import Dispatch
import Foundation

#if canImport(FoundationNetworking)
import FoundationNetworking
#endif

/// Returns the CEP's 8 digits, accepting the mask.
///
/// `"01001-000"`, `"01001000"` and `" 01001-000 "` all become `"01001000"`.
///
/// Throws ``CepisError/invalidCEP(_:)`` unless exactly 8 digits remain. The
/// client runs this **before** any request: a 7-digit CEP is the caller's bug,
/// and spending a round trip for the server to say so wastes both sides' time.
public func normalizeCEP(_ value: String) throws -> String {
    var cleaned = ""
    for character in value.trimmingCharacters(in: .whitespacesAndNewlines) {
        if character == "-" || character == "." || character == " " { continue }
        cleaned.append(character)
    }
    guard cleaned.count == 8, cleaned.allSatisfy({ $0.isASCII && $0.isNumber }) else {
        throw CepisError.invalidCEP("CEP must be 8 digits, got \"\(value)\"")
    }
    return cleaned
}

private struct HTTPOutcome: Sendable {
    let status: Int
    let body: Data
    let retryAfter: TimeInterval?
}

private final class TaskBox: @unchecked Sendable {
    private let lock = NSLock()
    private var task: URLSessionDataTask?
    private var cancelled = false

    func store(_ task: URLSessionDataTask) {
        lock.lock()
        defer { lock.unlock() }
        if cancelled {
            task.cancel()
        } else {
            self.task = task
        }
    }

    func cancel() {
        lock.lock()
        defer { lock.unlock() }
        cancelled = true
        task?.cancel()
    }
}

/// Client for the cep.is API.
///
/// Build one and reuse it: the configuration is immutable and no method keeps
/// state between calls, so it is safe to share across tasks and threads.
///
/// ```swift
/// let client = Client(timeout: 5, totalTimeout: 15)
/// let address = try await client.lookup("01001-000")
/// print(address.localidade, address.uf)
/// ```
///
/// ``Client/shared`` is a ready-made instance with the defaults, for scripts
/// and one-off lookups.
public struct Client: @unchecked Sendable {
    /// Where the public API lives: `https://cep.is`.
    public static let defaultBaseURL = URL(string: "https://cep.is")!

    /// The most CEPs the API accepts in one batch request.
    public static let maxBatch = 500

    /// This library's version, as it appears in the default `User-Agent`.
    public static let version = "0.1.0"

    /// A shared client with the default configuration.
    ///
    /// It is immutable; anyone needing different settings builds their own.
    public static let shared = Client()

    static let retriableStatus: Set<Int> = [429, 500, 502, 503, 504]

    /// Where the API lives. A trailing slash is stripped on construction, so
    /// it cannot produce the doubled separators that some proxies normalise
    /// and others 404.
    public let baseURL: URL

    /// Seconds allowed **per request**, as `URLRequest.timeoutInterval`
    /// defines it — not a cap on the whole call. That cap is ``totalTimeout``.
    public let timeout: TimeInterval

    /// How many attempts in total, counting the first. `1` disables retrying.
    public let maxAttempts: Int

    /// Base of the exponential wait, in seconds.
    public let backoffBase: TimeInterval

    /// Cap on each individual wait, in seconds. It applies to a server-sent
    /// `Retry-After` too.
    public let backoffMax: TimeInterval

    /// Cap on the whole call, attempts and waits included, or `nil` to leave
    /// ``maxAttempts`` as the only bound.
    public let totalTimeout: TimeInterval?

    /// The `User-Agent` sent with every request.
    public let userAgent: String

    private let session: URLSession

    /// Creates a client.
    ///
    /// - Parameters:
    ///   - baseURL: Where the API lives. Point it at a local instance (the API
    ///     repo brings one up on `http://localhost:8080` with `just up`) or at
    ///     a test server. It is the only parameter that usually changes
    ///     between environments.
    ///   - timeout: Seconds allowed per request. This is `URLRequest`'s
    ///     timeout, which measures inactivity rather than total elapsed time;
    ///     for a hard cap on the call use `totalTimeout`.
    ///   - maxAttempts: How many attempts in total, counting the first. The
    ///     default of `3` covers the common case (one transient failure)
    ///     without turning a real outage into a long wait.
    ///   - backoffBase: Base of the exponential wait, in seconds. The actual
    ///     wait is drawn from `[0, base * 2^(n-1)]` — *full jitter*. The
    ///     randomness is not decoration: without it, N clients that fail
    ///     together come back together and reproduce the spike that took the
    ///     service down.
    ///   - backoffMax: Cap on each individual wait, in seconds. It applies to
    ///     a server-sent `Retry-After` too, or a `Retry-After: 3600` from a
    ///     misconfigured proxy would park the caller for an hour.
    ///   - totalTimeout: Cap on the whole call, attempts and waits included.
    ///     Set it in code serving a web request: without it the worst case is
    ///     `maxAttempts * timeout` plus the sum of the backoffs.
    ///   - userAgent: Overrides the `User-Agent`. The default identifies the
    ///     library and its version, which is what lets the server diagnose an
    ///     old client misbehaving.
    ///   - session: The `URLSession` to send through. The shared session is
    ///     the right answer almost always.
    ///
    /// - Precondition: `maxAttempts >= 1` and `timeout > 0`.
    public init(
        baseURL: URL = Client.defaultBaseURL,
        timeout: TimeInterval = 10,
        maxAttempts: Int = 3,
        backoffBase: TimeInterval = 0.5,
        backoffMax: TimeInterval = 8,
        totalTimeout: TimeInterval? = nil,
        userAgent: String? = nil,
        session: URLSession = .shared
    ) {
        precondition(maxAttempts >= 1, "maxAttempts must be >= 1")
        precondition(timeout > 0, "timeout must be > 0")

        self.baseURL = Client.stripTrailingSlashes(baseURL)
        self.timeout = timeout
        self.maxAttempts = maxAttempts
        self.backoffBase = backoffBase
        self.backoffMax = backoffMax
        self.totalTimeout = totalTimeout
        self.userAgent = userAgent ?? "cepis-swift/\(Client.version) (+https://cep.is)"
        self.session = session
    }

    /// Looks up a single CEP.
    ///
    /// ```swift
    /// let address = try await client.lookup("01001-000")
    /// address.localidade  // "São Paulo"
    /// ```
    ///
    /// A 400 from the API surfaces as ``CepisError/invalidCEP(_:)`` carrying
    /// the API's own message: the format already passed local validation, so
    /// the server knows something this client does not.
    ///
    /// - Throws: ``CepisError/invalidCEP(_:)`` when the CEP is not 8 digits,
    ///   before any request is made; ``CepisError/cepNotFound(_:)`` when it is
    ///   well-formed but absent from the database;
    ///   ``CepisError/rateLimited(message:retryAfter:)``,
    ///   ``CepisError/serverError(message:status:)`` or
    ///   ``CepisError/networkError(message:)`` once every attempt has failed.
    public func lookup(_ cep: String) async throws -> Address {
        let number = try normalizeCEP(cep)
        let response = try await perform(path: "/\(number)")

        if response.status == 404 {
            throw CepisError.cepNotFound("CEP \(number) does not exist in the DNE database")
        }
        if response.status == 400 {
            throw CepisError.invalidCEP(
                Client.apiMessage(response.body, fallback: "CEP \(number) rejected"))
        }

        return try Client.decode(Address.self, from: response.body)
    }

    /// Looks up many CEPs in one batch.
    ///
    /// Returns an array of **the same length and in the same order** as the
    /// input. A CEP that does not exist becomes `nil` at its position — it does
    /// not vanish from the array, and it does not throw. That is the right
    /// difference from ``lookup(_:)``: in a batch of a thousand, one bad CEP
    /// must not cost the other nine hundred and ninety-nine.
    ///
    /// A malformed CEP also becomes `nil`, for the same reason. If you need to
    /// know *which* one was malformed, validate first with ``normalizeCEP(_:)``.
    ///
    /// Input is chunked into blocks of ``maxBatch`` automatically.
    ///
    /// - Warning: The batch carries **no coordinate**. The endpoint behind it
    ///   is CSV, and the CSV has no `geo` column. Every ``Address`` returned
    ///   here has `geo == nil`, including for CEPs that do have one. Need the
    ///   coordinate? Use ``lookup(_:)``, one at a time.
    public func lookupMany(_ ceps: [String]) async throws -> [Address?] {
        guard !ceps.isEmpty else { return [] }

        var result: [Address?] = []
        result.reserveCapacity(ceps.count)

        var start = 0
        while start < ceps.count {
            let end = min(start + Client.maxBatch, ceps.count)
            result.append(contentsOf: try await lookupChunk(Array(ceps[start..<end])))
            start = end
        }
        return result
    }

    /// Queries `GET /health`: does the API answer, and with which database.
    ///
    /// Does not throw for an API reporting itself degraded — that arrives in
    /// ``Health/status``, and checking health is exactly when you want the
    /// diagnosis rather than an error. Network failures still throw.
    public func health() async throws -> Health {
        let response = try await perform(path: "/health", acceptedStatus: [200, 503])
        return try Client.decode(Health.self, from: response.body)
    }

    private func lookupChunk(_ ceps: [String]) async throws -> [Address?] {
        let joined = ceps.joined(separator: ",")
        let encoded = joined.addingPercentEncoding(withAllowedCharacters: .alphanumerics) ?? joined
        let response = try await perform(path: "/csv", query: "ceps=\(encoded)")

        var rows = Client.addresses(fromCSV: Client.text(from: response.body))

        if rows.count > ceps.count {
            rows = Array(rows.prefix(ceps.count))
        } else if rows.count < ceps.count {
            rows.append(contentsOf: [Address?](repeating: nil, count: ceps.count - rows.count))
        }
        return rows
    }

    private func perform(
        path: String,
        query: String? = nil,
        acceptedStatus: Set<Int> = [200]
    ) async throws -> HTTPOutcome {
        let url = try makeURL(path: path, query: query)
        let deadline = totalTimeout.map { Client.monotonicNow() + $0 }

        var lastStatus: Int?
        var lastRetryAfter: TimeInterval?
        var lastFailure: String?

        var attempt = 1
        while attempt <= maxAttempts {
            if let deadline, Client.monotonicNow() >= deadline { break }

            do {
                let outcome = try await send(makeRequest(url: url))
                if acceptedStatus.contains(outcome.status)
                    || !Client.retriableStatus.contains(outcome.status)
                {
                    return outcome
                }
                lastStatus = outcome.status
                lastRetryAfter = outcome.retryAfter
                lastFailure = nil
            } catch let error as CepisError {
                throw error
            } catch {
                if Task.isCancelled { throw CancellationError() }
                lastStatus = nil
                lastRetryAfter = nil
                lastFailure = "\(error)"
            }

            if attempt == maxAttempts { break }

            var wait = backoff(attempt: attempt, retryAfter: lastRetryAfter)
            if let deadline {
                let remaining = deadline - Client.monotonicNow()
                if remaining <= 0 { break }
                wait = min(wait, remaining)
            }
            if wait > 0 {
                try await Task.sleep(nanoseconds: UInt64(wait * 1_000_000_000))
            }
            attempt += 1
        }

        let target = "\(url.absoluteString) (after \(maxAttempts) attempt(s))"
        if lastStatus == 429 {
            throw CepisError.rateLimited(
                message: "rate limit reached at \(target)", retryAfter: lastRetryAfter)
        }
        if let status = lastStatus {
            throw CepisError.serverError(
                message: "the API answered \(status) at \(target)", status: status)
        }
        throw CepisError.networkError(
            message: "could not reach \(target): "
                + (lastFailure ?? "the deadline set by totalTimeout elapsed"))
    }

    private func makeRequest(url: URL) -> URLRequest {
        var request = URLRequest(url: url)
        request.httpMethod = "GET"
        request.timeoutInterval = timeout
        request.cachePolicy = .reloadIgnoringLocalCacheData
        request.setValue(userAgent, forHTTPHeaderField: "User-Agent")
        request.setValue(
            "application/json, text/csv;q=0.9, */*;q=0.1", forHTTPHeaderField: "Accept")
        request.setValue("identity", forHTTPHeaderField: "Accept-Encoding")
        return request
    }

    private func send(_ request: URLRequest) async throws -> HTTPOutcome {
        let box = TaskBox()
        return try await withTaskCancellationHandler {
            try await withCheckedThrowingContinuation {
                (continuation: CheckedContinuation<HTTPOutcome, Error>) in
                let task = session.dataTask(with: request) { data, response, error in
                    if let error {
                        continuation.resume(throwing: error)
                        return
                    }
                    guard let http = response as? HTTPURLResponse else {
                        continuation.resume(
                            throwing: CepisError.invalidResponse(
                                "the answer did not come back as HTTP"))
                        return
                    }
                    continuation.resume(
                        returning: HTTPOutcome(
                            status: http.statusCode,
                            body: data ?? Data(),
                            retryAfter: Client.retryAfterSeconds(
                                http.value(forHTTPHeaderField: "Retry-After"))))
                }
                box.store(task)
                task.resume()
            }
        } onCancel: {
            box.cancel()
        }
    }

    private func backoff(attempt: Int, retryAfter: TimeInterval?) -> TimeInterval {
        if let retryAfter { return min(retryAfter, backoffMax) }
        let ceiling = min(backoffBase * pow(2, Double(attempt - 1)), backoffMax)
        guard ceiling > 0 else { return 0 }
        return Double.random(in: 0...ceiling)
    }

    private func makeURL(path: String, query: String?) throws -> URL {
        var text = baseURL.absoluteString + path
        if let query { text += "?\(query)" }
        guard let url = URL(string: text) else {
            throw CepisError.invalidResponse("could not build a request URL from \"\(text)\"")
        }
        return url
    }

    private static func stripTrailingSlashes(_ url: URL) -> URL {
        var text = url.absoluteString
        while text.hasSuffix("/") { text.removeLast() }
        return URL(string: text) ?? url
    }

    private static func monotonicNow() -> TimeInterval {
        Double(DispatchTime.now().uptimeNanoseconds) / 1_000_000_000
    }

    static func retryAfterSeconds(_ value: String?) -> TimeInterval? {
        guard let raw = value?.trimmingCharacters(in: .whitespaces), !raw.isEmpty else {
            return nil
        }
        if let seconds = Double(raw) { return max(0, seconds) }

        let formats = [
            "EEE, dd MMM yyyy HH:mm:ss zzz",
            "EEEE, dd-MMM-yy HH:mm:ss zzz",
            "EEE MMM d HH:mm:ss yyyy",
        ]
        for format in formats {
            let formatter = DateFormatter()
            formatter.locale = Locale(identifier: "en_US_POSIX")
            formatter.timeZone = TimeZone(identifier: "GMT")
            formatter.dateFormat = format
            if let date = formatter.date(from: raw) {
                return max(0, date.timeIntervalSinceNow)
            }
        }
        return nil
    }

    private static func decode<T: Decodable>(_ type: T.Type, from body: Data) throws -> T {
        do {
            return try JSONDecoder().decode(type, from: body)
        } catch {
            let preview = String(decoding: body.prefix(120), as: UTF8.self)
            throw CepisError.invalidResponse(
                "response is not valid JSON — something between you and the API is "
                    + "answering instead of it: \"\(preview)\"")
        }
    }

    private static func apiMessage(_ body: Data, fallback: String) -> String {
        struct APIError: Decodable {
            let mensagem: String?
        }
        if let decoded = try? JSONDecoder().decode(APIError.self, from: body),
            let message = decoded.mensagem, !message.isEmpty
        {
            return message
        }
        return fallback
    }

    private static func text(from body: Data) -> String {
        var bytes = body
        if bytes.starts(with: [0xEF, 0xBB, 0xBF]) {
            bytes = bytes.dropFirst(3)
        }
        return String(decoding: bytes, as: UTF8.self)
    }

    static func addresses(fromCSV text: String) -> [Address?] {
        let rows = CSV.parse(text)
        guard let header = rows.first else { return [] }

        var result: [Address?] = []
        for row in rows.dropFirst() {
            var fields: [String: String] = [:]
            for (index, name) in header.enumerated() {
                let key = name.trimmingCharacters(in: .whitespaces)
                fields[key] = index < row.count ? row[index] : ""
            }
            let localidade = (fields["localidade"] ?? "").trimmingCharacters(in: .whitespaces)
            result.append(localidade.isEmpty ? nil : Address(csvFields: fields))
        }
        return result
    }
}