// Package auth: argon2id password hashing, cookie sessions, login rate limiting.
package auth

import (
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/hex"
	"errors"
	"fmt"
	"strings"
	"sync"
	"time"

	"golang.org/x/crypto/argon2"

	"hamlab/internal/store"
)

const (
	argonTime    = 3
	argonMemory  = 64 * 1024
	argonThreads = 2
	argonKeyLen  = 32

	SessionTTL = 30 * 24 * time.Hour
)

func HashPassword(pw string) (string, error) {
	salt := make([]byte, 16)
	if _, err := rand.Read(salt); err != nil {
		return "", err
	}
	key := argon2.IDKey([]byte(pw), salt, argonTime, argonMemory, argonThreads, argonKeyLen)
	enc := base64.RawStdEncoding.EncodeToString
	return fmt.Sprintf("$argon2id$v=19$m=%d,t=%d,p=%d$%s$%s",
		argonMemory, argonTime, argonThreads, enc(salt), enc(key)), nil
}

func VerifyPassword(hash, pw string) bool {
	parts := strings.Split(hash, "$")
	if len(parts) != 6 || parts[1] != "argon2id" {
		return false
	}
	var m, t uint32
	var p uint8
	if _, err := fmt.Sscanf(parts[3], "m=%d,t=%d,p=%d", &m, &t, &p); err != nil {
		return false
	}
	salt, err := base64.RawStdEncoding.DecodeString(parts[4])
	if err != nil {
		return false
	}
	want, err := base64.RawStdEncoding.DecodeString(parts[5])
	if err != nil {
		return false
	}
	got := argon2.IDKey([]byte(pw), salt, t, m, p, uint32(len(want)))
	return subtle.ConstantTimeCompare(got, want) == 1
}

// ---- API tokens (hub → node) ----

// NewToken returns a random bearer token and its SHA-256 hex, which is what gets stored.
func NewToken() (token, hash string, err error) {
	b := make([]byte, 32)
	if _, err := rand.Read(b); err != nil {
		return "", "", err
	}
	token = base64.RawURLEncoding.EncodeToString(b)
	return token, HashToken(token), nil
}

func HashToken(token string) string {
	sum := sha256.Sum256([]byte(token))
	return hex.EncodeToString(sum[:])
}

// TokenMatches compares a presented token against stored hashes in constant time.
func TokenMatches(token string, hashes []string) bool {
	h := []byte(HashToken(token))
	ok := false
	for _, want := range hashes {
		if subtle.ConstantTimeCompare(h, []byte(want)) == 1 {
			ok = true
		}
	}
	return ok
}

// Sessions is a token store backed by the state file. Expiry is sliding.
type Sessions struct {
	st *store.Store
}

func NewSessions(st *store.Store) *Sessions {
	s := &Sessions{st: st}
	_ = st.Update(func(state *store.State) { pruneExpired(state) })
	return s
}

func pruneExpired(state *store.State) {
	now := time.Now()
	for tok, exp := range state.Sessions {
		if exp.Before(now) {
			delete(state.Sessions, tok)
		}
	}
}

func (s *Sessions) Create() (string, error) {
	b := make([]byte, 32)
	if _, err := rand.Read(b); err != nil {
		return "", err
	}
	tok := hex.EncodeToString(b)
	err := s.st.Update(func(state *store.State) {
		pruneExpired(state)
		state.Sessions[tok] = time.Now().Add(SessionTTL)
	})
	return tok, err
}

// Valid reports whether tok is live, sliding its expiry (persisted at most hourly).
func (s *Sessions) Valid(tok string) bool {
	if tok == "" {
		return false
	}
	ok := false
	extend := false
	s.st.View(func(state *store.State) {
		exp, found := state.Sessions[tok]
		if found && exp.After(time.Now()) {
			ok = true
			extend = time.Until(exp) < SessionTTL-time.Hour
		}
	})
	if ok && extend {
		_ = s.st.Update(func(state *store.State) {
			if _, found := state.Sessions[tok]; found {
				state.Sessions[tok] = time.Now().Add(SessionTTL)
			}
		})
	}
	return ok
}

func (s *Sessions) Delete(tok string) {
	_ = s.st.Update(func(state *store.State) { delete(state.Sessions, tok) })
}

// Limiter is a single global backoff: after 5 failures, each further failure
// doubles a lockout starting at 30s, capped at 10 minutes. Single-user box.
// Limiter locks out a client after repeated failed logins, per client address, with a
// global backstop so a botnet can't burn CPU on argon2 indefinitely. A lockout for one
// address never blocks another.
type Limiter struct {
	mu      sync.Mutex
	clients map[string]*bucket
	global  bucket
}

type bucket struct {
	failures  int
	lockUntil time.Time
	last      time.Time
}

var ErrLocked = errors.New("too many failed logins")

const (
	perClientLock = 5   // failures before a client is locked
	globalLock    = 100 // failures (all clients) in the window before everyone waits a minute
	globalWindow  = 5 * time.Minute
)

func (l *Limiter) get(key string) *bucket {
	if l.clients == nil {
		l.clients = map[string]*bucket{}
	}
	b, ok := l.clients[key]
	if !ok {
		b = &bucket{}
		l.clients[key] = b
	}
	// forget clients that have been quiet for an hour
	if len(l.clients) > 256 {
		for k, x := range l.clients {
			if time.Since(x.last) > time.Hour && time.Now().After(x.lockUntil) {
				delete(l.clients, k)
			}
		}
	}
	return b
}

func (l *Limiter) Allow(key string) error {
	l.mu.Lock()
	defer l.mu.Unlock()
	now := time.Now()
	if now.Before(l.global.lockUntil) {
		return ErrLocked
	}
	if now.Sub(l.global.last) > globalWindow {
		l.global.failures = 0
	}
	if b := l.get(key); now.Before(b.lockUntil) {
		return ErrLocked
	}
	return nil
}

func (l *Limiter) Fail(key string) {
	l.mu.Lock()
	defer l.mu.Unlock()
	now := time.Now()
	b := l.get(key)
	b.failures++
	b.last = now
	if b.failures >= perClientLock {
		d := 30 * time.Second << uint(b.failures-perClientLock)
		if d > 10*time.Minute {
			d = 10 * time.Minute
		}
		b.lockUntil = now.Add(d)
	}
	if now.Sub(l.global.last) > globalWindow {
		l.global.failures = 0
	}
	l.global.failures++
	l.global.last = now
	if l.global.failures >= globalLock {
		l.global.lockUntil = now.Add(time.Minute)
		l.global.failures = 0
	}
}

func (l *Limiter) Reset(key string) {
	l.mu.Lock()
	defer l.mu.Unlock()
	delete(l.clients, key)
}