import Elementary
import HTTPTypes
import Hummingbird
import NIOCore

/// A pre-rendered HTTP response for a fully static HTML page.
///
/// The document is rendered to bytes once, at initialization, and every ``response(for:)`` reuses those bytes — along with a fixed status and
/// precomputed headers — instead of re-rendering. This suits pages whose markup never changes between requests, such as the landing page and the
/// not-found page, avoiding a per-request Elementary render on hot paths.
///
/// A successful page also revalidates cheaply: its headers carry a weak entity tag derived from the rendered bytes and a `Cache-Control` that asks
/// clients to revalidate (`no-cache`), so a repeat visit costs a `304 Not Modified` instead of a full transfer — and a deploy that changes the page
/// changes the tag, propagating immediately.
///
/// ``LocalizedHTMLCollectionResponse`` builds on this type, caching one instance per supported language.
///
/// The body is written as an unsized stream (no `Content-Length`), mirroring `HTMLResponse`, so the response-compression middleware downstream
/// treats it exactly as it would a freshly rendered page.
public struct CachedHTMLResponse: Sendable {

    // MARK: Properties

    /// The page rendered to bytes once.
    private let buffer: ByteBuffer

    /// The weak entity tag of the rendered bytes, present on successful pages only.
    private let eTag: String?

    /// The headers applied to every response, precomputed once.
    private let headers: HTTPFields

    /// The status applied to every response.
    private let status: HTTPResponse.Status

    // MARK: Initializers

    /// Renders the given document to bytes once.
    ///
    /// A `200 OK` page gets the revalidation headers (`ETag` and `Cache-Control`); an error page does not, since a `304 Not Modified` only
    /// ever stands in for a success.
    /// - Parameters:
    ///   - status: the status applied to every response. Defaults to `.ok`.
    ///   - additionalHeaders: extra headers merged onto every response, alongside the content type.
    ///     Used to carry per-language signals such as `Content-Language` and `Vary`.
    ///   - document: the static HTML document to render and cache.
    public init(
        status: HTTPResponse.Status = .ok,
        additionalHeaders: HTTPFields = [:],
        document: some HTMLDocument
    ) {
        let buffer = ByteBuffer(string: document.render())
        var headers: HTTPFields = [
            .contentType: "text/html; charset=utf-8"
        ]
        var eTag: String?

        if status == .ok {
            var hash = FNV1aHash()

            hash.combine(buffer.readableBytesView)

            eTag = "W/\"\(hash.digest)\""

            headers[.eTag] = eTag
            headers[.cacheControl] = "public, no-cache"
        }

        for field in additionalHeaders {
            headers[field.name] = field.value
        }

        self.buffer = buffer
        self.eTag = eTag
        self.headers = headers
        self.status = status
    }

    // MARK: Methods

    /// Builds a response from the cached, pre-rendered bytes.
    ///
    /// A conditional request whose `If-None-Match` names the page's entity tag is answered with a
    /// bodyless `304 Not Modified`. Otherwise the full page is served, mirroring the
    /// `text/html; charset=utf-8` content type `HTMLResponse` produces and leaving the
    /// `Content-Length` unset so small pages remain eligible for compression.
    /// - Parameter request: the request the response answers.
    /// - Returns: the response carrying the cached HTML body, or its `304` revalidation.
    public func response(
        for request: Request
    ) -> Response {
        if
            let eTag,
            request.method == .get || request.method == .head,
            let match = request.headers[.ifNoneMatch],
            match == "*" || match.contains(eTag)
        {
            return Response(
                status: .notModified,
                headers: headers
            )
        }

        return Response(
            status: status,
            headers: headers,
            body: .init { [buffer] writer in
                try await writer.write(buffer)
                try await writer.finish(nil)
            }
        )
    }

}