// Package node — HTTP-level guards: body-size limits, timestamp windows, and
// a tiny per-IP token-bucket rate limiter.
//
// These are intentionally lightweight, dependency-free, and fail open if
// misconfigured. They do not replace proper production fronting (reverse
// proxy with rate-limit module), but they close the most obvious abuse
// vectors when the node is exposed directly.
package node

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

	"go-blockchain/blockchain"
)

// ─── Limits ──────────────────────────────────────────────────────────────────

// MaxTxRequestBytes caps the size of a single POST /api/tx body. A signed
// transaction with a modest payload sits well under 16 KiB; we allow 64 KiB
// to accommodate small WASM deploys submitted by trusted signers.
//
// Note: DEPLOY_CONTRACT with a large WASM binary uses the same endpoint and
// therefore the same cap. If you deploy bigger contracts, raise this cap on
// the nodes that accept deploys, or (better) add a dedicated upload route.
const MaxTxRequestBytes int64 = 64 * 1024

// TxTimestampSkew is the maximum accepted deviation between a transaction's
// declared timestamp and the node's current wall clock. Transactions outside
// the window are rejected at the API layer to harden against replay + clock
// skew attacks (we do not retain old txIDs forever, so a very old but
// otherwise valid tx could otherwise slip in after its dedup entry rotates).
const TxTimestampSkew = 1 * time.Hour

// ValidateTxTimestamp returns an error if tx.Timestamp is further than
// TxTimestampSkew from now. The zero-time is also rejected.
//
// Exported so the WS gateway (and other in-process callers) can reuse the
// same validation as the HTTP /api/tx path without importing the file-local
// helper.
func ValidateTxTimestamp(tx *blockchain.Transaction) error {
	if tx.Timestamp.IsZero() {
		return fmt.Errorf("timestamp is required")
	}
	now := time.Now().UTC()
	delta := tx.Timestamp.Sub(now)
	if delta < 0 {
		delta = -delta
	}
	if delta > TxTimestampSkew {
		return fmt.Errorf("timestamp %s is outside ±%s window of current time %s",
			tx.Timestamp.Format(time.RFC3339), TxTimestampSkew, now.Format(time.RFC3339))
	}
	return nil
}

// ─── Rate limiter ────────────────────────────────────────────────────────────

// ipRateLimiter is a best-effort token bucket keyed by source IP. Buckets are
// created lazily and garbage-collected by a background sweep.
type ipRateLimiter struct {
	rate        float64       // tokens added per second
	burst       float64       // bucket capacity
	sweepEvery  time.Duration // how often to GC inactive buckets
	inactiveTTL time.Duration // drop buckets idle longer than this

	mu      sync.Mutex
	buckets map[string]*bucket
	stopCh  chan struct{}
}

type bucket struct {
	tokens   float64
	lastSeen time.Time
}

// newIPRateLimiter constructs a limiter: each IP gets `burst` tokens that refill
// at `rate` per second.
func newIPRateLimiter(rate, burst float64) *ipRateLimiter {
	l := &ipRateLimiter{
		rate:        rate,
		burst:       burst,
		sweepEvery:  2 * time.Minute,
		inactiveTTL: 10 * time.Minute,
		buckets:     make(map[string]*bucket),
		stopCh:      make(chan struct{}),
	}
	go l.sweepLoop()
	return l
}

// Allow deducts one token for the given IP; returns false if the bucket is empty.
func (l *ipRateLimiter) Allow(ip string) bool {
	now := time.Now()
	l.mu.Lock()
	defer l.mu.Unlock()

	b, ok := l.buckets[ip]
	if !ok {
		b = &bucket{tokens: l.burst, lastSeen: now}
		l.buckets[ip] = b
	}
	elapsed := now.Sub(b.lastSeen).Seconds()
	b.tokens += elapsed * l.rate
	if b.tokens > l.burst {
		b.tokens = l.burst
	}
	b.lastSeen = now
	if b.tokens < 1 {
		return false
	}
	b.tokens--
	return true
}

func (l *ipRateLimiter) sweepLoop() {
	tk := time.NewTicker(l.sweepEvery)
	defer tk.Stop()
	for {
		select {
		case <-l.stopCh:
			return
		case now := <-tk.C:
			l.mu.Lock()
			for ip, b := range l.buckets {
				if now.Sub(b.lastSeen) > l.inactiveTTL {
					delete(l.buckets, ip)
				}
			}
			l.mu.Unlock()
		}
	}
}

// clientIP extracts the best-effort originating IP. X-Forwarded-For is
// respected but only the leftmost entry is trusted; do not rely on this
// behind untrusted proxies (configure a real reverse proxy for production).
func clientIP(r *http.Request) string {
	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
		if i := strings.IndexByte(xff, ','); i >= 0 {
			return strings.TrimSpace(xff[:i])
		}
		return strings.TrimSpace(xff)
	}
	if xr := r.Header.Get("X-Real-IP"); xr != "" {
		return strings.TrimSpace(xr)
	}
	ip, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		return r.RemoteAddr
	}
	return ip
}

// Package-level singletons (best-effort; DoS hardening is a defense-in-depth
// measure, not a primary security boundary).
var (
	// Allow up to 10 tx submissions/s per IP with a burst of 20.
	submitTxLimiter = newIPRateLimiter(10, 20)
	// Inbox / contacts polls: allow 20/s per IP with a burst of 40.
	readLimiter = newIPRateLimiter(20, 40)
)

// withSubmitTxGuards composes size + rate-limit protection around apiSubmitTx.
// It rejects oversize bodies and flooding IPs before any JSON decoding happens,
// so an attacker cannot consume CPU decoding 10 MB of nothing.
func withSubmitTxGuards(inner http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		if r.Method == http.MethodPost {
			ip := clientIP(r)
			if !submitTxLimiter.Allow(ip) {
				w.Header().Set("Retry-After", "2")
				jsonErr(w, fmt.Errorf("rate limit exceeded"), http.StatusTooManyRequests)
				return
			}
			r.Body = http.MaxBytesReader(w, r.Body, MaxTxRequestBytes)
		}
		inner.ServeHTTP(w, r)
	}
}

// withReadLimit is the equivalent for inbox / contacts read endpoints. It only
// applies rate limiting (no body size cap needed for GETs).
func withReadLimit(inner http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		if !readLimiter.Allow(clientIP(r)) {
			w.Header().Set("Retry-After", "1")
			jsonErr(w, fmt.Errorf("rate limit exceeded"), http.StatusTooManyRequests)
			return
		}
		inner.ServeHTTP(w, r)
	}
}

// ── Access-token gating ──────────────────────────────────────────────────────
//
// Single-node operators often want their node "private" — only they (and
// apps they explicitly configure) can submit transactions or read state
// through its HTTP API. Two flavours:
//
//   1. Semi-public: anyone can GET chain state (netstats, blocks, txs),
//      but only clients with a valid Bearer token can POST /api/tx or
//      send WS `submit_tx`. Default when `--api-token` is set.
//
//   2. Fully private: EVERY endpoint requires the token. Use `--api-private`
//      together with `--api-token`. Good for a personal node whose data
//      the operator considers sensitive (e.g. who they're chatting with).
//
// Both modes gate via a shared secret (HTTP `Authorization: Bearer <token>`
// or WS hello-time check). There's no multi-user access control here —
// operators wanting role-based auth should front the node with their
// usual reverse-proxy auth (mTLS, OAuth, basicauth). This is the
// "just keep randos off my box" level of access control.

var (
	// accessToken is the shared secret required by gated endpoints.
	// Empty = gating disabled.
	accessToken string
	// accessPrivate gates READ endpoints too, not just write. Only
	// applies when accessToken is non-empty.
	accessPrivate bool
)

// SetAPIAccess configures the token + private-mode flags. Called once at
// node startup. Pass empty token to disable gating entirely (public node).
func SetAPIAccess(token string, private bool) {
	accessToken = strings.TrimSpace(token)
	accessPrivate = private
}

// checkAccessToken returns nil if the request carries the configured
// Bearer token, or an error describing the problem. When accessToken
// is empty (public node) it always returns nil.
//
// Accepts `Authorization: Bearer <token>` or the `?token=<token>` query
// parameter — the latter lets operators open `https://node.example.com/
// api/netstats?token=...` directly in a browser while testing a private
// node without needing custom headers.
func checkAccessToken(r *http.Request) error {
	if accessToken == "" {
		return nil
	}
	// Header takes precedence.
	auth := r.Header.Get("Authorization")
	if strings.HasPrefix(auth, "Bearer ") {
		if strings.TrimPrefix(auth, "Bearer ") == accessToken {
			return nil
		}
		return fmt.Errorf("invalid bearer token")
	}
	if q := r.URL.Query().Get("token"); q != "" {
		if q == accessToken {
			return nil
		}
		return fmt.Errorf("invalid ?token= query param")
	}
	return fmt.Errorf("missing bearer token; pass Authorization: Bearer <token> or ?token=...")
}

// withWriteTokenGuard wraps a write handler (submit tx, etc.) so it
// requires the access token whenever one is configured. Independent of
// accessPrivate — writes are ALWAYS gated when a token is set.
func withWriteTokenGuard(inner http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		if err := checkAccessToken(r); err != nil {
			w.Header().Set("WWW-Authenticate", `Bearer realm="dchain"`)
			jsonErr(w, err, http.StatusUnauthorized)
			return
		}
		inner.ServeHTTP(w, r)
	}
}

// withReadTokenGuard gates a read endpoint. Only enforced when
// accessPrivate is true; otherwise falls through to the inner handler
// so public nodes keep working as before.
func withReadTokenGuard(inner http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		if accessToken != "" && accessPrivate {
			if err := checkAccessToken(r); err != nil {
				w.Header().Set("WWW-Authenticate", `Bearer realm="dchain"`)
				jsonErr(w, err, http.StatusUnauthorized)
				return
			}
		}
		inner.ServeHTTP(w, r)
	}
}

// AccessTokenForWS returns the current token + private-mode flag so the
// WS hub can apply the same policy to websocket submit_tx ops and, when
// private, to connection upgrades themselves.
func AccessTokenForWS() (token string, private bool) {
	return accessToken, accessPrivate
}