import Foundation

// MARK: - Service Protocol

protocol TrainingPlanServiceProtocol: Sendable {
    func listPlans(type: PlanType?, difficulty: Difficulty?) async throws -> [TrainingPlan]
    func getPlan(id: String) async throws -> TrainingPlan
    func generatePlan(request: GeneratePlanRequest) async throws -> TrainingPlan
    func followPlan(id: String) async throws
    func unfollowPlan(id: String) async throws
    func updateSessionStatus(sessionId: String, status: SessionStatus) async throws
}

// MARK: - Default Service

class TrainingPlanService: TrainingPlanServiceProtocol {
    private let baseURL: URL
    private let session: URLSession
    private let authToken: String?

    init(
        baseURL: URL = URL(string: "http://localhost:3000")!,
        session: URLSession = .shared,
        authToken: String? = nil
    ) {
        self.baseURL = baseURL
        self.session = session
        self.authToken = authToken
    }

    func listPlans(type: PlanType? = nil, difficulty: Difficulty? = nil) async throws -> [TrainingPlan] {
        var components = URLComponents(url: baseURL.appendingPathComponent("/api/training-plans"), resolvingAgainstBaseURL: true)!
        var queryItems: [URLQueryItem] = []
        if let type = type { queryItems.append(URLQueryItem(name: "type", value: type.rawValue)) }
        if let difficulty = difficulty { queryItems.append(URLQueryItem(name: "difficulty", value: difficulty.rawValue)) }
        components.queryItems = queryItems

        let request = try buildRequest(url: components.url!)
        let (data, response) = try await session.data(for: request)
        try validateResponse(response)

        let decoded = try JSONDecoder().decode(TrainingPlanListResponse.self, from: data)
        return decoded.plans
    }

    func getPlan(id: String) async throws -> TrainingPlan {
        let url = baseURL.appendingPathComponent("/api/training-plans/\(id)")
        let request = try buildRequest(url: url)
        let (data, response) = try await session.data(for: request)
        try validateResponse(response)

        let decoded = try JSONDecoder().decode(TrainingPlanDetailResponse.self, from: data)
        return decoded.plan
    }

    func generatePlan(request: GeneratePlanRequest) async throws -> TrainingPlan {
        let url = baseURL.appendingPathComponent("/api/training-plans/generate")
        var request = try buildRequest(url: url, method: .post)
        request.httpBody = try JSONEncoder().encode(request)

        let (data, response) = try await session.data(for: request)
        try validateResponse(response)

        let decoded = try JSONDecoder().decode(GeneratePlanResponse.self, from: data)
        return decoded.plan
    }

    func followPlan(id: String) async throws {
        let url = baseURL.appendingPathComponent("/api/training-plans/\(id)/follow")
        let request = try buildRequest(url: url, method: .post)
        let (_, response) = try await session.data(for: request)
        try validateResponse(response)
    }

    func unfollowPlan(id: String) async throws {
        let url = baseURL.appendingPathComponent("/api/training-plans/\(id)/follow")
        let request = try buildRequest(url: url, method: .delete)
        let (_, response) = try await session.data(for: request)
        try validateResponse(response)
    }

    func updateSessionStatus(sessionId: String, status: SessionStatus) async throws {
        let url = baseURL.appendingPathComponent("/api/training-plans/sessions/\(sessionId)/status")
        var request = try buildRequest(url: url, method: .patch)
        let body = try JSONEncoder().encode(["status": status.rawValue])
        request.httpBody = body

        let (_, response) = try await session.data(for: request)
        try validateResponse(response)
    }

    // MARK: - Helpers

    private func buildRequest(url: URL, method: HTTPMethod = .get, body: Data? = nil) throws -> URLRequest {
        var request = URLRequest(url: url)
        request.httpMethod = method.rawValue
        request.setValue("application/json", forHTTPHeaderField: "Content-Type")

        if let token = authToken {
            request.setValue("Bearer \(token)", forHTTPHeaderField: "Authorization")
        }

        if let body = body {
            request.httpBody = body
        }

        return request
    }

    private func validateResponse(_ response: URLResponse) throws {
        guard let httpResponse = response as? HTTPURLResponse else {
            throw TrainingPlanError.invalidResponse
        }

        guard (200...299).contains(httpResponse.statusCode) else {
            switch httpResponse.statusCode {
            case 401: throw TrainingPlanError.unauthorized
            case 403: throw TrainingPlanError.forbidden
            case 404: throw TrainingPlanError.notFound
            case 429: throw TrainingPlanError.rateLimited
            case 500...599: throw TrainingPlanError.serverError(httpResponse.statusCode)
            default: throw TrainingPlanError.httpError(httpResponse.statusCode)
            }
        }
    }
}

// MARK: - Error Types

enum TrainingPlanError: LocalizedError {
    case invalidResponse
    case unauthorized
    case forbidden
    case notFound
    case rateLimited
    case serverError(Int)
    case httpError(Int)
    case decodingError(Error)

    var errorDescription: String {
        switch self {
        case .invalidResponse: return "Invalid server response"
        case .unauthorized: return "Unauthorized — please log in again"
        case .forbidden: return "Forbidden — check permissions"
        case .notFound: return "Training plan not found"
        case .rateLimited: return "Too many requests — try again shortly"
        case .serverError(let code): return "Server error (\(code))"
        case .httpError(let code): return "HTTP error (\(code))"
        case .decodingError(let error): return "Decoding error: \(error.localizedDescription)"
        }
    }
}