import Foundation

public final class ExistingLyricsRevisionDirector: Sendable {
    private let aiService: any AIService

    public init(aiService: any AIService) {
        self.aiService = aiService
    }

    public func reviseExistingLyrics(
        in project: SongProject,
        sourceLyrics: Lyrics? = nil,
        instruction: String,
        mode: LyricsRevisionMode,
        conversation: [AIConversationMessage] = [],
        localeIdentifier: String? = nil,
        revisedAt: Date = Date()
    ) async throws -> SongProjectUpdateResult {
        guard mode != .writeFromIdea else {
            throw ExistingLyricsRevisionDirectorError.unsupportedMode(mode)
        }

        let trimmedInstruction = instruction.trimmingCharacters(in: .whitespacesAndNewlines)
        guard !trimmedInstruction.isEmpty else {
            throw ExistingLyricsRevisionDirectorError.emptyInstruction
        }

        let lyricsToRevise = sourceLyrics ?? project.lyrics
        guard !lyricsToRevise.text.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty else {
            throw ExistingLyricsRevisionDirectorError.emptySourceLyrics
        }

        guard !project.isUpdateScopeLocked(.lyrics) else {
            return SongProjectUpdateResult(project: project)
        }

        let request = LyricsRevisionRequest(
            context: AIRequestContext(
                userInstruction: trimmedInstruction,
                conversation: conversation,
                localeIdentifier: localeIdentifier
            ),
            project: project,
            sourceLyrics: lyricsToRevise,
            mode: mode
        )
        let result = try await aiService.reviseLyrics(from: request)

        var proposedProject = project
        proposedProject.lyrics = result.lyrics
        return SongProjectAIUpdateMerger().apply(
            snapshot: SongProjectSnapshot(project: proposedProject),
            changedScopes: [.lyrics],
            notes: result.notes,
            to: project,
            updatedAt: revisedAt
        )
    }
}

public enum ExistingLyricsRevisionDirectorError: Error, Equatable, Sendable {
    case emptyInstruction
    case emptySourceLyrics
    case unsupportedMode(LyricsRevisionMode)
}