// HTTPS + JSON calls to the WristCode gateway. docs/api.md lists every route.

import Foundation

enum GatewayError: LocalizedError, Equatable {
    /// 401: the device token is missing or revoked. The app shows the pairing screen.
    case unauthorized
    /// Any other non-2xx status, with the gateway's `{ "error": ... }` message when it sent one.
    case http(status: Int, message: String?)
    case transport(String)
    case decoding(String)

    var errorDescription: String? {
        switch self {
        case .unauthorized:
            "This watch is no longer paired."
        case let .http(status, message):
            message ?? Self.defaultMessage(status)
        case let .transport(message):
            message
        case let .decoding(message):
            "The gateway sent something unexpected. \(message)"
        }
    }

    /// True for 409, when the request was stale, such as an approval someone already answered.
    var isStale: Bool {
        if case .http(409, _) = self { return true }
        return false
    }

    private static func defaultMessage(_ status: Int) -> String {
        switch status {
        case 404: "Not found."
        case 409: "Already handled."
        case 429: "Too many attempts. Wait a minute."
        case 503: "That host is offline."
        default: "The gateway returned \(status)."
        }
    }
}

/// Everything the watch asks of a gateway. `GatewayClient` talks to the real one and `DemoGateway`
/// serves fixtures, so every screen works without a live gateway.
protocol GatewayAPI: Sendable {
    func feed() async throws -> Feed
    func summary() async throws -> Summary
    func thread(hostId: String, threadId: String) async throws -> ThreadDetail
    func sendMessage(hostId: String, threadId: String, text: String) async throws
    func respond(hostId: String, threadId: String, request: RespondRequest) async throws
    func stop(hostId: String, threadId: String) async throws
    func projects() async throws -> ProjectsResponse
    func newThread(_ request: NewThreadRequest) async throws -> NewThreadResponse
    func registerPush(_ registration: PushRegistration) async throws
    func status() async throws -> StatusResponse
    /// Connects the host again from its first route.
    func reconnect(hostId: String) async throws
    func testPush() async throws -> PushResult
    /// Revokes this watch's device token.
    func unpair() async throws
}

struct GatewayClient: GatewayAPI {
    let baseURL: URL
    let token: String?
    var session: URLSession = .shared

    /// `POST /v1/pair`. The only call made without a device token.
    static func pair(baseURL: URL, code: String, deviceName: String) async throws -> PairResponse {
        let client = GatewayClient(baseURL: baseURL, token: nil)
        return try await client.call("POST", ["v1", "pair"], body: PairRequest(code: code, deviceName: deviceName))
    }

    func feed() async throws -> Feed {
        try await call("GET", ["v1", "feed"])
    }

    func summary() async throws -> Summary {
        try await call("GET", ["v1", "summary"])
    }

    func thread(hostId: String, threadId: String) async throws -> ThreadDetail {
        try await call("GET", ["v1", "threads", hostId, threadId])
    }

    func sendMessage(hostId: String, threadId: String, text: String) async throws {
        try await callIgnoringBody("POST", ["v1", "threads", hostId, threadId, "messages"], body: SendMessageRequest(text: text))
    }

    func respond(hostId: String, threadId: String, request: RespondRequest) async throws {
        try await callIgnoringBody("POST", ["v1", "threads", hostId, threadId, "respond"], body: request)
    }

    func stop(hostId: String, threadId: String) async throws {
        try await callIgnoringBody("POST", ["v1", "threads", hostId, threadId, "stop"], body: nil as SendMessageRequest?)
    }

    func projects() async throws -> ProjectsResponse {
        try await call("GET", ["v1", "projects"])
    }

    func newThread(_ request: NewThreadRequest) async throws -> NewThreadResponse {
        try await call("POST", ["v1", "threads"], body: request)
    }

    func registerPush(_ registration: PushRegistration) async throws {
        try await callIgnoringBody("PUT", ["v1", "device", "push"], body: registration)
    }

    func status() async throws -> StatusResponse {
        try await call("GET", ["v1", "status"])
    }

    func reconnect(hostId: String) async throws {
        try await callIgnoringBody("POST", ["v1", "hosts", hostId, "reconnect"], body: nil as SendMessageRequest?)
    }

    func testPush() async throws -> PushResult {
        try await call("POST", ["v1", "device", "test-push"])
    }

    func unpair() async throws {
        try await callIgnoringBody("DELETE", ["v1", "device"], body: nil as SendMessageRequest?)
    }

    // MARK: - Plumbing

    private func call<Response: Decodable>(_ method: String, _ path: [String]) async throws -> Response {
        try await call(method, path, body: nil as SendMessageRequest?)
    }

    private func call<Body: Encodable, Response: Decodable>(
        _ method: String, _ path: [String], body: Body?
    ) async throws -> Response {
        let data = try await perform(method, path, body: body)
        do {
            return try JSON.decoder().decode(Response.self, from: data)
        } catch {
            throw GatewayError.decoding(String(describing: error))
        }
    }

    private func callIgnoringBody<Body: Encodable>(_ method: String, _ path: [String], body: Body?) async throws {
        _ = try await perform(method, path, body: body)
    }

    private func perform<Body: Encodable>(_ method: String, _ path: [String], body: Body?) async throws -> Data {
        // appending(component:) percent-encodes each segment, slashes included.
        let url = path.reduce(baseURL) { $0.appending(component: $1) }
        var request = URLRequest(url: url, timeoutInterval: 20)
        request.httpMethod = method
        request.setValue("application/json", forHTTPHeaderField: "Accept")
        if let token {
            request.setValue("Bearer \(token)", forHTTPHeaderField: "Authorization")
        }
        if let body {
            request.setValue("application/json", forHTTPHeaderField: "Content-Type")
            request.httpBody = try JSON.encoder().encode(body)
        }

        let data: Data
        let response: URLResponse
        do {
            (data, response) = try await session.data(for: request)
        } catch let error as URLError {
            throw GatewayError.transport(error.localizedDescription)
        }

        guard let http = response as? HTTPURLResponse else {
            throw GatewayError.transport("No HTTP response.")
        }
        guard (200 ..< 300).contains(http.statusCode) else {
            if http.statusCode == 401 { throw GatewayError.unauthorized }
            let message = try? JSON.decoder().decode(ErrorResponse.self, from: data).error
            throw GatewayError.http(status: http.statusCode, message: message)
        }
        return data
    }
}