// Package mcp is the MCP edge for the cart: a Model Context Protocol server
// exposing the cart grain as tools so an agent can inspect and mutate carts
// (list items, change quantities, remove items, clear carts, apply vouchers,
// manage users, etc.).
//
// Transport (JSON-RPC 2.0 over streamable HTTP), dispatch, and the schema/result
// helpers live in platform/mcp; this package only defines the cart-specific
// tools and wires them to the grain pool. Mounted by cmd/cart under /mcp.
package mcp

import (
	"context"

	"git.k6n.net/mats/go-cart-actor/pkg/actor"
	"git.k6n.net/mats/go-cart-actor/pkg/cart"
	pmcp "git.k6n.net/mats/platform/mcp"
	"net/http"

	"google.golang.org/protobuf/proto"
)

const (
	serverName    = "cart"
	serverVersion = "0.1.0"
)

// Applier is the minimal grain-pool interface the cart MCP needs: read a
// cart's current state (Get) and apply a mutation (Apply). The
// SimpleGrainPool[cart.CartGrain] in cmd/cart satisfies it directly.
type Applier interface {
	Get(ctx context.Context, id uint64) (*cart.CartGrain, error)
	Apply(ctx context.Context, id uint64, mutation ...proto.Message) (*actor.MutationResult[cart.CartGrain], error)
}

// Server is the MCP edge over the cart grain pool.
type Server struct {
	applier Applier
	mcp     *pmcp.Server
}

// New builds an MCP server exposing the cart grain as tools.
func New(applier Applier) *Server {
	s := &Server{applier: applier}
	s.mcp = pmcp.NewServer(pmcp.Info{Name: serverName, Version: serverVersion}, s.buildTools()...)
	return s
}

// Handler returns the streamable-HTTP handler to mount (e.g. at /mcp).
func (s *Server) Handler() http.Handler { return s.mcp.Handler() }