import Elementary
import Foundation

/// A page of a website: an HTML document with the shared scaffolding assembled around the page's content.
///
/// A conforming page supplies its locale, its title, the stylesheets and scripts it needs, its head metadata, and its content; the protocol assembles the
/// rest of the document around them: the viewport declaration, the summary, canonical, and social card tags, the structured data script, the analytics
/// tracker script, and the metadata followed by the stylesheet links and the deferred script tags in the head, and the content as the body.
public protocol Page: HTMLDocument, Sendable {

    // MARK: Associated types

    /// The type of the page's markup.
    associatedtype Content: HTML

    /// The type of the page's head metadata markup.
    associatedtype Metadata: HTML

    // MARK: Properties

    /// The analytics tracker embedded as a deferred script in the document head, or `nil` (the default) to omit it.
    var analytics: Analytics? { get }

    /// The version token appended to the page's asset URLs, or `nil` to leave them unversioned.
    var assetVersion: String? { get }

    /// The canonical URL the page is served at, rendered as a `link rel="canonical"` tag in the document head, or `nil` (the default) to omit the tag.
    var canonicalURL: String? { get }

    /// The page's markup, rendered as the document body.
    @HTMLBuilder
    var content: Content { get }

    /// The locale the page content is localized to.
    var locale: Locale { get }

    /// The markup placed in the document head before the ``stylesheets`` links: icon and manifest links, extra meta tags, and the like.
    @HTMLBuilder
    var metadata: Metadata { get }

    /// The scripts loaded from the document head, in order.
    ///
    /// Rendered as `defer`red tags: the downloads start while the head is parsed, and the scripts still execute in order only after the
    /// document is fully parsed — the same semantics end-of-body tags would give, minus the late download start.
    var scripts: [any Asset] { get }

    /// The card controlling the page's link previews, rendered as Open Graph and Twitter meta tags in the document head, or `nil` (the default)
    /// to omit them.
    var socialCard: SocialCard? { get }

    /// The page's structured data, rendered as a JSON-LD script in the document head, or `nil` (the default) to omit it.
    var structuredData: StructuredData? { get }

    /// The stylesheets linked in the document head, in order.
    var stylesheets: [any Asset] { get }

    /// The page's summary, rendered as a `meta name="description"` tag in the document head, or `nil` (the default) to omit the tag.
    var summary: String? { get }

}

// MARK: - Implementations

public extension Page {
    
    // MARK: Computed
    
    /// The canonical URL is omitted unless the page provides one.
    var canonicalURL: String? {
        nil
    }
    
    /// The page ``content``; the ``scripts`` load deferred from the ``head``.
    @HTMLBuilder
    var body: some HTML {
        content
    }
    
    /// The viewport declaration, the ``summary``, ``canonicalURL``, and ``socialCard`` tags, the ``structuredData`` script and the
    /// ``analytics`` tracker script (when provided), and the ``metadata`` followed by the ``stylesheets`` links and the deferred
    /// ``scripts`` tags, placed in the document head.
    ///
    /// The charset declaration is omitted: Elementary's `HTMLDocument` scaffolding already emits `<meta charset="UTF-8">` before this markup,
    /// and HTML5 allows only one.
    ///
    /// The structured data is an inert data block — browsers never execute it, so a site's `Content-Security-Policy` does not apply to it —
    /// that search engines read for the organization's name, logo, and profiles. The analytics tracker, by contrast, is an executable script the
    /// policy must allow, and it is `defer`red so it never delays the page render; each behavior flag renders its `data-` attribute only when enabled.
    @HTMLBuilder
    var head: some HTML {
        meta(
            .name(.viewport),
            .content("width=device-width, initial-scale=1")
        )
        
        if let summary {
            meta(
                .name(.description),
                .content(summary)
            )
        }
        
        if let canonicalURL {
            link(
                .rel("canonical"),
                .href(canonicalURL)
            )
        }

        if let socialCard {
            for tag in socialCard.tags {
                meta(
                    .custom(
                        name: tag.attribute.rawValue,
                        value: tag.name.rawValue
                    ),
                    .content(tag.content)
                )
            }
        }

        if let structuredData {
            script(.custom(
                name: "type",
                value: "application/ld+json"
            )) {
                HTMLRaw(structuredData.payload)
            }
        }

        if let analytics {
            script(
                .defer,
                .src(analytics.scriptURL)
            ) {}
            .attributes(contentsOf: analytics.attributes.map {
                .custom(
                    name: $0.name,
                    value: $0.value
                )
            })
        }

        metadata
        
        for file in stylesheets {
            link(
                .rel(.stylesheet),
                .href(file.urlPath(
                    for: .css,
                    version: assetVersion
                ))
            )
        }

        for file in scripts {
            script(
                .defer,
                .src(file.urlPath(
                    for: .js,
                    version: assetVersion
                ))
            ) {}
        }
    }
    
    /// The analytics tracker is omitted unless the page provides one.
    var analytics: Analytics? {
        nil
    }

    /// The social card is omitted unless the page provides one.
    var socialCard: SocialCard? {
        nil
    }

    /// The structured data is omitted unless the page provides one.
    var structuredData: StructuredData? {
        nil
    }

    /// The summary is omitted unless the page provides one.
    var summary: String? {
        nil
    }
    
}