package auth

import (
	"context"
	"crypto/rand"
	"crypto/sha256"
	"encoding/base64"
	"errors"
	"fmt"
	"time"

	"github.com/google/uuid"
	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgxpool"
)

// Session is the application-side view of a row in the sessions table.
type Session struct {
	ID        uuid.UUID
	UserID    uuid.UUID
	ExpiresAt time.Time
	RevokedAt *time.Time
}

// ErrSessionNotFound is returned when no (valid, non-expired, non-revoked)
// session matches the given token.
var ErrSessionNotFound = errors.New("session not found")

// SessionConfig controls token generation and persistence.
type SessionConfig struct {
	TokenBytes int           // entropy of the raw token before base64url encoding
	TTL        time.Duration // validity window from creation
}

// SessionStore wraps database access for the sessions table and handles opaque
// token generation. Only the SHA-256 hash of a token is ever stored.
type SessionStore struct {
	pool *pgxpool.Pool
	cfg  SessionConfig
}

// NewSessionStore constructs a SessionStore. tokenBytes must be >= 16.
func NewSessionStore(pool *pgxpool.Pool, cfg SessionConfig) *SessionStore {
	if cfg.TokenBytes < 16 {
		cfg.TokenBytes = 32
	}
	return &SessionStore{pool: pool, cfg: cfg}
}

// Create issues a new session for the user and returns the raw token (to send to
// the client exactly once) together with the persisted Session row.
func (s *SessionStore) Create(ctx context.Context, userID uuid.UUID, userAgent, ip string) (token string, sess Session, err error) {
	raw := make([]byte, s.cfg.TokenBytes)
	if _, err = rand.Read(raw); err != nil {
		return "", Session{}, fmt.Errorf("generate session token: %w", err)
	}
	token = base64.RawURLEncoding.EncodeToString(raw)
	tokenHash := hashToken(token)
	expiresAt := time.Now().Add(s.cfg.TTL)

	const q = `
		INSERT INTO sessions (user_id, token_hash, expires_at, user_agent, ip)
		VALUES ($1, $2, $3, NULLIF($4, ''), NULLIF($5, ''))
		RETURNING id, user_id, expires_at, revoked_at`
	err = s.pool.QueryRow(ctx, q, userID, tokenHash, expiresAt, userAgent, ip).
		Scan(&sess.ID, &sess.UserID, &sess.ExpiresAt, &sess.RevokedAt)
	if err != nil {
		return "", Session{}, fmt.Errorf("insert session: %w", err)
	}
	return token, sess, nil
}

// Lookup returns the active session for a raw token. It rejects expired and
// revoked sessions and updates last_seen_at (best-effort, errors logged by caller).
func (s *SessionStore) Lookup(ctx context.Context, token string) (Session, error) {
	if token == "" {
		return Session{}, ErrSessionNotFound
	}
	const q = `
		UPDATE sessions
		   SET last_seen_at = now()
		 WHERE token_hash = $1
		   AND revoked_at IS NULL
		   AND expires_at > now()
		 RETURNING id, user_id, expires_at, revoked_at`
	var sess Session
	err := s.pool.QueryRow(ctx, q, hashToken(token)).Scan(
		&sess.ID, &sess.UserID, &sess.ExpiresAt, &sess.RevokedAt)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return Session{}, ErrSessionNotFound
		}
		return Session{}, fmt.Errorf("lookup session: %w", err)
	}
	return sess, nil
}

// Revoke marks the session matching token as revoked. Missing tokens are a no-op
// so logout stays idempotent.
func (s *SessionStore) Revoke(ctx context.Context, token string) error {
	if token == "" {
		return nil
	}
	const q = `UPDATE sessions SET revoked_at = now() WHERE token_hash = $1 AND revoked_at IS NULL`
	_, err := s.pool.Exec(ctx, q, hashToken(token))
	if err != nil {
		return fmt.Errorf("revoke session: %w", err)
	}
	return nil
}

// hashToken returns the lowercase hex SHA-256 digest of a raw token. The hash is
// what we store; the raw token never touches the database.
func hashToken(token string) string {
	sum := sha256.Sum256([]byte(token))
	return base64.RawURLEncoding.EncodeToString(sum[:])
}