package main

import (
	"errors"
	"fmt"
	"net"
	"net/http"
	"strings"
	"sync"
	"time"

	"golang.org/x/time/rate"
	"greenlight.debuggingjon.dev/internal/data"
	"greenlight.debuggingjon.dev/internal/validator"
)

func (app *application) recoverPanic(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// Create a deferred function (which will always be run in the event of a panic
		// as Go unwinds the stack).
		defer func() {
			// Use the builtin recover function to check if there has been a panic or
			// not.
			if err := recover(); err != nil {
				// If there was a panic, set a "Connection: close" header on the
				// response. This acts as a trigger to make Go's HTTP server
				// automatically close the current connection after a response has been
				// sent.
				w.Header().Set("Connection", "close")
				// The value returned by recover() has the type interface{}, so we use
				// fmt.Errorf() to normalize it into an error and call our
				// serverErrorResponse() helper. In turn, this will log the error using
				// our custom Logger type at the ERROR level and send the client a 500
				// Internal Server Error response.
				app.serverErrorResponse(w, r, fmt.Errorf("%s", err))
			}
		}()

		next.ServeHTTP(w, r)
	})
}

func (app *application) rateLimit(next http.Handler) http.Handler {
	// Define a client struct to hold the rate limiter and last seen time for each
	// client.
	type client struct {
		limiter  *rate.Limiter
		lastSeen time.Time
	}

	var (
		mu sync.Mutex
		// Update the map so the values are pointers to a client struct.
		clients = make(map[string]*client)
	)

	// Launch a background goroutine which removes old entries from the
	// clients map once
	// every minute.
	go func() {
		for {
			time.Sleep(time.Minute)

			// Lock the mutex to prevent any rate limiter checks from happening while
			// the cleanup is taking place.
			mu.Lock()

			// Loop through all clients. If they haven't been seen within the last three
			// minutes, delete the corresponding entry from the map.
			for ip, client := range clients {
				if time.Since(client.lastSeen) > 3*time.Minute {
					delete(clients, ip)
				}
			}

			// Importantly, unlock the mutex when the cleanup is complete.
			mu.Unlock()
		}
	}()

	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// Only carry out the check if rate limiting is enabled.
		if app.config.limiter.enabled {
			ip, _, err := net.SplitHostPort(r.RemoteAddr)
			if err != nil {
				app.serverErrorResponse(w, r, err)
				return
			}

			mu.Lock()

			if _, found := clients[ip]; !found {
				clients[ip] = &client{
					// Use the requests-per-second and burst values from the config
					// struct.
					limiter: rate.NewLimiter(rate.Limit(app.config.limiter.rps), app.config.limiter.burst),
				}
			}

			clients[ip].lastSeen = time.Now()

			if !clients[ip].limiter.Allow() {
				mu.Unlock()
				app.rateLimitExceededResponse(w, r)
				return
			}

			mu.Unlock()
		}

		next.ServeHTTP(w, r)
	})
}

func (app *application) authenticate(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		// Add the "Vary: Authorization" header to the response. This indicates to any
		// caches that the response may vary based on the value of the Authorization
		// header in the request.
		w.Header().Add("Vary", "Authorization")

		// Retrieve the value of the Authorization header from the request. This will
		// return the empty string "" if there is no such header found.
		authorizationHeader := r.Header.Get("Authorization")

		// If there is no Authorization header found, use the contextSetUser() helper
		// that we just made to add the AnonymousUser to the request context. Then we
		// call the next handler in the chain and return without executing any of the
		// code below.
		if authorizationHeader == "" {
			r = app.contextSetUser(r, data.AnonymousUser)
			next.ServeHTTP(w, r)
			return
		}

		// Otherwise, we expect the value of the Authorization header to be in the format
		// "Bearer <token>". We try to split this into its constituent parts, and if the
		// header isn't in the expected format we return a 401 Unauthorized response
		// using the invalidAuthenticationTokenResponse() helper (which we will create
		// in a moment).
		headerParts := strings.Split(authorizationHeader, " ")
		if len(headerParts) != 2 || headerParts[0] != "Bearer" {
			app.invalidAuthenticationTokenResponse(w, r)
			return
		}

		// Extract the actual authentication token from the header parts.
		token := headerParts[1]

		// Validate the token to make sure it is in a sensible format.
		v := validator.New()

		// If the token isn't valid, use the invalidAuthenticationTokenResponse()
		// helper to send a response, rather than the failedValidationResponse() helper
		// that we'd normally use.
		if data.ValidateTokenPlaintext(v, token); !v.Valid() {
			app.invalidAuthenticationTokenResponse(w, r)
			return
		}

		// Retrieve the details of the user associated with the authentication token,
		// again calling the invalidAuthenticationTokenResponse() helper if no
		// matching record was found. IMPORTANT: Notice that we are using
		// ScopeAuthentication as the first parameter here.
		user, err := app.models.Users.GetForToken(data.ScopeAuthentication, token)
		if err != nil {
			switch {
			case errors.Is(err, data.ErrRecordNotFound):
				app.invalidAuthenticationTokenResponse(w, r)
			default:
				app.serverErrorResponse(w, r, err)
			}
			return
		}

		// Call the contextSetUser() helper to add the user information to the request
		// context.
		r = app.contextSetUser(r, user)

		// Call the next handler in the chain.
		next.ServeHTTP(w, r)
	})
}

// anonymous.
func (app *application) requireAuthenticatedUser(next http.HandlerFunc) http.HandlerFunc {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		user := app.contextGetUser(r)

		if user.IsAnonymous() {
			app.authenticationRequiredResponse(w, r)
			return
		}

		next.ServeHTTP(w, r)
	})
}

// Checks that a user is both authenticated and activated.
func (app *application) requireActivatedUser(next http.HandlerFunc) http.HandlerFunc {
	// Rather than returning this http.HandlerFunc we assign it to the variable fn.
	fn := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		user := app.contextGetUser(r)

		// Check that a user is activated.
		if !user.Activated {
			app.inactiveAccountResponse(w, r)
			return
		}

		next.ServeHTTP(w, r)
	})

	// Wrap fn with the requireAuthenticatedUser() middleware before returning it.
	return app.requireAuthenticatedUser(fn)
}

// Note that the first parameter for the middleware function is the permission code that
// we require the user to have.
func (app *application) requirePermission(code string, next http.HandlerFunc) http.HandlerFunc {
	fn := func(w http.ResponseWriter, r *http.Request) {
		// Retrieve the user from the request context.
		user := app.contextGetUser(r)

		// Get the slice of permissions for the user.
		permissions, err := app.models.Permissions.GetAllForUser(user.ID)
		if err != nil {
			app.serverErrorResponse(w, r, err)
			return
		}

		// Check if the slice includes the required permission. If it doesn't, then
		// return a 403 Forbidden response.
		if !permissions.Include(code) {
			app.notPermittedResponse(w, r)
			return
		}

		// Otherwise they have the required permission so we call the next handler in
		// the chain.
		next.ServeHTTP(w, r)
	}

	// Wrap this with the requireActivatedUser() middleware before returning it.
	return app.requireActivatedUser(fn)
}

func (app *application) enableCORS(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Add("Vary", "Origin")

		// Add the "Vary: Access-Control-Request-Method" header.
		w.Header().Add("Vary", "Access-Control-Request-Method")

		origin := r.Header.Get("Origin")

		if origin != "" && len(app.config.cors.trustedOrigins) != 0 {
			for i := range app.config.cors.trustedOrigins {
				if origin == app.config.cors.trustedOrigins[i] {
					w.Header().Set("Access-Control-Allow-Origin", origin)

					// Check if the request has the HTTP method OPTIONS and contains the
					// "Access-Control-Request-Method" header. If it does, then we treat
					// it as a preflight request.
					if r.Method == http.MethodOptions && r.Header.Get("Access-Control-Request-Method") != "" {
						// Set the necessary preflight response headers, as discussed
						// previously.
						w.Header().Set("Access-Control-Allow-Methods", "OPTIONS, PUT, PATCH, DELETE")
						w.Header().Set("Access-Control-Allow-Headers", "Authorization, Content-Type")

						// Write the headers along with a 200 OK status and return from
						// the                        // the middleware with no further action.
						w.WriteHeader(http.StatusOK)
						return
					}
				}
			}
		}

		next.ServeHTTP(w, r)
	})
}