package restapi

import (
	"bytes"
	"errors"
	"io"
	"math"
	"net"
	"net/http"
	"path"
	"strconv"
	"strings"
	"sync"
	"time"

	"galaxy/gateway/internal/config"

	"github.com/gin-gonic/gin"
	"golang.org/x/time/rate"
)

const (
	errorCodeRequestTooLarge = "request_too_large"
	errorCodeRateLimited     = "rate_limited"

	publicRESTIPBucketKeySegment = "/ip="
)

var errRequestBodyTooLarge = errors.New("request body exceeds the configured limit")

// PublicMalformedRequestReason identifies the stable malformed-request counter
// dimension recorded by the public REST anti-abuse middleware.
type PublicMalformedRequestReason string

const (
	// PublicMalformedRequestReasonEmptyBody records a missing request body.
	PublicMalformedRequestReasonEmptyBody PublicMalformedRequestReason = "empty_body"

	// PublicMalformedRequestReasonMalformedJSON records syntactically malformed
	// JSON.
	PublicMalformedRequestReasonMalformedJSON PublicMalformedRequestReason = "malformed_json"

	// PublicMalformedRequestReasonInvalidJSONValue records JSON values whose
	// types do not match the expected request schema.
	PublicMalformedRequestReasonInvalidJSONValue PublicMalformedRequestReason = "invalid_json_value"

	// PublicMalformedRequestReasonUnknownField records JSON objects with fields
	// outside the documented schema.
	PublicMalformedRequestReasonUnknownField PublicMalformedRequestReason = "unknown_field"

	// PublicMalformedRequestReasonMultipleJSONObjects records requests that
	// contain more than one JSON object.
	PublicMalformedRequestReasonMultipleJSONObjects PublicMalformedRequestReason = "multiple_json_objects"

	// PublicMalformedRequestReasonOversizedBody records requests whose bodies
	// exceed the configured class limit.
	PublicMalformedRequestReasonOversizedBody PublicMalformedRequestReason = "oversized_body"
)

// PublicRateLimitDecision describes the outcome returned by a public REST
// limiter for one request bucket reservation attempt.
type PublicRateLimitDecision struct {
	// Allowed reports whether the request may proceed immediately.
	Allowed bool

	// RetryAfter is the minimum delay the client should wait before retrying
	// when Allowed is false.
	RetryAfter time.Duration
}

// PublicRequestLimiter applies public REST rate-limit policy to a concrete
// bucket key.
type PublicRequestLimiter interface {
	// Reserve evaluates key under policy and returns whether the request may
	// proceed immediately.
	Reserve(key string, policy config.PublicRateLimitConfig) PublicRateLimitDecision
}

// PublicRequestObserver captures low-cardinality public REST anti-abuse
// telemetry.
type PublicRequestObserver interface {
	// RecordMalformedRequest records one malformed request in class for reason.
	RecordMalformedRequest(class PublicRouteClass, reason PublicMalformedRequestReason)
}

type noopPublicRequestObserver struct{}

func (noopPublicRequestObserver) RecordMalformedRequest(PublicRouteClass, PublicMalformedRequestReason) {
}

type inMemoryPublicRequestLimiter struct {
	now             func() time.Time
	cleanupInterval time.Duration

	mu          sync.Mutex
	entries     map[string]*publicRateLimiterEntry
	nextCleanup time.Time
}

type publicRateLimiterEntry struct {
	limiter   *rate.Limiter
	limit     rate.Limit
	burst     int
	expiresAt time.Time
}

func newInMemoryPublicRequestLimiter() *inMemoryPublicRequestLimiter {
	return &inMemoryPublicRequestLimiter{
		now:             time.Now,
		cleanupInterval: time.Minute,
		entries:         make(map[string]*publicRateLimiterEntry),
	}
}

func (l *inMemoryPublicRequestLimiter) Reserve(key string, policy config.PublicRateLimitConfig) PublicRateLimitDecision {
	now := l.now()
	limit := rate.Limit(float64(policy.Requests) / policy.Window.Seconds())

	l.mu.Lock()
	defer l.mu.Unlock()

	l.cleanupExpiredBucketsLocked(now)

	entry, ok := l.entries[key]
	if !ok || entry.limit != limit || entry.burst != policy.Burst {
		entry = &publicRateLimiterEntry{
			limiter: rate.NewLimiter(limit, policy.Burst),
			limit:   limit,
			burst:   policy.Burst,
		}
		l.entries[key] = entry
	}

	entry.expiresAt = now.Add(publicRateLimiterEntryTTL(policy.Window))

	reservation := entry.limiter.ReserveN(now, 1)
	if !reservation.OK() {
		return PublicRateLimitDecision{
			Allowed:    false,
			RetryAfter: policy.Window,
		}
	}

	retryAfter := reservation.DelayFrom(now)
	if retryAfter > 0 {
		return PublicRateLimitDecision{
			Allowed:    false,
			RetryAfter: retryAfter,
		}
	}

	return PublicRateLimitDecision{Allowed: true}
}

func (l *inMemoryPublicRequestLimiter) cleanupExpiredBucketsLocked(now time.Time) {
	if !l.nextCleanup.IsZero() && now.Before(l.nextCleanup) {
		return
	}

	for key, entry := range l.entries {
		if !entry.expiresAt.After(now) {
			delete(l.entries, key)
		}
	}

	l.nextCleanup = now.Add(l.cleanupInterval)
}

func publicRateLimiterEntryTTL(window time.Duration) time.Duration {
	if window < time.Minute {
		return time.Minute
	}

	return 2 * window
}

func withPublicAntiAbuse(policy config.PublicHTTPAntiAbuseConfig, limiter PublicRequestLimiter, observer PublicRequestObserver) gin.HandlerFunc {
	return func(c *gin.Context) {
		class, ok := PublicRouteClassFromContext(c.Request.Context())
		if !ok {
			class = PublicRouteClassPublicMisc
		}

		allowedMethods := allowedMethodsForRequestShape(c.Request)
		if len(allowedMethods) > 0 && !isAllowedMethod(c.Request.Method, allowedMethods) {
			c.Header("Allow", strings.Join(allowedMethods, ", "))
			abortWithError(c, http.StatusMethodNotAllowed, errorCodeMethodNotAllowed, "request method is not allowed for this route")
			return
		}

		classPolicy := publicRoutePolicyForClass(policy, class)
		bodyBytes, err := bufferRequestBody(c.Request, classPolicy.MaxBodyBytes)
		if err != nil {
			switch {
			case errors.Is(err, errRequestBodyTooLarge):
				observer.RecordMalformedRequest(class, PublicMalformedRequestReasonOversizedBody)
				abortWithError(c, http.StatusRequestEntityTooLarge, errorCodeRequestTooLarge, "request body exceeds the configured limit")
			default:
				abortWithError(c, http.StatusInternalServerError, errorCodeInternalError, "internal server error")
			}
			return
		}

		clientIP := clientIPFromRemoteAddr(c.Request.RemoteAddr)
		if decision := limiter.Reserve(publicRESTIPBucketKey(class, clientIP), classPolicy.RateLimit); !decision.Allowed {
			abortRateLimited(c, decision.RetryAfter)
			return
		}

		identity, err := extractPublicAuthIdentity(c.Request.URL.Path, bodyBytes)
		switch {
		case err == nil:
			identityPolicy := publicAuthIdentityPolicyForPath(c.Request.URL.Path, policy)
			if decision := limiter.Reserve(publicAuthIdentityBucketKey(class, identity.kind, identity.value), identityPolicy.RateLimit); !decision.Allowed {
				abortRateLimited(c, decision.RetryAfter)
				return
			}
		case errors.Is(err, errPublicAuthIdentityNotApplicable):
		default:
			if reason, malformed := malformedRequestReasonFromError(err); malformed {
				observer.RecordMalformedRequest(class, reason)
			}
		}

		c.Next()
	}
}

func publicRoutePolicyForClass(policy config.PublicHTTPAntiAbuseConfig, class PublicRouteClass) config.PublicRoutePolicyConfig {
	switch class.Normalized() {
	case PublicRouteClassPublicAuth:
		return policy.PublicAuth
	case PublicRouteClassBrowserBootstrap:
		return policy.BrowserBootstrap
	case PublicRouteClassBrowserAsset:
		return policy.BrowserAsset
	default:
		return policy.PublicMisc
	}
}

func publicAuthIdentityPolicyForPath(requestPath string, policy config.PublicHTTPAntiAbuseConfig) config.PublicAuthIdentityPolicyConfig {
	switch requestPath {
	case "/api/v1/public/auth/send-email-code":
		return policy.SendEmailCodeIdentity
	case "/api/v1/public/auth/confirm-email-code":
		return policy.ConfirmEmailCodeIdentity
	default:
		return config.PublicAuthIdentityPolicyConfig{}
	}
}

func allowedMethodsForRequestShape(r *http.Request) []string {
	switch {
	case isPublicAuthPath(r.URL.Path):
		return []string{http.MethodPost}
	case isProbePath(r.URL.Path):
		return []string{http.MethodGet}
	case matchesBrowserAssetRequestShape(r):
		return []string{http.MethodGet, http.MethodHead}
	case matchesBrowserBootstrapRequestShape(r):
		return []string{http.MethodGet, http.MethodHead}
	default:
		return nil
	}
}

func isAllowedMethod(method string, allowedMethods []string) bool {
	for _, allowedMethod := range allowedMethods {
		if method == allowedMethod {
			return true
		}
	}

	return false
}

func isPublicAuthPath(requestPath string) bool {
	switch requestPath {
	case "/api/v1/public/auth/send-email-code", "/api/v1/public/auth/confirm-email-code":
		return true
	default:
		return false
	}
}

func isProbePath(requestPath string) bool {
	switch requestPath {
	case "/healthz", "/readyz":
		return true
	default:
		return false
	}
}

func matchesBrowserBootstrapRequestShape(r *http.Request) bool {
	if r.URL.Path == "/" {
		return true
	}

	return strings.Contains(strings.ToLower(r.Header.Get("Accept")), "text/html")
}

func matchesBrowserAssetRequestShape(r *http.Request) bool {
	if strings.HasPrefix(r.URL.Path, "/assets/") {
		return true
	}

	switch strings.ToLower(path.Ext(r.URL.Path)) {
	case ".js", ".mjs", ".css", ".map", ".png", ".jpg", ".jpeg", ".gif", ".svg", ".ico", ".woff", ".woff2", ".json", ".webmanifest":
		return true
	default:
		return false
	}
}

func bufferRequestBody(r *http.Request, maxBodyBytes int64) ([]byte, error) {
	if r == nil {
		return nil, nil
	}

	if r.Body == nil {
		r.Body = io.NopCloser(bytes.NewReader(nil))
		return nil, nil
	}

	bodyBytes, err := io.ReadAll(io.LimitReader(r.Body, maxBodyBytes+1))
	closeErr := r.Body.Close()
	if err != nil {
		return nil, err
	}
	if closeErr != nil {
		return nil, closeErr
	}
	if int64(len(bodyBytes)) > maxBodyBytes {
		return nil, errRequestBodyTooLarge
	}

	r.Body = io.NopCloser(bytes.NewReader(bodyBytes))

	return bodyBytes, nil
}

func abortRateLimited(c *gin.Context, retryAfter time.Duration) {
	c.Header("Retry-After", retryAfterHeaderValue(retryAfter))
	abortWithError(c, http.StatusTooManyRequests, errorCodeRateLimited, "request rate limit exceeded")
}

func retryAfterHeaderValue(delay time.Duration) string {
	seconds := int64(math.Ceil(delay.Seconds()))
	if seconds < 1 {
		seconds = 1
	}

	return strconv.FormatInt(seconds, 10)
}

func clientIPFromRemoteAddr(remoteAddr string) string {
	host, _, err := net.SplitHostPort(strings.TrimSpace(remoteAddr))
	if err == nil {
		return host
	}

	remoteAddr = strings.TrimSpace(remoteAddr)
	if remoteAddr == "" {
		return "unknown"
	}

	return remoteAddr
}

func publicRESTIPBucketKey(class PublicRouteClass, clientIP string) string {
	return class.BaseBucketKey() + publicRESTIPBucketKeySegment + clientIP
}

func publicAuthIdentityBucketKey(class PublicRouteClass, identityKind string, identityValue string) string {
	return class.BaseBucketKey() + "/" + identityKind + "=" + identityValue
}