import Foundation

protocol CommentGenerationServiceProtocol: Sendable {
    func generateComment(from draft: CommentDraft) async throws -> GeneratedComment
}

enum CommentGenerationServiceFactory {
    static func makeDefault() -> any CommentGenerationServiceProtocol {
        if let apiKey = AIConfiguration.apiKey {
            return OpenAICommentGenerationService(apiKey: apiKey)
        }
        return MockCommentGenerationService()
    }
}

enum CommentGenerationError: LocalizedError {
    case emptySubreddit
    case invalidSubreddit
    case emptyPostTitle
    case emptyParentComment
    case apiUnavailable(String)
    case invalidResponse

    var errorDescription: String? {
        switch self {
        case .emptySubreddit:
            "Enter a subreddit to tailor the comment."
        case .invalidSubreddit:
            "Subreddit names must be 3–21 characters and use only letters, numbers, or underscores."
        case .emptyPostTitle:
            "Enter the post title for thread context."
        case .emptyParentComment:
            "Paste the parent comment you're replying to."
        case .apiUnavailable(let message):
            message
        case .invalidResponse:
            "The AI returned an unexpected response. Please try again."
        }
    }
}

// MARK: - OpenAI

struct OpenAICommentGenerationService: CommentGenerationServiceProtocol {
    let apiKey: String
    var model: String = "gpt-4o-mini"

    func generateComment(from draft: CommentDraft) async throws -> GeneratedComment {
        try CommentDraftValidator.validateForGeneration(from: draft)

        let subreddit = PostDraftValidator.normalizedSubreddit(draft.subreddit)
        let prompt = buildPrompt(draft: draft, subreddit: subreddit)

        var request = URLRequest(url: URL(string: "https://api.openai.com/v1/chat/completions")!)
        request.httpMethod = "POST"
        request.setValue("Bearer \(apiKey)", forHTTPHeaderField: "Authorization")
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")

        let body: [String: Any] = [
            "model": model,
            "temperature": 0.85,
            "response_format": ["type": "json_object"],
            "messages": [
                [
                    "role": "system",
                    "content": """
                    You write authentic Reddit comments. Respond with JSON only using keys:
                    body (string, primary comment, max 10000 chars),
                    variants (array of objects with body, score 0-100, reasoning strings).
                    Generate exactly \(draft.variantCount.rawValue) variants ranked by fit.
                    Write like a real Redditor: conversational, community-aware, no corporate tone.
                    Use markdown when requested. Never start with "As an AI" or similar.
                    Match subreddit culture. Be respectful even when disagreeing.
                    """,
                ],
                ["role": "user", "content": prompt],
            ],
        ]
        request.httpBody = try JSONSerialization.data(withJSONObject: body)

        let (data, response) = try await URLSession.shared.data(for: request)
        guard let http = response as? HTTPURLResponse else {
            throw CommentGenerationError.apiUnavailable("Couldn't reach the AI service.")
        }

        if http.statusCode == 401 {
            throw CommentGenerationError.apiUnavailable("Invalid OpenAI API key.")
        }
        guard (200 ... 299).contains(http.statusCode) else {
            let message = String(data: data, encoding: .utf8) ?? "HTTP \(http.statusCode)"
            throw CommentGenerationError.apiUnavailable("AI request failed: \(message)")
        }

        return try parseResponse(data)
    }

    private func buildPrompt(draft: CommentDraft, subreddit: String) -> String {
        var lines = [
            "Subreddit: r/\(subreddit)",
            "Comment type: \(draft.commentType.title)",
            "Post title: \(draft.postTitle.trimmingCharacters(in: .whitespaces))",
            "Intent: \(draft.intent.title) — \(draft.intent.subtitle)",
            "Tone: \(draft.tone.title)",
            "Length: \(draft.length.title) (\(draft.length.wordRange))",
            "Markdown: \(draft.useMarkdown ? "yes" : "plain text only")",
        ]

        if !draft.topic.trimmingCharacters(in: .whitespaces).isEmpty {
            lines.append("Focus / angle: \(draft.topic.trimmingCharacters(in: .whitespaces))")
        }
        if !draft.postBody.trimmingCharacters(in: .whitespaces).isEmpty {
            lines.append("Post excerpt: \(draft.postBody.trimmingCharacters(in: .whitespaces))")
        }
        if draft.commentType == .reply {
            lines.append("Parent comment to reply to: \(draft.parentComment.trimmingCharacters(in: .whitespaces))")
            if draft.includeQuote {
                lines.append("Optionally quote relevant part of parent with > blockquote.")
            }
        }

        return lines.joined(separator: "\n")
    }

    private func parseResponse(_ data: Data) throws -> GeneratedComment {
        struct APIEnvelope: Decodable {
            struct Choice: Decodable {
                struct Message: Decodable {
                    let content: String
                }
                let message: Message
            }
            let choices: [Choice]
        }

        struct VariantPayload: Decodable {
            let body: String
            let score: Int
            let reasoning: String
        }

        struct Payload: Decodable {
            let body: String
            let variants: [VariantPayload]
        }

        let envelope = try JSONDecoder().decode(APIEnvelope.self, from: data)
        guard let content = envelope.choices.first?.message.content.data(using: .utf8) else {
            throw CommentGenerationError.invalidResponse
        }

        let payload = try JSONDecoder().decode(Payload.self, from: content)
        let body = String(payload.body.prefix(CommentDraftValidator.maxCommentLength))
        let variants = payload.variants.map {
            CommentVariant(
                body: String($0.body.prefix(CommentDraftValidator.maxCommentLength)),
                score: min(100, max(0, $0.score)),
                reasoning: $0.reasoning
            )
        }

        guard !body.isEmpty else { throw CommentGenerationError.invalidResponse }
        return GeneratedComment(body: body, variants: variants)
    }
}

// MARK: - Mock

struct MockCommentGenerationService: CommentGenerationServiceProtocol {
    func generateComment(from draft: CommentDraft) async throws -> GeneratedComment {
        try CommentDraftValidator.validateForGeneration(from: draft)

        try await Task.sleep(for: .milliseconds(900))

        let subreddit = PostDraftValidator.normalizedSubreddit(draft.subreddit)
        let postTitle = draft.postTitle.trimmingCharacters(in: .whitespaces)
        let body = makePrimaryComment(draft: draft, subreddit: subreddit, postTitle: postTitle)
        let variants = makeVariants(draft: draft, subreddit: subreddit, postTitle: postTitle)

        return GeneratedComment(body: body, variants: variants)
    }

    private func makePrimaryComment(draft: CommentDraft, subreddit: String, postTitle: String) -> String {
        let opener: String = switch draft.commentType {
        case .topLevel:
            "Great post — "
        case .reply:
            draft.includeQuote && !draft.parentComment.isEmpty
                ? "> \(draft.parentComment.prefix(120))\(draft.parentComment.count > 120 ? "…" : "")\n\n"
                : ""
        }

        let core: String = switch draft.intent {
        case .addValue:
            """
            I've been lurking r/\(subreddit) for a while and this is one of the better takes on **\(postTitle)** I've seen.

            A few things that worked for me:
            - Start small and iterate — don't overthink the first step
            - The community wiki/FAQ is underrated
            - Happy to share more detail if anyone's interested

            Solid discussion thread.
            """
        case .agree:
            """
            100% agree. This matches my experience with \(postTitle.lowercased()) and I'm glad someone finally said it clearly.

            The part about taking it one step at a time really resonates. Thanks for posting this to r/\(subreddit).
            """
        case .disagree:
            """
            Respectfully disagree on one point here. I think the common advice around \(postTitle.lowercased()) oversimplifies things.

            In my experience the nuance matters a lot — context, timing, and individual circumstances change the answer. Not trying to be contrarian, just adding another data point from someone who's been through it.
            """
        case .question:
            """
            Genuine question — when you say this applies broadly, do you mean for beginners or everyone in r/\(subreddit)?

            I've had mixed results depending on setup. Would love to hear what worked for others on **\(postTitle)**.
            """
        case .funny:
            """
            r/\(subreddit) never disappoints. Read the title, thought "yeah same," then read the post and now I'm questioning every life choice.

            Saving this for the next time someone asks about \(postTitle.lowercased()). 😅
            """
        case .empathetic:
            """
            Thanks for sharing this — it's hard to post about \(postTitle.lowercased()) and I appreciate the honesty.

            You're not alone in feeling this way. Took me a long time to figure out and the r/\(subreddit) community helped more than I expected.
            """
        case .technical:
            """
            For anyone looking for the technical breakdown on **\(postTitle)**:

            1. **Root cause** — usually configuration or environment mismatch
            2. **Fix** — verify prerequisites first, then apply the minimal change
            3. **Verify** — reproduce the original issue and confirm it's resolved

            Happy to go deeper in a follow-up if useful.
            """
        case .summary:
            """
            **TL;DR:** \(postTitle) — main takeaway is to start simple, validate early, and lean on community resources in r/\(subreddit).

            Worth a read if you're new to this.
            """
        }

        let trimmed = adjustLength(core, length: draft.length)
        return String((opener + trimmed).prefix(CommentDraftValidator.maxCommentLength))
    }

    private func makeVariants(draft: CommentDraft, subreddit: String, postTitle: String) -> [CommentVariant] {
        let base = makePrimaryComment(draft: draft, subreddit: subreddit, postTitle: postTitle)
        let altIntents: [CommentIntent] = [.agree, .question, .summary, .addValue, .funny]
            .filter { $0 != draft.intent }
            .prefix(draft.variantCount.rawValue - 1)
            .map { $0 }

        var variants: [CommentVariant] = [
            CommentVariant(
                body: base,
                score: 92,
                reasoning: "Best match for \(draft.intent.title) intent and \(draft.tone.title.lowercased()) tone."
            ),
        ]

        for (index, intent) in altIntents.enumerated() {
            var altDraft = draft
            altDraft.intent = intent
            let body = makePrimaryComment(draft: altDraft, subreddit: subreddit, postTitle: postTitle)
            variants.append(
                CommentVariant(
                    body: body,
                    score: 88 - index * 4,
                    reasoning: "Alternative \(intent.title.lowercased()) angle for r/\(subreddit)."
                )
            )
        }

        return Array(variants.prefix(draft.variantCount.rawValue))
    }

    private func adjustLength(_ text: String, length: CommentLength) -> String {
        switch length {
        case .short:
            let sentences = text.components(separatedBy: ". ")
            return sentences.prefix(2).joined(separator: ". ") + (sentences.count > 1 ? "." : "")
        case .medium:
            return text
        case .long:
            return text + "\n\nEdit: thanks for the awards kind strangers — didn't expect this to blow up."
        }
    }
}