import Domain
import Store

public actor SyncEngine {
    private let library: any Library
    private let batchSize = 500

    public init(library: any Library) {
        self.library = library
    }

    public func sync(_ source: any LibrarySource) async throws {
        do {
            try await pull(source, since: library.cursor(for: source.id))
        } catch SourceError.cannotCalculateChanges {
            try await pull(source, since: nil)
        }
    }

    private func pull(_ source: any LibrarySource, since cursor: Cursor?) async throws {
        var batch: [Change] = []
        for try await change in source.changes(since: cursor) {
            if case let .checkpoint(next) = change {
                await library.apply(batch)
                batch.removeAll(keepingCapacity: true)
                await library.setCursor(next, for: source.id)
            } else {
                batch.append(change)
                if batch.count >= batchSize {
                    await library.apply(batch)
                    batch.removeAll(keepingCapacity: true)
                }
            }
        }
        if !batch.isEmpty {
            await library.apply(batch)
        }
    }
}