// Package session implements the Email Agent's server-side session store and
// the signed cookie format used to reference a session.
//
// Sessions live in the process (there is one user and one container), so the
// cookie only ever carries an opaque, HMAC-signed identifier. Nothing about the
// user or the session state is stored client-side.
package session

import (
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/hex"
	"strings"
	"sync"
	"time"
)

// CookieName is the name of the session cookie.
const CookieName = "email_agent_session"

// DefaultTTL is how long a session stays valid.
const DefaultTTL = 30 * 24 * time.Hour

// Session is a single logged-in session.
type Session struct {
	ID        string
	User      string
	CreatedAt time.Time
	ExpiresAt time.Time
}

// Store is an in-memory, concurrency-safe session store with lazy expiry.
type Store struct {
	mu       sync.Mutex
	sessions map[string]Session
	ttl      time.Duration
	now      func() time.Time
}

// NewStore returns an empty store. A non-positive ttl falls back to DefaultTTL.
func NewStore(ttl time.Duration) *Store {
	if ttl <= 0 {
		ttl = DefaultTTL
	}
	return &Store{
		sessions: make(map[string]Session),
		ttl:      ttl,
		now:      time.Now,
	}
}

// Create makes a new session for user and returns it.
func (s *Store) Create(user string) (Session, error) {
	id, err := RandomToken(32)
	if err != nil {
		return Session{}, err
	}
	now := s.now()
	sess := Session{
		ID:        id,
		User:      user,
		CreatedAt: now,
		ExpiresAt: now.Add(s.ttl),
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	s.gcLocked()
	s.sessions[id] = sess
	return sess, nil
}

// Get returns the session for id if it exists and has not expired.
func (s *Store) Get(id string) (Session, bool) {
	s.mu.Lock()
	defer s.mu.Unlock()
	sess, ok := s.sessions[id]
	if !ok {
		return Session{}, false
	}
	if s.now().After(sess.ExpiresAt) {
		delete(s.sessions, id)
		return Session{}, false
	}
	return sess, true
}

// Delete removes a session.
func (s *Store) Delete(id string) {
	s.mu.Lock()
	defer s.mu.Unlock()
	delete(s.sessions, id)
}

// Len returns the number of live (not-yet-swept) sessions.
func (s *Store) Len() int {
	s.mu.Lock()
	defer s.mu.Unlock()
	return len(s.sessions)
}

func (s *Store) gcLocked() {
	now := s.now()
	for id, sess := range s.sessions {
		if now.After(sess.ExpiresAt) {
			delete(s.sessions, id)
		}
	}
}

// Manager pairs a session store with the HMAC key that signs cookie values.
type Manager struct {
	store *Store
	key   []byte
}

// NewManager builds a Manager. An empty key is replaced with a random key so
// the manager is always safe to use; the caller should normally supply one.
func NewManager(key []byte, ttl time.Duration) (*Manager, error) {
	if len(key) == 0 {
		k, err := randomBytes(32)
		if err != nil {
			return nil, err
		}
		key = k
	}
	return &Manager{store: NewStore(ttl), key: key}, nil
}

// New creates a session and returns it with the signed cookie value to set.
func (m *Manager) New(user string) (Session, string, error) {
	sess, err := m.store.Create(user)
	if err != nil {
		return Session{}, "", err
	}
	return sess, m.sign(sess.ID), nil
}

// Lookup resolves a signed cookie value to a live session.
func (m *Manager) Lookup(cookieValue string) (Session, bool) {
	id, ok := m.verify(cookieValue)
	if !ok {
		return Session{}, false
	}
	return m.store.Get(id)
}

// Destroy deletes the session referenced by a signed cookie value.
func (m *Manager) Destroy(cookieValue string) {
	if id, ok := m.verify(cookieValue); ok {
		m.store.Delete(id)
	}
}

func (m *Manager) sign(id string) string {
	mac := hmac.New(sha256.New, m.key)
	mac.Write([]byte(id))
	return id + "." + hex.EncodeToString(mac.Sum(nil))
}

func (m *Manager) verify(value string) (string, bool) {
	i := strings.LastIndexByte(value, '.')
	if i <= 0 || i == len(value)-1 {
		return "", false
	}
	id, sigHex := value[:i], value[i+1:]
	sig, err := hex.DecodeString(sigHex)
	if err != nil {
		return "", false
	}
	mac := hmac.New(sha256.New, m.key)
	mac.Write([]byte(id))
	if subtle.ConstantTimeCompare(sig, mac.Sum(nil)) != 1 {
		return "", false
	}
	return id, true
}

// RandomToken returns a URL-safe random string carrying n bytes of entropy.
func RandomToken(n int) (string, error) {
	b, err := randomBytes(n)
	if err != nil {
		return "", err
	}
	return base64.RawURLEncoding.EncodeToString(b), nil
}

func randomBytes(n int) ([]byte, error) {
	b := make([]byte, n)
	if _, err := rand.Read(b); err != nil {
		return nil, err
	}
	return b, nil
}