcache.db.UstadCacheDb
import com.ustadmobile.libcache.db.entities.DownloadJobItem
import com.ustadmobile.libcache.db.entities.PinnedPublication
import com.ustadmobile.libcache.db.entities.TransferJobItemStatus
import com.ustadmobile.libcache.util.LaunchNoVarySearchConstants
import com.ustadmobile.libcache.util.withWriterTransaction
import io.ktor.client.HttpClient
import io.ktor.client.call.body
import io.ktor.client.request.get
import io.ktor.client.request.head
import io.ktor.client.request.header
import io.ktor.http.contentLength
import io.ktor.http.headersOf
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.async
import kotlinx.coroutines.awaitAll
import kotlinx.coroutines.channels.Channel
import kotlinx.coroutines.channels.produce
import kotlinx.coroutines.coroutineScope
import world.respect.lib.opds.model.OpdsPublication
import world.respect.lib.opds.model.ReadiumLink
import world.respect.lib.opds.model.findLearningUnitAcquisitionLinks
import world.respect.libutil.ext.resolve
import java.util.concurrent.CopyOnWriteArrayList

/**
 * Fetches the publication manifest (over http) and goes through all of the resource links.
 * For each resource:
 *
 *  a) Create a DownloadJobItem
 *  b) Add a RetentionLock
 *  c) If the resource does not have a size provided in the ReadiumLink, then make an HTTP HEAD request
 *     to get the size and update the TransferJobItem accordingly.
 *  d) Enqueue RunDownloadJob to run the actual download.
 */
@OptIn(ExperimentalCoroutinesApi::class)
class PinPublicationPrepareUseCase(
    private val httpClient: HttpClient,
    private val db: UstadCacheDb,
    private val cache: UstadCache,
    private val enqueueRunDownloadJobUseCase: EnqueueRunDownloadJobUseCase,
) {


    /**
     * This is here because some manifests contain spaces in hrefwhich should NOT be there.
     */
    fun String.cleanHref(): String {
        return this.replace(" ", "%20")
    }

    /**
     *
     */
    suspend operator fun invoke(
        downloadJobUid: Int
    ) {
        val downloadJob = db.downloadJobDao.findByUid(downloadJobUid)
            ?: throw IllegalArgumentException("No transfer job with uid $downloadJobUid")
        val manifestJobItem = db.downloadJobItemDao.findPendingByJobUid(downloadJobUid).first()
        db.downloadJobDao.updateStatus(
            jobUid = downloadJobUid,
            status = TransferJobItemStatus.STATUS_PREPARING_INT,
        )

        val manifestUrl = downloadJob.djPubManifestUrl
            ?: throw IllegalArgumentException("no manifest url")

        val publication: OpdsPublication = httpClient.get(manifestUrl).body()

        val resourceAndAcquireJobItems = buildList {
            val acquisitionLinks = publication.findLearningUnitAcquisitionLinks()

            acquisitionLinks.forEach { acquisitionLink ->
                cache.setExtraResponseHeaders(
                    url = manifestUrl.resolve(acquisitionLink.href),
                    extraResponseHeaders = headersOf(
                        "No-Vary-Search" to listOf(LaunchNoVarySearchConstants.LAUNCH_LINK_NO_VARY_HEADER)
                    )
                )
            }

            val linksToDownload = buildList {
                publication.resources?.also { addAll(it) }
                addAll(acquisitionLinks)
                add(ReadiumLink(href = manifestUrl.toString()))
                publication.images?.also { addAll(it) }
            }

            addAll(
                linksToDownload.map { resource ->
                    DownloadJobItem(
                        djiDjUid = downloadJobUid,
                        djiUrl = manifestUrl.resolve(resource.href.cleanHref()),
                        djiTotalSize = (resource.size ?: 0).toLong()
                    )
                }
            )

            add(manifestJobItem)
        }.distinctBy { item -> item.djiUrl }

        val downloadJobItemWithSize: MutableList<DownloadJobItem> = CopyOnWriteArrayList()

        coroutineScope {
            val jobItemProducer = produce(
                capacity = Channel.UNLIMITED
            ) {
                resourceAndAcquireJobItems.forEach { send(it) }
                close()
            }

            val jobs = (1.. PARALLEL_SIZE_FETCH_LIMIT).map {
                async {
                    for (item in jobItemProducer) {
                        val jobItemWithSize = if(item.djiTotalSize <= 0) {
                            val headResponse = httpClient.head(item.djiUrl) {
                                header("cache-control", "no-cache, no-store")
                            }
                            val contentLength = headResponse.contentLength()

                            item.copy(
                                djiTotalSize = contentLength ?: 0
                            )
                        }else {
                            item
                        }
                        downloadJobItemWithSize.add(jobItemWithSize)
                    }
                }
            }

            jobs.awaitAll()
        }

        cache.addRetentionLocks(
            downloadJobItemWithSize.map {
                EntryLockRequest(
                    url = it.djiUrl.toString(),
                    publicationUid = downloadJob.djPubManifestHash,
                )
            }
        )

        db.withWriterTransaction {
            db.downloadJobItemDao.upsertList(downloadJobItemWithSize)
            db.pinnedPublicationDao.insert(
                PinnedPublication(
                    ppUrlHash = downloadJob.djPubManifestHash,
                    title = downloadJob.djName ?: ""
                )
            )
        }

        enqueueRunDownloadJobUseCase(downloadJobUid)
    }


    companion object {

        const val PARALLEL_SIZE_FETCH_LIMIT = 8

        const val DEFAULT_MAX_ATTEMPTS = 5

    }
}