CHGenerator/Sources/Generator/Generators/PageHeadGenerator.swift

import Foundation

struct PageHeadGenerator {

    // TODO: Add to configuration
    static let linkPreviewDesiredImageWidth = 1600

    let factory: TemplateFactory

    private let results: GenerationResultsHandler

    init(factory: TemplateFactory, results: GenerationResultsHandler) {
        self.factory = factory
        self.results = results
    }

    func generate(page: Element, language: String, headers: RequiredHeaders = .init()) -> String {
        let metadata = page.localized(for: language)

        var content = [PageHeadTemplate.Key : String]()
        content[.author] = page.author
        content[.title] = metadata.linkPreviewTitle
        content[.description] = metadata.linkPreviewDescription
        if let image = page.linkPreviewImage(for: language) {
            // Note: Generate separate destination link for the image,
            // since we don't want a single large image for thumbnails.
            // Warning: Link preview source path must be relative to root
            let linkPreviewImageName = "thumbnail-link.\(image.lastComponentAfter("."))"
            let sourceImagePath = page.pathRelativeToRootForContainedInputFile(image)
            let destinationImagePath = page.pathRelativeToRootForContainedInputFile(linkPreviewImageName)
            results.requireSingleImage(
                source: sourceImagePath,
                destination: destinationImagePath,
                requiredBy: page.path,
                width: PageHeadGenerator.linkPreviewDesiredImageWidth)
            content[.image] =  factory.html.linkPreviewImage(file: linkPreviewImageName)
        }
        content[.customPageContent] = results.getContentOfOptionalFile(at: page.additionalHeadContentPath, source: page.path)

        let head = (content[.customPageContent].unwrapped { [$0] } ?? []) + headers
            .map { $0.rawValue }
            .sorted()
            .map { option in
                let scriptPath = "assets/js/\(option)"
                let relative = page.relativePathToOtherSiteElement(file: scriptPath)
                return factory.html.scriptInclude(path: relative)
            }

        content[.customPageContent] = head.joined(separator: "\n")
        return factory.pageHead.generate(content)
    }
}