package membership

import (
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"fmt"
	"net/http"
	"strconv"
	"sync"
	"time"

	cs "fn-registry/functions/cybersecurity"

	"github.com/enmanuel/unibus/pkg/frame"
)

// AuthMode is the control-plane authentication rollout state (feature flag
// bus-auth). It governs how the HTTP middleware treats a request whose signature
// is missing, invalid, replayed, skewed, or from an unregistered identity.
//
//	AuthOff     — do not verify anything (legacy behavior; default).
//	AuthSoft    — verify and LOG rejections, but let the request through. Lets
//	              clients migrate to signing without an outage.
//	AuthEnforce — reject unauthenticated requests with 401.
type AuthMode int

const (
	AuthOff AuthMode = iota
	AuthSoft
	AuthEnforce
)

func (m AuthMode) String() string {
	switch m {
	case AuthOff:
		return "off"
	case AuthSoft:
		return "soft"
	case AuthEnforce:
		return "enforce"
	default:
		return "unknown"
	}
}

// ParseAuthMode maps the bus-auth flag string to an AuthMode.
func ParseAuthMode(s string) (AuthMode, error) {
	switch s {
	case "off", "":
		return AuthOff, nil
	case "soft":
		return AuthSoft, nil
	case "enforce":
		return AuthEnforce, nil
	default:
		return AuthOff, fmt.Errorf("membership: invalid bus-auth mode %q (want off|soft|enforce)", s)
	}
}

// Control-plane signature headers. The client signs the canonical bytes of the
// request and presents these; the server reconstructs the canonical bytes and
// verifies. See canonicalRequest for the exact byte layout.
const (
	hdrPub   = "X-Unibus-Pub"   // signer Ed25519 public key, lowercase hex
	hdrTs    = "X-Unibus-Ts"    // unix seconds (string)
	hdrNonce = "X-Unibus-Nonce" // 16 random bytes, std base64
	hdrSig   = "X-Unibus-Sig"   // Ed25519 signature over canonical, std base64
)

// Anti-replay parameters. A request is accepted only if its timestamp is within
// clockSkew of now; nonces are remembered for nonceTTL so a captured request
// cannot be replayed inside its acceptance window. nonceTTL must be >= the full
// acceptance window (2*clockSkew) so a replay can never outlive its memory.
const (
	clockSkew = 30 * time.Second
	nonceTTL  = 60 * time.Second
	// maxNonceCacheEntries bounds the replay cache so it cannot grow without limit
	// (audit H7). With IsAuthorized now gating insertion, only authorized traffic
	// is cached, so this ceiling is only approached under a legitimate burst; at
	// the cap the oldest nonce is evicted (its TTL is nearly up anyway).
	maxNonceCacheEntries = 100_000
)

// CanonicalRequest returns the exact bytes that are signed and verified for a
// control-plane request:
//
//	method "\n" path "\n" ts "\n" nonce "\n" hex(sha256(body))
//
// path is the request URI (path plus raw query) so query parameters (endpoint,
// epoch) are covered by the signature. It is exported so the client library and
// tests sign with the identical construction — the one place this format lives.
func CanonicalRequest(method, path, ts, nonce string, body []byte) []byte {
	sum := sha256.Sum256(body)
	return []byte(method + "\n" + path + "\n" + ts + "\n" + nonce + "\n" + hex.EncodeToString(sum[:]))
}

// nonceCache remembers recently-seen nonces to reject replays. It is an
// in-memory store guarded by a mutex — sufficient for a single membershipd
// process (the spec's chosen tradeoff over a server-issued nonce round-trip). A
// distributed deployment would need a shared store (tracked for issue 0003).
//
// Pruning is O(expired), not O(n): because the TTL is constant, insertion order
// equals expiry order, so the oldest entries (front of `order`) are exactly the
// ones that expire first (audit H7 — the previous full-map scan under the mutex
// was a CPU-amplification vector). A size cap bounds memory.
type nonceCache struct {
	mu    sync.Mutex
	seen  map[string]time.Time // nonce -> expiry
	order []string             // nonces in insertion order == expiry order
	ttl   time.Duration
	cap   int
}

func newNonceCache(ttl time.Duration, capacity int) *nonceCache {
	return &nonceCache{seen: make(map[string]time.Time), ttl: ttl, cap: capacity}
}

// rememberOrReject records nonce and returns true if it was unseen, or false if
// it is a replay (still live in the cache).
func (n *nonceCache) rememberOrReject(nonce string, now time.Time) bool {
	n.mu.Lock()
	defer n.mu.Unlock()

	// Prune expired entries from the front (oldest first). The first live entry
	// ends the scan — everything behind it was inserted later and is newer.
	cut := 0
	for cut < len(n.order) {
		exp, ok := n.seen[n.order[cut]]
		if !ok {
			cut++ // already evicted by the cap path below
			continue
		}
		if !exp.Before(now) {
			break
		}
		delete(n.seen, n.order[cut])
		cut++
	}
	if cut > 0 {
		n.order = append(n.order[:0], n.order[cut:]...)
	}

	if exp, ok := n.seen[nonce]; ok && !exp.Before(now) {
		return false // a live replay
	}

	// Bound memory: at capacity, evict the oldest entry (its TTL is nearly up).
	for len(n.seen) >= n.cap && len(n.order) > 0 {
		oldest := n.order[0]
		n.order = n.order[1:]
		delete(n.seen, oldest)
	}

	n.seen[nonce] = now.Add(n.ttl)
	n.order = append(n.order, nonce)
	return true
}

// authResult is what a successful authentication yields: the verified signing
// key (hex), the endpoint id derived from it, and the authorized user record.
// Handlers use endpoint for membership authorization (only a member of a room
// may read its metadata/keys); user is available for role checks.
type authResult struct {
	pubHex   string
	endpoint string
	user     User
}

// authenticate verifies the signature headers on r against body and the user
// allowlist. It returns an error describing the first failing check; the
// middleware decides whether that error blocks (enforce) or only logs (soft).
//
// Order matters: cheap, non-cryptographic checks (header presence, key shape,
// clock skew) run first; the Ed25519 verification runs before the replay cache
// is touched so an attacker cannot poison the cache with unsigned nonces; the
// allowlist lookup runs last.
func (s *Server) authenticate(r *http.Request, body []byte, now time.Time) (authResult, error) {
	pubHex := r.Header.Get(hdrPub)
	ts := r.Header.Get(hdrTs)
	nonce := r.Header.Get(hdrNonce)
	sigB64 := r.Header.Get(hdrSig)
	if pubHex == "" || ts == "" || nonce == "" || sigB64 == "" {
		return authResult{}, fmt.Errorf("missing auth headers")
	}

	pub, err := hex.DecodeString(pubHex)
	if err != nil || len(pub) != 32 {
		return authResult{}, fmt.Errorf("malformed %s (want 32-byte Ed25519 hex)", hdrPub)
	}

	tsInt, err := strconv.ParseInt(ts, 10, 64)
	if err != nil {
		return authResult{}, fmt.Errorf("malformed %s", hdrTs)
	}
	if d := now.Unix() - tsInt; d > int64(clockSkew/time.Second) || d < -int64(clockSkew/time.Second) {
		return authResult{}, fmt.Errorf("timestamp out of range (skew %ds)", d)
	}

	sig, err := base64.StdEncoding.DecodeString(sigB64)
	if err != nil {
		return authResult{}, fmt.Errorf("malformed %s", hdrSig)
	}

	canonical := CanonicalRequest(r.Method, r.URL.RequestURI(), ts, nonce, body)
	if !cs.VerifyEd25519(pub, canonical, sig) {
		return authResult{}, fmt.Errorf("invalid signature")
	}

	// Authorize BEFORE touching the replay cache (audit H7): an unregistered
	// identity can mint valid signatures for free, so caching its nonces would let
	// it poison/grow the cache pre-auth. Only authorized identities are remembered.
	if !s.store.IsAuthorized(pubHex) {
		return authResult{}, fmt.Errorf("identity not authorized")
	}

	user, err := s.store.GetUser(pubHex)
	if err != nil {
		// IsAuthorized passed but the row vanished (race with revoke): fail closed.
		return authResult{}, fmt.Errorf("identity not authorized")
	}

	// Anti-replay last: a replayed request from an authorized identity is still
	// rejected here (the nonce is already live in the cache from its first use).
	if !s.nonces.rememberOrReject(nonce, now) {
		return authResult{}, fmt.Errorf("replayed nonce")
	}

	return authResult{pubHex: pubHex, endpoint: frame.EndpointID(pub), user: user}, nil
}