package middleware

import (
	"math"
	"net/http"
	"net/netip"
	"slices"
	"strings"
	"time"

	"github.com/go-chi/httprate"
)

const (
	// loginRateLimit is the maximum number of login attempts
	// per interval.
	loginRateLimit = 5

	// loginRateInterval is the time window for the rate limit.
	loginRateInterval = 1 * time.Minute

	// passwordChangeRateLimit is the maximum number of password
	// change attempts per interval. Each attempt verifies the
	// current password, so the endpoint must be rate-limited
	// like any other password-based authentication endpoint.
	passwordChangeRateLimit = 5

	// passwordChangeRateInterval is the time window for the
	// password change rate limit.
	passwordChangeRateInterval = 1 * time.Minute

	// receiverRateInterval is the time window for the webhook
	// receiver rate limit. The configured limit is expressed in
	// requests per minute.
	receiverRateInterval = 1 * time.Minute

	// receiverAggregateMultiplier scales the configured
	// per-entrypoint receiver limit into the aggregate limit one
	// client IP may spend across the whole /webhook/* route. Ten
	// entrypoints' worth lets a single sender address drive several
	// entrypoints at their full rate, while still capping what one
	// address costs the unauthenticated receiver.
	receiverAggregateMultiplier = 10

	// maxForwardedHops bounds how many X-Forwarded-For entries the
	// chain walk examines. Real chains are one to three hops, but a
	// client can pad the header up to MaxHeaderBytes, so without a
	// bound every request pays a walk proportional to whatever the
	// client sent.
	maxForwardedHops = 64
)

// normalizeAddr strips the IPv4-in-IPv6 wrapper and any zone from
// addr so that comparisons and bucket keys are canonical.
func normalizeAddr(addr netip.Addr) netip.Addr {
	return addr.Unmap().WithZone("")
}

// isTrustedProxy reports whether addr belongs to a network the
// operator listed in TRUSTED_PROXIES. The list is empty by default,
// so by default nothing is trusted.
func (m *Middleware) isTrustedProxy(addr netip.Addr) bool {
	for _, prefix := range m.params.Config.TrustedProxies {
		if prefix.Contains(addr) {
			return true
		}
	}

	return false
}

// forwardedClientAddr returns the client address named by this
// request's X-Forwarded-For chain. It is consulted only for requests
// whose direct peer is a trusted proxy.
//
// X-Forwarded-For is the only header read. X-Real-IP and
// True-Client-IP are deliberately ignored: the reverse proxies in
// common use append to X-Forwarded-For and pass any other header the
// client sent through untouched, so believing a single-valued header
// would let a client behind the trusted proxy name its own bucket —
// the very bypass this gating exists to close.
//
// The chain is walked right to left, because the rightmost entry is
// the one the nearest proxy appended and everything to its left may
// have been written by the client. The first hop that is not itself
// a trusted proxy is the client. A hop that cannot be read as a bare
// address ends the walk: past it the chain is not the shape assumed
// here, so the caller falls back to the peer address.
//
// Only the last maxForwardedHops entries are examined. A longer chain
// is padding, and running out of hops falls back to the peer address
// the same way an unreadable hop does.
//
// The entries are cut off the right end of each header value in place
// rather than split out of it: the receiver is unauthenticated and a
// client can pad the header up to MaxHeaderBytes, so splitting would
// allocate in proportion to the padding (about 8 MB for a 1 MB
// header) before the cap could discard any of it. Multiple header
// values are walked in reverse for the same reason, since joining
// them copies the whole chain.
func (m *Middleware) forwardedClientAddr(
	r *http.Request,
) (netip.Addr, bool) {
	seen := 0

	for _, value := range slices.Backward(
		r.Header.Values("X-Forwarded-For"),
	) {
		for last := false; !last && seen < maxForwardedHops; seen++ {
			hop := value

			comma := strings.LastIndexByte(value, ',')
			if comma < 0 {
				last = true
			} else {
				hop, value = value[comma+1:], value[:comma]
			}

			hop = strings.TrimSpace(hop)
			if hop == "" {
				continue
			}

			addr, err := netip.ParseAddr(hop)
			if err != nil {
				return netip.Addr{}, false
			}

			if addr = normalizeAddr(addr); !m.isTrustedProxy(addr) {
				return addr, true
			}
		}
	}

	return netip.Addr{}, false
}

// rateLimitKey is the client identity every rate limiter in this
// package buckets on. Forwarded headers are honoured only when the
// direct peer (RemoteAddr) is inside the configured trusted-proxy
// set; otherwise the peer address itself is the key. Without that
// gate any client could mint a fresh bucket per request, or starve
// another client's bucket, by picking an X-Forwarded-For value —
// which makes every limit here decorative against a deliberate
// attacker.
func (m *Middleware) rateLimitKey(r *http.Request) (string, error) {
	return m.clientKey(r), nil
}

// clientKey computes the bucket key described on rateLimitKey.
func (m *Middleware) clientKey(r *http.Request) string {
	peer, err := netip.ParseAddr(ipFromHostPort(r.RemoteAddr))
	if err != nil {
		// Not an address we can reason about; key on the raw
		// value, the most specific identity left. On a
		// Unix-socket listener every peer carries the same
		// RemoteAddr and so shares one bucket, which is the
		// fail-closed direction.
		return r.RemoteAddr
	}

	peer = normalizeAddr(peer)
	if !m.isTrustedProxy(peer) {
		return peer.String()
	}

	if addr, ok := m.forwardedClientAddr(r); ok {
		return addr.String()
	}

	return peer.String()
}

// tooManyRequests returns the 429 handler shared by every limiter:
// it logs the rejection with logMessage and answers with
// responseMessage. httprate adds the Retry-After header (RFC 6585).
func (m *Middleware) tooManyRequests(
	logMessage, responseMessage string,
) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		m.log.Warn(logMessage, "path", r.URL.Path)
		http.Error(w, responseMessage, http.StatusTooManyRequests)
	}
}

// LoginRateLimit returns middleware that enforces per-IP rate
// limiting on login attempts using go-chi/httprate. Only POST
// requests are rate-limited; GET requests (rendering the login
// form) pass through unaffected. When the rate limit is exceeded,
// a 429 Too Many Requests response is returned. Clients are
// identified by rateLimitKey.
func (m *Middleware) LoginRateLimit() func(http.Handler) http.Handler {
	return m.postRateLimit(
		loginRateLimit,
		loginRateInterval,
		"login rate limit exceeded",
		"Too many login attempts. Please try again later.",
	)
}

// PasswordChangeRateLimit returns middleware that enforces
// per-IP rate limiting on password change attempts. The change
// endpoint verifies the current password, so without a limit a
// stolen session could be used to brute-force it; the limit
// matches the login endpoint's.
func (m *Middleware) PasswordChangeRateLimit() func(http.Handler) http.Handler {
	return m.postRateLimit(
		passwordChangeRateLimit,
		passwordChangeRateInterval,
		"password change rate limit exceeded",
		"Too many password change attempts. "+
			"Please try again later.",
	)
}

// postRateLimit builds middleware that enforces a per-IP rate
// limit on POST requests only; all other methods pass through
// unaffected. Requests over the limit receive a 429 with the
// given response message, and each rejection is logged with the
// given log message. Clients are identified by rateLimitKey.
func (m *Middleware) postRateLimit(
	limit int,
	interval time.Duration,
	logMessage, responseMessage string,
) func(http.Handler) http.Handler {
	limiter := httprate.Limit(
		limit,
		interval,
		httprate.WithKeyFuncs(m.rateLimitKey),
		httprate.WithLimitHandler(
			m.tooManyRequests(logMessage, responseMessage),
		),
	)

	return func(next http.Handler) http.Handler {
		limited := limiter(next)

		return http.HandlerFunc(func(
			w http.ResponseWriter,
			r *http.Request,
		) {
			// Only rate-limit POST requests.
			if r.Method != http.MethodPost {
				next.ServeHTTP(w, r)

				return
			}

			limited.ServeHTTP(w, r)
		})
	}
}

// ReceiverRateLimit returns middleware that rate-limits the public
// webhook receiver endpoint with two limits in series.
//
// The inner limit is per client IP per request path: the path
// contains the entrypoint UUID, so each sender is limited per
// entrypoint without affecting other senders or other entrypoints.
// It is Config.ReceiverRateLimit requests per minute.
//
// That limit alone bounds nothing in aggregate. The route pattern
// /webhook/{uuid} matches any single segment, so a client that
// invents a fresh path per request mints a fresh bucket per request
// and never refills one — and every such request still reaches the
// handler's entrypoint lookup before it 404s. The outer limit is
// therefore keyed on the client IP alone, capping what one address
// can spend across the whole route however it varies the path.
//
// Requests over either limit receive a 429. Clients are identified
// by rateLimitKey.
func (m *Middleware) ReceiverRateLimit() func(http.Handler) http.Handler {
	perEntrypoint := httprate.Limit(
		m.params.Config.ReceiverRateLimit,
		receiverRateInterval,
		httprate.WithKeyFuncs(
			m.rateLimitKey,
			httprate.KeyByEndpoint,
		),
		httprate.WithLimitHandler(m.tooManyRequests(
			"webhook receiver rate limit exceeded",
			"Too many requests. Please slow down.",
		)),
	)

	aggregate := httprate.Limit(
		receiverAggregateLimit(m.params.Config.ReceiverRateLimit),
		receiverRateInterval,
		httprate.WithKeyFuncs(m.rateLimitKey),
		httprate.WithLimitHandler(m.tooManyRequests(
			"webhook receiver aggregate rate limit exceeded",
			"Too many requests. Please slow down.",
		)),
	)

	return func(next http.Handler) http.Handler {
		return aggregate(perEntrypoint(next))
	}
}

// receiverAggregateLimit is the per-IP aggregate limit derived from
// the configured per-entrypoint limit. The operator sets the latter
// and nothing bounds it from above, so the multiplication is
// saturated rather than allowed to wrap into a negative limit that
// would reject every request.
func receiverAggregateLimit(perEntrypoint int) int {
	if perEntrypoint > math.MaxInt/receiverAggregateMultiplier {
		return math.MaxInt
	}

	return perEntrypoint * receiverAggregateMultiplier
}