import HTTPTypes
import Hummingbird
import HummingbirdCompression
import Logging

/// Compresses the responses that are not already encoded, and passes the ones that are through untouched.
///
/// Hummingbird's `ResponseCompressionMiddleware` appends to `Content-Encoding` without checking for one, so a pre-compressed page (see
/// ``CachedHTMLResponse``) would go out as `gzip, gzip` — a client decodes once and renders the inner gzip stream. This stands in for that
/// middleware and delegates to it, so the threshold, the negotiation and the compressor stay its behaviour.
///
/// - Note: `Context` is the request context the middleware is resolved against.
public struct CompressionMiddleware<Context: RequestContext>: Sendable {

    // MARK: Properties

    /// The middleware the unencoded responses are handed to.
    private let compression: ResponseCompressionMiddleware<Context>

    // MARK: Initializers

    /// Creates a compression middleware.
    /// - Parameter minimumResponseSizeToCompress: the smallest response body, in bytes, that is compressed at all.
    public init(
        minimumResponseSizeToCompress: Int
    ) {
        self.compression = .init(
            minimumResponseSizeToCompress: minimumResponseSizeToCompress
        )
    }

}

// MARK: - RouterMiddleware

extension CompressionMiddleware: RouterMiddleware {

    // MARK: Functions

    /// Passes the request down the chain and compresses the response on the way back up, unless it already names an encoding.
    /// - Parameters:
    ///   - request: the incoming request.
    ///   - context: the context the request is resolved against.
    ///   - next: the next responder in the middleware chain.
    /// - Returns: the downstream response, compressed when it was not already.
    /// - Throws: any error thrown downstream.
    public func handle(
        _ request: Request,
        context: Context,
        next: (Request, Context) async throws -> Response
    ) async throws -> Response {
        let response = try await next(
            request,
            context
        )

        guard response.headers[.contentEncoding] == nil else {
            return response
        }

        // The response is already in hand, so the delegate gets it rather than the chain — `next` runs once.
        return try await compression.handle(
            request,
            context: context
        ) { _, _ in
            response
        }
    }

}