// Package app wires configuration into the running control/data plane. In
// Phase 1 it builds the Router from static upstream config; later phases feed
// instances from discovery + the reconciler instead.
package app

import (
	"fmt"
	"log/slog"
	"os"

	"gitea.thetempleofdoom.com/drjones/mcp-gateway-nexus/internal/config"
	"gitea.thetempleofdoom.com/drjones/mcp-gateway-nexus/internal/domain"
	"gitea.thetempleofdoom.com/drjones/mcp-gateway-nexus/internal/mcp"
	"gitea.thetempleofdoom.com/drjones/mcp-gateway-nexus/internal/router"
	"gitea.thetempleofdoom.com/drjones/mcp-gateway-nexus/internal/version"
)

// nexusInfo identifies Nexus to upstream MCP servers.
func nexusInfo() mcp.Implementation {
	return mcp.Implementation{Name: "mcp-nexus", Version: version.Version}
}

// BuildRouter constructs a Router with an upstream client per config entry.
// Transports are created but not connected; call ConnectAll afterwards.
func BuildRouter(cfg *config.Config, log *slog.Logger) (*router.Router, error) {
	r := router.New(nexusInfo(), log)
	for _, u := range cfg.Upstreams {
		transport, err := buildTransport(u)
		if err != nil {
			return nil, fmt.Errorf("upstream %q: %w", u.Namespace, err)
		}
		client := mcp.NewClient(transport, nexusInfo())
		r.Add(u.Namespace, client)
	}
	return r, nil
}

func buildTransport(u config.UpstreamConfig) (mcp.Transport, error) {
	switch domain.TransportKind(u.Transport) {
	case domain.TransportStdio:
		return mcp.NewStdioTransport(mcp.StdioConfig{
			Command: u.Command,
			Args:    u.Args,
			Env:     u.Env,
			Dir:     u.Dir,
			Stderr:  prefixWriter{ns: u.Namespace},
		})
	case domain.TransportHTTP:
		return mcp.NewHTTPTransport(mcp.HTTPConfig{
			Endpoint: u.Endpoint,
			Headers:  u.Headers,
		})
	default:
		return nil, fmt.Errorf("unknown transport %q", u.Transport)
	}
}

// prefixWriter tags upstream stderr lines with their namespace on our stderr.
type prefixWriter struct{ ns string }

func (p prefixWriter) Write(b []byte) (int, error) {
	fmt.Fprintf(os.Stderr, "[upstream:%s] %s", p.ns, b)
	return len(b), nil
}