final class PageGenerator {

    private let content: Content

    init(content: Content) {
        self.content = content
    }

    private func makeHeaders(requiredItems: Set<KnownHeaderElement>) -> Set<HeaderElement> {
        var result = content.defaultPageHeaders
        for item in requiredItems {
            guard let header = item.header(content: content) else {
                print("Missing header \(item)")
                #warning("Add warning on missing file assignment")
                continue
            }
            result.insert(header)
        }
        return result
    }

    func generate(page: Page, language: ContentLanguage, results: PageGenerationResults) -> String? {
        let contentGenerator = PageContentParser(
            content: content,
            language: language, results: results)

        guard let rawPageContent = content.storage.pageContent(for: page.id, language: language) else {
            return nil
        }

        let pageContent = contentGenerator.generatePage(from: rawPageContent)

        let localized = page.localized(in: language)

        let tags: [FeedEntryData.Tag] = page.tags.map { tag in
            .init(name: tag.localized(in: language).name,
                  url: content.absoluteUrlToTag(tag, language: language))
        }

        let headers = makeHeaders(requiredItems: results.requiredHeaders)
        results.require(files: headers.compactMap { $0.file })

        let fullPage = ContentPage(
            language: language,
            dateString: page.dateText(in: language),
            title: localized.title,
            tags: tags,
            linkTitle: localized.linkPreviewTitle ?? localized.title,
            description: localized.linkPreviewDescription ?? "",
            navigationBarLinks: content.navigationBar(in: language),
            pageContent: pageContent,
            headers: headers,
            footers: results.requiredFooters.sorted(),
            icons: results.requiredIcons)
            .content

        return fullPage
    }
}