CHGenerator/WebsiteGenerator/Content/LanguageContainer.swift

import Foundation

protocol LanguageIdentifiable {

    var languageIdentifier: String { get }

    var title: String { get }
}

protocol LanguageContainer {

    associatedtype LocalizedContainer: LanguageIdentifiable

    var languages: [LocalizedContainer] { get }

}

protocol LocalizedMetadataContainer {

    associatedtype MetadataType: LanguageContainer

    var metadata: MetadataType { get }

    func hasContent(for language: String) -> Bool
}

// MARK: Default implementations

extension LocalizedMetadataContainer {

    func hasContent(for language: String) -> Bool {
        true
    }
}

// MARK: Extensions

extension LocalizedMetadataContainer {

    func localized(for language: String) -> MetadataType.LocalizedContainer {
        metadata.localized(for: language)
    }

    /**
     The localized title of the element.

     This title is used as large text in overview pages, or as the `<h1>` title on pages. If no separate link preview title is specified using a localized `linkPreview.title`, then this value is also used for link previews.
     */
    func title(for language: String) -> String {
        localized(for: language).title
    }


    func nextLanguage(for languageIdentifier: String) -> String? {
        let langs = metadata.languages.map { $0.languageIdentifier }
        guard let index = langs.firstIndex(of: languageIdentifier) else {
            return nil
        }
        for i in 1..<langs.count {
            let next = langs[(index + i) % langs.count]
            guard hasContent(for: next) else {
                continue
            }
            guard next != languageIdentifier else {
                return nil
            }

            return next
        }
        return nil
    }
}

extension LanguageContainer {

    var languageIdentifiers: [String] {
        languages.map { $0.languageIdentifier }
    }

    #warning("Throw better error for missing language")
    func localized(for language: String) -> LocalizedContainer {
        languages.first { $0.languageIdentifier == language }!
    }

    /**
     The localized title of the element.

     This title is used as large text in overview pages, or as the `<h1>` title on pages. If no separate link preview title is specified using a localized `linkPreview.title`, then this value is also used for link previews.
     */
    func title(for language: String) -> String {
        localized(for: language).title
    }
}

extension LocalizedMetadataContainer where Self: SiteElement, Self.MetadataType.LocalizedContainer: LinkPreviewMetadataProvider {

    private func linkPreviewImageFileName(for language: String) -> String? {
        if let fileName = localized(for: language).linkPreview?.image {
            return fileName
        }
        // Check for the existence of a localized thumbnail
        let fileName = Self.thumbnailFileNameLocalized(for: language)
        if inputFolder.appendingPathComponent(fileName).exists {
            return fileName
        }
        let defaultThumbnail = Self.defaultThumbnailFileName
        if inputFolder.appendingPathComponent(defaultThumbnail).exists {
            return defaultThumbnail
        }
        return nil
    }

    func linkPreviewImage(for language: String) -> String? {
        guard let fileName = linkPreviewImageFileName(for: language) else {
            return nil
        }
        return "/\(path)/\(fileName)"
    }
}