import AVFoundation
import Foundation

enum DubbingCompositorError: LocalizedError {
    case noAudioTracks
    case cannotReadDuration

    var errorDescription: String? {
        switch self {
        case .noAudioTracks:
            return "录音没有音频轨道"
        case .cannotReadDuration:
            return "无法读取录音时长"
        }
    }
}

enum DubbingCompositor {
    static func merge(urls: [URL], outputURL: URL) async throws {
        try await mergeSync(urls: urls, outputURL: outputURL)
    }

    private static func mergeSync(urls: [URL], outputURL: URL) async throws {
        try? FileManager.default.removeItem(at: outputURL)
        let composition = AVMutableComposition()
        var currentTime = CMTime.zero
        for url in urls {
            let source = AVURLAsset(url: url)
            let tracks = try await source.loadTracks(withMediaType: .audio)
            guard let track = tracks.first else {
                throw DubbingCompositorError.noAudioTracks
            }
            let duration = try await source.load(.duration)
            guard duration.seconds.isFinite, duration.seconds > 0 else {
                throw DubbingCompositorError.cannotReadDuration
            }
            let timeRange = CMTimeRange(start: .zero, duration: duration)
            let compositionTrack = composition.addMutableTrack(
                withMediaType: .audio,
                preferredTrackID: kCMPersistentTrackID_Invalid
            )
            guard let compositionTrack else {
                throw DubbingCompositorError.noAudioTracks
            }
            try compositionTrack.insertTimeRange(timeRange, of: track, at: currentTime)
            currentTime = currentTime + duration
        }

        guard let exporter = AVAssetExportSession(
            asset: composition,
            presetName: AVAssetExportPresetAppleM4A
        ) else {
            throw DubbingCompositorError.noAudioTracks
        }
        exporter.outputURL = outputURL
        exporter.outputFileType = .m4a
        await exporter.export()
        guard exporter.status == .completed else {
            throw exporter.error ?? DubbingCompositorError.noAudioTracks
        }
    }
}