CHGenerator/WebsiteGenerator/Generators/PageHeadGenerator.swift

import Foundation

protocol PageHeadInfoProvider {

    var author: String { get }

    var linkPreviewTitle: String { get }

    var linkPreviewDescription: String { get }

    var linkPreviewImage: String? { get }

    var customHeadContent: String? { get }
}

struct PageHeadInfo: PageHeadInfoProvider {

    let author: String

    let linkPreviewTitle: String

    let linkPreviewDescription: String

    let linkPreviewImage: String?

    let customHeadContent: String?
}

struct PageHeadGenerator {

    let template: PageHeadTemplate

    let files: FileProcessor

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

    func generate(page: PageHeadInfoProvider) throws -> String {
        var content = [PageHeadTemplate.Key : String]()
        content[.author] = page.author
        content[.title] = page.linkPreviewTitle
        content[.description] = page.linkPreviewDescription
        if let image = page.linkPreviewImage {
            // 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 linkPreviewImagePath = image.insert("-link", beforeLast: ".")
            try files.requireImage(
                source: image,
                destination: linkPreviewImagePath,
                width: Site.linkPreviewDesiredImageWidth)
            #warning("Make link preview image path absolute")
            content[.image] = "<meta property=\"og:image\" content=\"\(linkPreviewImagePath)\" />"
        }
        content[.customPageContent] = page.customHeadContent

        return template.generate(content)
    }
}