package mcp

import (
	"encoding/json"
	"fmt"
)

// rpcRequest is a JSON-RPC 2.0 request. Notifications omit ID so the field is
// tagged omitempty. Params is any so callers pass a struct or a JSON object.
type rpcRequest struct {
	JSONRPC string `json:"jsonrpc"`
	ID      any    `json:"id,omitempty"`
	Method  string `json:"method"`
	Params  any    `json:"params,omitempty"`
}

// rpcResponse is a JSON-RPC 2.0 response. Result is kept raw until the caller
// decodes it into the concrete result type. Method is populated only for
// server-initiated notifications (no id), so transports can route them to the
// client's notification handler.
type rpcResponse struct {
	JSONRPC string          `json:"jsonrpc"`
	ID      any             `json:"id,omitempty"`
	Method  string          `json:"method,omitempty"`
	Result  json.RawMessage `json:"result,omitempty"`
	Error   *rpcError       `json:"error,omitempty"`
}

// rpcError is a JSON-RPC error object. It satisfies error so it can be wrapped
// and surfaced with %w.
type rpcError struct {
	Code    int    `json:"code"`
	Message string `json:"message"`
	Data    any    `json:"data,omitempty"`
}

// Error implements error.
func (e *rpcError) Error() string {
	return fmt.Sprintf("code %d: %s", e.Code, e.Message)
}

// encodeRequest marshals a JSON-RPC request. An id of nil produces a
// notification (no id field).
func encodeRequest(id any, method string, params any) ([]byte, error) {
	return json.Marshal(rpcRequest{JSONRPC: "2.0", ID: id, Method: method, Params: params})
}

// decodeIDString reports whether res carries the string id expected for a JSON
// string id request.
func decodeIDString(res rpcResponse, id string) bool {
	s, ok := res.ID.(string)
	return ok && s == id
}