SyncFlow/app/src/main/kotlin/com/syncflow/domain/sync/SyncEngine.kt

package com.syncflow.domain.sync

import android.content.Context
import android.net.Uri
import com.syncflow.data.db.SyncConflictDao
import com.syncflow.data.db.SyncEventDao
import com.syncflow.data.db.SyncFileStateDao
import com.syncflow.data.db.SyncPairDao
import com.syncflow.data.db.entities.SyncConflictEntity
import com.syncflow.data.db.entities.SyncEventEntity
import com.syncflow.data.db.entities.SyncFileStateEntity
import com.syncflow.data.providers.CloudProvider
import com.syncflow.domain.model.ConflictStrategy
import com.syncflow.domain.model.DeleteBehavior
import com.syncflow.domain.model.RemoteFile
import com.syncflow.domain.model.SyncDirection
import com.syncflow.domain.model.SyncEventType
import com.syncflow.domain.model.SyncPair
import com.syncflow.domain.model.SyncStatus
import dagger.hilt.android.qualifiers.ApplicationContext
import kotlinx.coroutines.*
import kotlinx.coroutines.sync.Semaphore
import kotlinx.coroutines.sync.withPermit
import timber.log.Timber
import java.io.File
import java.time.Instant
import javax.inject.Inject

class SyncEngine @Inject constructor(
    private val syncPairDao: SyncPairDao,
    private val fileStateDao: SyncFileStateDao,
    private val conflictDao: SyncConflictDao,
    private val eventDao: SyncEventDao,
    @ApplicationContext private val context: Context,
) {
    suspend fun sync(pair: SyncPair, provider: CloudProvider): SyncResult {
        syncPairDao.updateStatus(pair.id, SyncStatus.SYNCING)
        logEvent(pair.id, SyncEventType.SYNC_STARTED, null, null, 0)

        return try {
            val result = performSync(pair, provider)
            val finalStatus = when {
                result.failedFiles > 0 && result.conflicts > 0 -> SyncStatus.CONFLICT
                result.failedFiles > 0 -> SyncStatus.PARTIAL
                result.conflicts > 0 -> SyncStatus.CONFLICT
                else -> SyncStatus.SUCCESS
            }
            syncPairDao.updateSyncResult(pair.id, Instant.now(), finalStatus, result.conflicts)
            logEvent(pair.id, SyncEventType.SYNC_COMPLETED, null, "↑${result.uploaded} ↓${result.downloaded} ✕${result.deleted} ✗${result.failedFiles}", result.bytesTransferred)
            result
        } catch (e: Exception) {
            Timber.e(e, "Sync failed for pair ${pair.id}")
            syncPairDao.updateSyncResult(pair.id, Instant.now(), SyncStatus.FAILED, 0)
            logEvent(pair.id, SyncEventType.SYNC_FAILED, null, e.message, 0)
            SyncResult(failedFiles = 1, error = e)
        }
    }

    private fun makeAccessor(localPath: String): LocalAccessor =
        if (localPath.startsWith("content://"))
            LocalAccessor.Saf(Uri.parse(localPath), context.contentResolver)
        else
            LocalAccessor.JavaFile(File(localPath))

    private suspend fun performSync(
        pair: SyncPair,
        provider: CloudProvider,
        isRetry: Boolean = false,
    ): SyncResult {
        val accessor = makeAccessor(pair.localPath)
        var knownStates = fileStateDao.getForPair(pair.id).associateBy { it.relativePath }
        val remoteFiles = provider.listFiles(pair.remotePath).getOrThrow()
            .associateBy { it.path.removePrefix(pair.remotePath).trimStart('/') }
        val localFiles = accessor.walkFiles(pair)

        // Self-healing: if every known-state path is absent from the current local scan but
        // the local folder does have files, the localPath was changed without clearing state.
        // The stale records would cause every old file to look like "DELETE_REMOTE" and every
        // new file to re-upload indefinitely. Wipe and retry once as a fresh initial sync.
        if (!isRetry && knownStates.isNotEmpty() && localFiles.isNotEmpty() &&
            knownStates.keys.none { it in localFiles }) {
            Timber.w("SyncEngine: stale folder states detected for pair ${pair.id} — resetting")
            fileStateDao.deleteForPair(pair.id)
            return performSync(pair, provider, isRetry = true)
        }

        val allPaths = (localFiles.keys + remoteFiles.keys + knownStates.keys).toSet()
        val hasPriorSyncState = knownStates.isNotEmpty()
        val semaphore = Semaphore(4)

        // Each async block returns its outcome; no shared mutable state across coroutines.
        data class FileOutcome(
            val uploaded: Int = 0, val downloaded: Int = 0, val deleted: Int = 0,
            val skipped: Int = 0, val failed: Int = 0, val conflicts: Int = 0,
            val bytesTransferred: Long = 0L,
            val newState: SyncFileStateEntity? = null,
        )

        val outcomes: List<FileOutcome> = coroutineScope {
            allPaths.map { rel ->
                async {
                    semaphore.withPermit {
                        val local = localFiles[rel]
                        val remote = remoteFiles[rel]
                        val known = knownStates[rel]
                        val decision = syncDecide(pair.syncDirection, pair.conflictStrategy, pair.deleteBehavior, local, remote, known, hasPriorSyncState)

                        when (decision) {
                            SyncDecision.UPLOAD -> {
                                var uploadedRemoteFile: RemoteFile? = null
                                val bytes = runCatching {
                                    ensureRemoteDirs(provider, pair.remotePath, rel)
                                    accessor.openInputStream(rel)?.use { stream ->
                                        uploadedRemoteFile = provider.uploadFile(stream, "${pair.remotePath}/$rel", local!!.sizeBytes) { }.getOrThrow()
                                    }
                                    local!!.sizeBytes
                                }.getOrElse { e ->
                                    Timber.e(e, "Upload failed: $rel")
                                    logEvent(pair.id, SyncEventType.FILE_SKIPPED, rel, e.message, 0)
                                    return@withPermit FileOutcome(failed = 1)
                                }
                                logEvent(pair.id, SyncEventType.FILE_UPLOADED, rel, null, bytes)
                                FileOutcome(uploaded = 1, bytesTransferred = bytes,
                                    newState = buildState(pair.id, rel, local!!, remoteAfterTransfer = uploadedRemoteFile))
                            }
                            SyncDecision.DOWNLOAD -> {
                                val bytes = runCatching {
                                    accessor.createOutputStream(rel)?.use { stream ->
                                        provider.downloadFile("${pair.remotePath}/$rel", stream) { }
                                    }
                                    remote!!.sizeBytes
                                }.getOrElse { e ->
                                    Timber.e(e, "Download failed: $rel")
                                    logEvent(pair.id, SyncEventType.FILE_SKIPPED, rel, e.message, 0)
                                    return@withPermit FileOutcome(failed = 1)
                                }
                                // Read the actual local mtime written by the OS/SAF after download.
                                val localMtime = runCatching { accessor.lastModifiedMs(rel) }
                                    .getOrDefault(System.currentTimeMillis()).takeIf { it > 0L }
                                    ?: System.currentTimeMillis()
                                logEvent(pair.id, SyncEventType.FILE_DOWNLOADED, rel, null, bytes)
                                FileOutcome(downloaded = 1, bytesTransferred = bytes,
                                    newState = buildState(pair.id, rel,
                                        LocalFileInfo(rel, remote!!.sizeBytes, localMtime),
                                        remoteAfterTransfer = remote,
                                        storeLocalMtime = false))
                            }
                            SyncDecision.DELETE_LOCAL -> {
                                val deleted = accessor.delete(rel)
                                if (!deleted) Timber.w("SyncEngine: DELETE_LOCAL failed (silent) for $rel")
                                fileStateDao.delete(pair.id, rel)
                                logEvent(pair.id, SyncEventType.FILE_DELETED, rel, "local", 0)
                                FileOutcome(deleted = 1)
                            }
                            SyncDecision.DELETE_REMOTE -> {
                                runCatching { provider.deleteFile("${pair.remotePath}/$rel") }
                                    .onFailure { e -> Timber.e(e, "SyncEngine: DELETE_REMOTE failed for $rel") }
                                fileStateDao.delete(pair.id, rel)
                                logEvent(pair.id, SyncEventType.FILE_DELETED, rel, "remote", 0)
                                FileOutcome(deleted = 1)
                            }
                            SyncDecision.CONFLICT -> {
                                conflictDao.insert(SyncConflictEntity(
                                    syncPairId = pair.id,
                                    relativePath = rel,
                                    localModifiedAt = local?.lastModifiedMs?.let { Instant.ofEpochMilli(it) } ?: Instant.EPOCH,
                                    localSizeBytes = local?.sizeBytes ?: 0L,
                                    remoteModifiedAt = remote?.modifiedAt ?: Instant.EPOCH,
                                    remoteSizeBytes = remote?.sizeBytes ?: 0L,
                                    resolution = null,
                                    detectedAt = Instant.now(),
                                ))
                                logEvent(pair.id, SyncEventType.CONFLICT_DETECTED, rel, null, 0)
                                FileOutcome(conflicts = 1)
                            }
                            SyncDecision.SKIP -> {
                                // Save state whenever both sides are present and state is absent or
                                // incomplete (post-upload null metadata). Without a baseline record,
                                // a subsequent local deletion would look like an unseen remote file
                                // and be re-downloaded instead of triggering DELETE_REMOTE.
                                val saveState = local != null && remote != null && (
                                    known == null ||
                                    known.remoteModifiedAt == null || known.localModifiedAt == null
                                )
                                if (saveState) {
                                    FileOutcome(skipped = 1, newState = buildState(pair.id, rel, local, remoteAfterTransfer = remote))
                                } else {
                                    FileOutcome(skipped = 1)
                                }
                            }
                        }
                    }
                }
            }.awaitAll()
        }

        fileStateDao.upsertAll(outcomes.mapNotNull { it.newState })
        return SyncResult(
            uploaded = outcomes.sumOf { it.uploaded },
            downloaded = outcomes.sumOf { it.downloaded },
            deleted = outcomes.sumOf { it.deleted },
            skipped = outcomes.sumOf { it.skipped },
            failedFiles = outcomes.sumOf { it.failed },
            conflicts = outcomes.sumOf { it.conflicts },
            bytesTransferred = outcomes.sumOf { it.bytesTransferred },
        )
    }

    private suspend fun ensureRemoteDirs(provider: CloudProvider, remotePairPath: String, rel: String) {
        val parts = rel.replace('\\', '/').split('/')
        var currentPath = remotePairPath
        for (part in parts.dropLast(1)) {
            currentPath = "$currentPath/$part"
            provider.createDirectory(currentPath).onFailure { e ->
                Timber.w("MKCOL $currentPath: ${e.message}")
            }
        }
    }

    private fun buildState(
        pairId: Long,
        rel: String,
        local: LocalFileInfo?,
        remoteAfterTransfer: RemoteFile?,
        storeLocalMtime: Boolean = true,
    ) = SyncFileStateEntity(
        syncPairId = pairId,
        relativePath = rel,
        // When storeLocalMtime=false, leave localModifiedAt null so the SKIP reconciliation
        // pass on the next sync reads it from the walkFiles cursor (avoids SAF stale-mtime loops).
        localModifiedAt = if (storeLocalMtime) local?.lastModifiedMs?.let { Instant.ofEpochMilli(it) } else null,
        localSizeBytes = local?.sizeBytes ?: 0L,
        localHash = null,
        remoteModifiedAt = remoteAfterTransfer?.modifiedAt,
        remoteSizeBytes = remoteAfterTransfer?.sizeBytes ?: 0L,
        remoteEtag = remoteAfterTransfer?.etag,
        lastSyncedAt = Instant.now(),
        syncedHash = null,
    )

    private suspend fun logEvent(pairId: Long, type: SyncEventType, file: String?, msg: String?, bytes: Long) {
        eventDao.insert(SyncEventEntity(syncPairId = pairId, timestamp = Instant.now(), eventType = type, filePath = file, message = msg, bytesTransferred = bytes))
    }
}

// Top-level so unit tests can call it directly without instantiating SyncEngine.
internal fun syncDecide(
    direction: SyncDirection,
    conflictStrategy: ConflictStrategy,
    deleteBehavior: DeleteBehavior,
    local: LocalFileInfo?,
    remote: RemoteFile?,
    known: SyncFileStateEntity?,
    hasPriorSyncState: Boolean = false,
): SyncDecision {
    val localExists = local != null
    val remoteExists = remote != null

    // Treat null known timestamps as "not yet recorded" — don't treat as changed.
    // The SKIP reconciliation pass will fill them in on the next sync.
    // Use second-precision for both sides: FAT32 has 2-second mtime resolution, WebDAV
    // RFC-1123 has 1-second resolution, so millisecond comparison causes phantom "changed"
    // detections and rewrite loops after a fresh download/upload.
    val localChanged = known == null ||
        (localExists && known.localModifiedAt != null &&
            local!!.lastModifiedMs / 1000 != known.localModifiedAt.epochSecond)
    val remoteChanged = known == null ||
        (remoteExists && known.remoteModifiedAt != null &&
            (remote!!.etag != known.remoteEtag ||
             remote.modifiedAt.epochSecond != known.remoteModifiedAt.epochSecond))

    return when {
        !localExists && !remoteExists -> SyncDecision.SKIP

        localExists && !remoteExists -> when {
            known == null -> when (direction) {
                SyncDirection.UPLOAD_ONLY, SyncDirection.TWO_WAY -> SyncDecision.UPLOAD
                else -> SyncDecision.SKIP
            }
            else -> when {
                deleteBehavior == DeleteBehavior.KEEP -> SyncDecision.SKIP
                direction == SyncDirection.DOWNLOAD_ONLY || direction == SyncDirection.TWO_WAY -> SyncDecision.DELETE_LOCAL
                else -> SyncDecision.SKIP
            }
        }

        !localExists && remoteExists -> when {
            known == null -> {
                // No state record: could be a new remote file OR a file whose state was lost.
                // Downloading is always safer than deleting — if the user deleted the local
                // copy intentionally, the state record will still exist (known != null) and
                // the else-branch below correctly deletes the remote copy.
                when (direction) {
                    SyncDirection.DOWNLOAD_ONLY, SyncDirection.TWO_WAY -> SyncDecision.DOWNLOAD
                    else -> SyncDecision.SKIP
                }
            }
            else -> when {
                deleteBehavior == DeleteBehavior.KEEP -> SyncDecision.SKIP
                direction == SyncDirection.UPLOAD_ONLY || direction == SyncDirection.TWO_WAY -> SyncDecision.DELETE_REMOTE
                else -> SyncDecision.SKIP
            }
        }

        localChanged && remoteChanged -> when (direction) {
            SyncDirection.UPLOAD_ONLY -> SyncDecision.UPLOAD
            SyncDirection.DOWNLOAD_ONLY -> SyncDecision.DOWNLOAD
            SyncDirection.TWO_WAY -> when (conflictStrategy) {
                ConflictStrategy.KEEP_LOCAL -> SyncDecision.UPLOAD
                ConflictStrategy.KEEP_REMOTE -> SyncDecision.DOWNLOAD
                ConflictStrategy.KEEP_NEWEST -> if ((local?.lastModifiedMs ?: 0L) >= (remote?.modifiedAt?.toEpochMilli() ?: 0L)) SyncDecision.UPLOAD else SyncDecision.DOWNLOAD
                ConflictStrategy.KEEP_LARGEST -> if ((local?.sizeBytes ?: 0L) >= (remote?.sizeBytes ?: 0L)) SyncDecision.UPLOAD else SyncDecision.DOWNLOAD
                ConflictStrategy.KEEP_BOTH -> SyncDecision.CONFLICT
                ConflictStrategy.ASK -> SyncDecision.CONFLICT
            }
        }

        localChanged -> when (direction) {
            SyncDirection.UPLOAD_ONLY, SyncDirection.TWO_WAY -> SyncDecision.UPLOAD
            else -> SyncDecision.SKIP
        }
        remoteChanged -> when (direction) {
            SyncDirection.DOWNLOAD_ONLY, SyncDirection.TWO_WAY -> SyncDecision.DOWNLOAD
            else -> SyncDecision.SKIP
        }
        else -> SyncDecision.SKIP
    }
}

enum class SyncDecision { UPLOAD, DOWNLOAD, DELETE_LOCAL, DELETE_REMOTE, CONFLICT, SKIP }

data class SyncResult(
    val uploaded: Int = 0,
    val downloaded: Int = 0,
    val deleted: Int = 0,
    val skipped: Int = 0,
    val failedFiles: Int = 0,
    val conflicts: Int = 0,
    val bytesTransferred: Long = 0L,
    val error: Exception? = null,
)