CHGenerator/WebsiteGenerator/Generators/PageHeadGenerator.swift

import Foundation

struct PageHeadGenerator {

    static let linkPreviewDesiredImageWidth = 1600

    let template: PageHeadTemplate

    init(factory: TemplateFactory) {
        self.template = factory.pageHead
    }

    func generate(page: Element, language: String) -> 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 = image.insert("-link", beforeLast: ".")
            let sourceImagePath = page.pathRelativeToRootForContainedInputFile(image)
            let destinationImagePath = page.pathRelativeToRootForContainedInputFile(linkPreviewImageName)
            files.requireImage(
                source: sourceImagePath,
                destination: destinationImagePath,
                width: PageHeadGenerator.linkPreviewDesiredImageWidth)
            content[.image] = "<meta property=\"og:image\" content=\"\(linkPreviewImageName)\" />"
        }
        content[.customPageContent] = page.customHeadContent()

        return template.generate(content)
    }
}