import Foundation
import Localization

/// Negotiates the best supported language for a request from its `Accept-Language` header.
///
/// Bound to the module's catalog languages via ``LanguageList``, an instance is invoked like a
/// function — through ``callAsFunction(forAcceptLanguage:)`` — to resolve a header value to a
/// supported language identifier, falling back to the default language.
struct NegotiateLanguage {

    // MARK: Properties

    /// The supported languages and default language, derived from the module's String Catalog.
    private let list: LanguageList

    // MARK: Initializers

    /// Creates a language negotiator backed by the module's String Catalog.
    init() {
        self.list = .init(bundle: .module)
    }

    // MARK: Methods

    /// Picks the best supported language for the given `Accept-Language` header value.
    ///
    /// Invoked by calling the instance directly, for example `negotiate(forAcceptLanguage: header)`.
    /// The header is split into its language tags (dropping any `q` weights), then matched against
    /// the supported languages with `Bundle.preferredLocalizations(from:forPreferences:)`. When the
    /// header is absent or matches nothing, the default language is returned.
    /// - Parameter acceptLanguage: the raw `Accept-Language` header value, if any.
    /// - Returns: the identifier of the supported language to serve.
    func callAsFunction(
        forAcceptLanguage acceptLanguage: String?
    ) -> String {
        guard let acceptLanguage else {
            return list.default
        }

        let tags = tags(from: acceptLanguage)

        guard !tags.isEmpty else {
            return list.default
        }

        let preferred = Bundle.preferredLocalizations(
            from: list.all,
            forPreferences: tags
        )

        return preferred.first
            ?? list.default
    }

}

// MARK: - Helpers

private extension NegotiateLanguage {

    // MARK: Methods

    /// Extracts the ordered language tags from an `Accept-Language` header value.
    ///
    /// Each comma-separated entry is reduced to its language tag by dropping the `;q=` weight, and
    /// blank entries are removed. The original order is kept, which mirrors descending preference
    /// closely enough for `preferredLocalizations(from:forPreferences:)` to resolve correctly.
    /// - Parameter acceptLanguage: the raw `Accept-Language` header value.
    /// - Returns: the ordered, weight-stripped language tags.
    func tags(
        from acceptLanguage: String
    ) -> [String] {
        acceptLanguage
            .split(separator: ",")
            .map { entry in
                entry
                    .split(separator: ";")
                    .first
                    .map(String.init)?
                    .trimmingCharacters(in: .whitespaces) ?? ""
            }
            .filter { !$0.isEmpty }
    }

}