import Foundation

/// A backend that resolves localized strings for an explicit locale and reports the languages it serves.
///
/// This is the seam that decouples ``Localize`` and ``LanguageList`` from *how* localizations are stored
/// and resolved. The shipping implementation, ``StringCatalog``, reads a raw `.xcstrings` catalog so it
/// behaves identically on Darwin and Linux. A future backend — for example one built on
/// `String(localized:)` for a native Apple app that needs plural and device variations — can conform
/// without changing any caller.
///
/// Resolution never fails: an implementation returns the key itself when it has no localization for it,
/// mirroring Foundation's `String(localized:)`. This is the contract both a dictionary lookup and the
/// native API can honour, since the native API cannot distinguish a missing key from a translation that
/// happens to equal the key.
protocol CatalogResolving: Sendable {

    // MARK: Properties

    /// The source (development) language, used as the final fallback when a locale has no localization.
    var sourceLanguage: String { get }

    /// Every language the backend can resolve strings for, including the source language.
    var languages: Set<String> { get }

    // MARK: Methods

    /// Resolves a catalog key in the given locale.
    /// - Parameters:
    ///   - key: the catalog key to look up.
    ///   - locale: the locale to resolve the key in.
    /// - Returns: the localized string, falling back to the source language, then to the key itself.
    func string(
        for key: String,
        in locale: Locale
    ) -> String

}