import AVFoundation
import Speech

/// The transcribing service that transcribes recorded audio into text on device.
///
/// The service runs the given recorded audio file through a `SpeechAnalyzer` with a `SpeechTranscriber` module, joining the finalized
/// results into the returned transcription. The transcription happens in the locale given at each call — or rather in the closest equivalent
/// the transcriber supports. The speech model assets for that locale are downloaded and installed on first use; every transcription after
/// that happens entirely offline.
public struct AudioTranscribing: Transcribing {

    // MARK: Initializers

    /// Creates an audio transcribing service.
    public init() {}

    // MARK: Methods

    /// Transcribes the given recorded audio file into text, deleting the file when the transcription finishes.
    ///
    /// - Parameters:
    ///   - audio: The location of the recorded audio file to transcribe.
    ///   - locale: The locale of the spoken language to transcribe.
    /// - Returns: The transcription of the recorded audio.
    /// - Throws: ``AudioTranscribingError/localeNotSupported`` when the transcriber supports no equivalent of the given locale,
    /// or any error thrown while installing the speech model assets, reading the audio file, or analyzing its contents.
    public func callAsFunction(
        _ audio: URL,
        locale: Locale
    ) async throws -> Transcription {
        defer {
            try? FileManager.default.removeItem(at: audio)
        }

        guard let locale = await SpeechTranscriber.supportedLocale(
            equivalentTo: locale
        ) else {
            throw AudioTranscribingError.localeNotSupported
        }

        let transcriber = SpeechTranscriber(
            locale: locale,
            preset: .transcription
        )

        if let request = try await AssetInventory.assetInstallationRequest(supporting: [transcriber]) {
            try await request.downloadAndInstall()
        }

        let analyzer = SpeechAnalyzer(modules: [transcriber])

        async let text = transcriber.results.reduce(into: "") { text, result in
            text += String(result.text.characters)
        }

        let file = try AVAudioFile(forReading: audio)

        if let lastSampleTime = try await analyzer.analyzeSequence(from: file) {
            try await analyzer.finalizeAndFinish(through: lastSampleTime)
        } else {
            await analyzer.cancelAndFinishNow()
        }

        return try await .init(text: text)
    }

}

// MARK: - Errors

/// The errors thrown by ``AudioTranscribing``.
public enum AudioTranscribingError: Error {
    /// The transcriber supports no equivalent of the locale the transcription was requested with.
    case localeNotSupported
}