// Copyright (C) 2017 Michael J. Fromberger. All Rights Reserved.

// Package jhttp implements a bridge from HTTP to JSON-RPC.  This permits
// requests to be submitted to a JSON-RPC server using HTTP as a transport.
package jhttp

import (
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"

	"github.com/creachadair/jrpc2"
	"github.com/creachadair/jrpc2/server"
)

// A Bridge is a http.Handler that bridges requests to a JSON-RPC server.
//
// By default, the bridge accepts only HTTP POST requests with the complete
// JSON-RPC request message in the body, with Content-Type application/json.
// Either a single request object or a list of request objects is supported.
//
// If the HTTP request method is not "POST", the bridge reports 405 (Method Not
// Allowed). If the Content-Type is not application/json, the bridge reports
// 415 (Unsupported Media Type).
//
// If a ParseRequest hook is set, these requirements are disabled, and the hook
// is entirely responsible for checking request structure.
//
// If a ParseGETRequest hook is set, HTTP "GET" requests are handled by a
// Getter using that hook; otherwise "GET" requests are handled as above.
//
// If the request completes, whether or not there is an error, the HTTP
// response is 200 (OK) for ordinary requests or 204 (No Response) for
// notifications, and the response body contains the JSON-RPC response.
//
// The bridge attaches the inbound HTTP request to the context passed to the
// client, allowing an EncodeContext callback to retrieve state from the HTTP
// headers. Use jhttp.HTTPRequest to retrieve the request from the context.
type Bridge struct {
	local    server.Local
	parseReq func(*http.Request) ([]*jrpc2.Request, error)
	getter   *Getter
}

// ServeHTTP implements the required method of http.Handler.
func (b Bridge) ServeHTTP(w http.ResponseWriter, req *http.Request) {
	// If a GET hook is defined, allow GET requests.
	if req.Method == "GET" && b.getter != nil {
		b.getter.ServeHTTP(w, req)
		return
	}

	// If no parse hook is defined, insist that the method is POST and the
	// content-type is application/json. Setting a hook disables these checks.
	if b.parseReq == nil {
		if req.Method != "POST" {
			w.WriteHeader(http.StatusMethodNotAllowed)
			return
		}
		if req.Header.Get("Content-Type") != "application/json" {
			w.WriteHeader(http.StatusUnsupportedMediaType)
			return
		}
	}
	if err := b.serveInternal(w, req); err != nil {
		w.WriteHeader(http.StatusInternalServerError)
		fmt.Fprintln(w, err.Error())
	}
}

func (b Bridge) serveInternal(w http.ResponseWriter, req *http.Request) error {
	// The HTTP request requires a response, but the server will not reply if
	// all the requests are notifications. Check whether we have any calls
	// needing a response, and choose whether to wait for a reply based on that.
	//
	// Note that we are forgiving about a missing version marker in a request,
	// since we can't tell at this point whether the server is willing to accept
	// messages like that.
	jreq, err := b.parseHTTPRequest(req)
	if err != nil && err != jrpc2.ErrInvalidVersion {
		return err
	}

	// Because the bridge shares the JSON-RPC client between potentially many
	// HTTP clients, we must virtualize the ID space for requests to preserve
	// the HTTP client's assignment of IDs.
	//
	// To do this, we keep track of the inbound ID for each request so that we
	// can map the responses back. This takes advantage of the fact that the
	// *jrpc2.Client detangles batch order so that responses come back in the
	// same order (modulo notifications) even if the server response did not
	// preserve order.

	// Generate request specifications for the client.
	var inboundID []string                // for requests
	spec := make([]jrpc2.Spec, len(jreq)) // requests & notifications
	for i, req := range jreq {
		spec[i] = jrpc2.Spec{
			Method: req.Method(),
			Notify: req.IsNotification(),
		}
		if req.HasParams() {
			var p json.RawMessage
			req.UnmarshalParams(&p)
			spec[i].Params = p
		}
		if !spec[i].Notify {
			inboundID = append(inboundID, req.ID())
		}
	}

	// Attach the HTTP request to the client context, so the encoder can see it.
	ctx := context.WithValue(req.Context(), httpReqKey{}, req)
	rsps, err := b.local.Client.Batch(ctx, spec)
	if err != nil {
		return err
	}

	// If all the requests were notifications, report success without responses.
	if len(rsps) == 0 {
		w.WriteHeader(http.StatusNoContent)
		return nil
	}

	// Otherwise, map the responses back to their original IDs, and marshal the
	// response back into the body.
	for i, rsp := range rsps {
		rsp.SetID(inboundID[i])
	}

	return b.encodeResponses(rsps, w)
}

func (b Bridge) parseHTTPRequest(req *http.Request) ([]*jrpc2.Request, error) {
	if b.parseReq != nil {
		return b.parseReq(req)
	}
	body, err := io.ReadAll(req.Body)
	if err != nil {
		return nil, err
	}
	return jrpc2.ParseRequests(body)
}

func (b Bridge) encodeResponses(rsps []*jrpc2.Response, w http.ResponseWriter) error {
	// If there is only a single reply, send it alone; otherwise encode a batch.
	// Per the spec (https://www.jsonrpc.org/specification#batch), this is OK;
	// we are not required to respond to a batch with an array:
	//
	//   The Server SHOULD respond with an Array containing the corresponding
	//   Response objects
	//
	data, err := marshalResponses(rsps)
	if err != nil {
		return err
	}
	writeJSON(w, http.StatusOK, json.RawMessage(data))
	return nil
}

// Close closes the channel to the server, waits for the server to exit, and
// reports its exit status.
func (b Bridge) Close() error {
	if b.getter != nil {
		b.getter.Close()
	}
	return b.local.Close()
}

// NewBridge constructs a new Bridge that starts a server on mux and dispatches
// HTTP requests to it.  The server will run until the bridge is closed.
//
// Note that a bridge is not able to push calls or notifications from the
// server back to the remote client. The bridge client is shared by multiple
// active HTTP requests, and has no way to know which of the callers the push
// should be forwarded to. You can enable push on the bridge server and set
// hooks on the bridge client as usual, but the remote client will not see push
// messages from the server.
func NewBridge(mux jrpc2.Assigner, opts *BridgeOptions) Bridge {
	b := Bridge{
		local: server.NewLocal(mux, &server.LocalOptions{
			Client: opts.clientOptions(),
			Server: opts.serverOptions(),
		}),
		parseReq: opts.parseRequest(),
	}
	if pget := opts.parseGETRequest(); pget != nil {
		g := NewGetter(mux, &GetterOptions{
			Client:       opts.clientOptions(),
			Server:       opts.serverOptions(),
			ParseRequest: pget,
		})
		b.getter = &g
	}
	return b
}

// BridgeOptions are optional settings for a Bridge. A nil pointer is ready for
// use and provides default values as described.
type BridgeOptions struct {
	// Options for the bridge client (default nil).
	Client *jrpc2.ClientOptions

	// Options for the bridge server (default nil).
	Server *jrpc2.ServerOptions

	// If non-nil, this function is called to parse JSON-RPC requests from the
	// HTTP request body. If this function reports an error, the request fails.
	// By default, the bridge uses jrpc2.ParseRequests on the HTTP request body.
	//
	// Setting this hook disables the default requirement that the request
	// method be POST and the content-type be application/json.
	ParseRequest func(*http.Request) ([]*jrpc2.Request, error)

	// If non-nil, this function is used to parse a JSON-RPC method name and
	// parameters from the URL of an HTTP GET request. If this function reports
	// an error, the request fails.
	//
	// If this hook is set, all GET requests are handled by a Getter using this
	// parse function, and are not passed to a ParseRequest hook even if one is
	// defined.
	ParseGETRequest func(*http.Request) (string, interface{}, error)
}

func (o *BridgeOptions) clientOptions() *jrpc2.ClientOptions {
	if o == nil {
		return nil
	}
	return o.Client
}

func (o *BridgeOptions) serverOptions() *jrpc2.ServerOptions {
	if o == nil {
		return nil
	}
	return o.Server
}

func (o *BridgeOptions) parseRequest() func(*http.Request) ([]*jrpc2.Request, error) {
	if o == nil {
		return nil
	}
	return o.ParseRequest
}

func (o *BridgeOptions) parseGETRequest() func(*http.Request) (string, interface{}, error) {
	if o == nil {
		return nil
	}
	return o.ParseGETRequest
}

type httpReqKey struct{}

// HTTPRequest returns the HTTP request associated with ctx, or nil. The
// context passed to the JSON-RPC client by the Bridge will contain this value.
func HTTPRequest(ctx context.Context) *http.Request {
	req, ok := ctx.Value(httpReqKey{}).(*http.Request)
	if ok {
		return req
	}
	return nil
}

// marshalResponses encodes a batch of JSON-RPC responses into JSON.
func marshalResponses(rsps []*jrpc2.Response) ([]byte, error) {
	if len(rsps) == 1 {
		return json.Marshal(rsps[0])
	}
	return json.Marshal(rsps)
}