// Package internalhttp provides the private probe HTTP listener used by the
// runnable Notification Service process.
package internalhttp

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
	"net"
	"net/http"
	"strconv"
	"sync"
	"time"

	"galaxy/notification/internal/telemetry"

	"go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp"
	"go.opentelemetry.io/otel/attribute"
)

const jsonContentType = "application/json; charset=utf-8"

const (
	// HealthzPath is the private liveness probe route.
	HealthzPath = "/healthz"

	// ReadyzPath is the private readiness probe route.
	ReadyzPath = "/readyz"
)

// Config describes the private internal HTTP listener owned by Notification
// Service.
type Config struct {
	// Addr is the TCP listen address used by the private probe HTTP server.
	Addr string

	// ReadHeaderTimeout bounds how long the listener may spend reading request
	// headers before the server rejects the connection.
	ReadHeaderTimeout time.Duration

	// ReadTimeout bounds how long the listener may spend reading one request.
	ReadTimeout time.Duration

	// IdleTimeout bounds how long the listener keeps an idle keep-alive
	// connection open.
	IdleTimeout time.Duration
}

// Validate reports whether cfg contains a usable private HTTP listener
// configuration.
func (cfg Config) Validate() error {
	switch {
	case cfg.Addr == "":
		return errors.New("internal HTTP addr must not be empty")
	case cfg.ReadHeaderTimeout <= 0:
		return errors.New("internal HTTP read header timeout must be positive")
	case cfg.ReadTimeout <= 0:
		return errors.New("internal HTTP read timeout must be positive")
	case cfg.IdleTimeout <= 0:
		return errors.New("internal HTTP idle timeout must be positive")
	default:
		return nil
	}
}

// Dependencies describes the collaborators used by the private probe
// transport layer.
type Dependencies struct {
	// Logger writes structured listener lifecycle logs. When nil, slog.Default
	// is used.
	Logger *slog.Logger

	// Telemetry records low-cardinality probe metrics and lifecycle events.
	Telemetry *telemetry.Runtime
}

// Server owns the private probe HTTP listener exposed by Notification
// Service.
type Server struct {
	cfg Config

	handler http.Handler
	logger  *slog.Logger
	metrics *telemetry.Runtime

	stateMu  sync.RWMutex
	server   *http.Server
	listener net.Listener
}

// NewServer constructs one private probe HTTP server for cfg and deps.
func NewServer(cfg Config, deps Dependencies) (*Server, error) {
	if err := cfg.Validate(); err != nil {
		return nil, fmt.Errorf("new internal HTTP server: %w", err)
	}

	logger := deps.Logger
	if logger == nil {
		logger = slog.Default()
	}

	return &Server{
		cfg:     cfg,
		handler: newHandler(logger, deps.Telemetry),
		logger:  logger.With("component", "internal_http"),
		metrics: deps.Telemetry,
	}, nil
}

// Run binds the configured listener and serves the private probe surface until
// Shutdown closes the server.
func (server *Server) Run(ctx context.Context) error {
	if ctx == nil {
		return errors.New("run internal HTTP server: nil context")
	}
	if err := ctx.Err(); err != nil {
		return err
	}

	listener, err := net.Listen("tcp", server.cfg.Addr)
	if err != nil {
		return fmt.Errorf("run internal HTTP server: listen on %q: %w", server.cfg.Addr, err)
	}

	httpServer := &http.Server{
		Handler:           server.handler,
		ReadHeaderTimeout: server.cfg.ReadHeaderTimeout,
		ReadTimeout:       server.cfg.ReadTimeout,
		IdleTimeout:       server.cfg.IdleTimeout,
	}

	server.stateMu.Lock()
	server.server = httpServer
	server.listener = listener
	server.stateMu.Unlock()

	server.logger.Info("notification internal HTTP server started", "addr", listener.Addr().String())
	server.metrics.RecordInternalHTTPEvent(context.Background(), "started")

	defer func() {
		server.stateMu.Lock()
		server.server = nil
		server.listener = nil
		server.stateMu.Unlock()
	}()

	err = httpServer.Serve(listener)
	switch {
	case err == nil:
		return nil
	case errors.Is(err, http.ErrServerClosed):
		server.logger.Info("notification internal HTTP server stopped")
		server.metrics.RecordInternalHTTPEvent(context.Background(), "stopped")
		return nil
	default:
		return fmt.Errorf("run internal HTTP server: serve on %q: %w", server.cfg.Addr, err)
	}
}

// Shutdown gracefully stops the private probe HTTP server within ctx.
func (server *Server) Shutdown(ctx context.Context) error {
	if ctx == nil {
		return errors.New("shutdown internal HTTP server: nil context")
	}

	server.stateMu.RLock()
	httpServer := server.server
	server.stateMu.RUnlock()

	if httpServer == nil {
		return nil
	}

	if err := httpServer.Shutdown(ctx); err != nil && !errors.Is(err, http.ErrServerClosed) {
		return fmt.Errorf("shutdown internal HTTP server: %w", err)
	}

	return nil
}

func newHandler(logger *slog.Logger, metrics *telemetry.Runtime) http.Handler {
	mux := http.NewServeMux()
	mux.HandleFunc("GET "+HealthzPath, handleHealthz)
	mux.HandleFunc("GET "+ReadyzPath, handleReadyz)

	return otelhttp.NewHandler(withObservability(mux, metrics), "notification.internal_http")
}

func withObservability(next http.Handler, metrics *telemetry.Runtime) http.Handler {
	return http.HandlerFunc(func(writer http.ResponseWriter, request *http.Request) {
		startedAt := time.Now()
		recorder := &statusRecorder{
			ResponseWriter: writer,
			statusCode:     http.StatusOK,
		}

		next.ServeHTTP(recorder, request)

		route := request.Pattern
		switch recorder.statusCode {
		case http.StatusMethodNotAllowed:
			route = "method_not_allowed"
		case http.StatusNotFound:
			route = "not_found"
		case 0:
			route = "unmatched"
		}
		if route == "" {
			route = "unmatched"
		}

		metrics.RecordInternalHTTPRequest(
			request.Context(),
			[]attribute.KeyValue{
				attribute.String("route", route),
				attribute.String("method", request.Method),
				attribute.String("status_code", strconv.Itoa(recorder.statusCode)),
			},
			time.Since(startedAt),
		)
	})
}

func handleHealthz(writer http.ResponseWriter, _ *http.Request) {
	writeStatusResponse(writer, http.StatusOK, "ok")
}

func handleReadyz(writer http.ResponseWriter, _ *http.Request) {
	writeStatusResponse(writer, http.StatusOK, "ready")
}

func writeStatusResponse(writer http.ResponseWriter, statusCode int, status string) {
	writer.Header().Set("Content-Type", jsonContentType)
	writer.WriteHeader(statusCode)
	_ = json.NewEncoder(writer).Encode(statusResponse{Status: status})
}

type statusResponse struct {
	Status string `json:"status"`
}

type statusRecorder struct {
	http.ResponseWriter
	statusCode int
}

func (recorder *statusRecorder) WriteHeader(statusCode int) {
	recorder.statusCode = statusCode
	recorder.ResponseWriter.WriteHeader(statusCode)
}