import Foundation
import HTTPTypes
import Hummingbird

/// Appends header names to the `Vary` header of every response passing through.
///
/// Placed just above the response-compression middleware, it marks each response as varying on `Accept-Encoding`: the static files and pre-rendered
/// pages are served with `Cache-Control: public`, so without the signal a shared cache could store a compressed body and hand it to a client that
/// never advertised support for the encoding.
///
/// Names already present on a response's `Vary` header — such as the `Accept-Language` the localized pages carry — are kept, and duplicates are not
/// added.
public struct VaryMiddleware<Context: RequestContext>: Sendable {

    // MARK: Properties

    /// The header names appended to every response's `Vary` header.
    private let names: [String]

    // MARK: Initializers

    /// Creates a vary middleware.
    /// - Parameter fields: the header names appended to every response's `Vary` header. Defaults to `Accept-Encoding`, the request header
    ///   the response-compression middleware acts on.
    public init(
        fields: [HTTPField.Name] = [.acceptEncoding]
    ) {
        self.names = fields.map(\.rawName)
    }

}

// MARK: - RouterMiddleware

extension VaryMiddleware: RouterMiddleware {

    // MARK: Functions

    /// Passes the request down the chain and appends the configured names to the response's `Vary` header on the way back up.
    /// - 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 with the `Vary` names applied.
    /// - Throws: any error thrown downstream.
    public func handle(
        _ request: Request,
        context: Context,
        next: (Request, Context) async throws -> Response
    ) async throws -> Response {
        var response = try await next(
            request,
            context
        )

        var vary = response.headers[.vary]?
            .split(separator: ",")
            .map { $0.trimmingCharacters(in: .whitespaces) } ?? []

        for name in names where !vary.contains(where: {
            $0.caseInsensitiveCompare(name) == .orderedSame
        }) {
            vary.append(name)
        }

        response.headers[.vary] = vary.joined(separator: ", ")

        return response
    }

}