package auth

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

	"github.com/mithral/vellum/internal/core"
)

// ErrNoSession means no session exists for the presented token, or it expired.
var ErrNoSession = errors.New("auth: no such session")

// Session is a logged-in browser or device.
//
// The token itself is NEVER stored. Only its SHA-256 is, so a database leak
// does not hand out live sessions.
type Session struct {
	ID          string
	UserID      int64
	DeviceLabel string
	IP          string
	UserAgent   string
	CreatedAt   time.Time
	LastSeen    time.Time
	ExpiresAt   time.Time
}

// newSessionToken returns a fresh opaque token and its storage hash.
//
// The token is 32 bytes of CSPRNG output, base64url-encoded. Hashing with
// SHA-256 (not argon2) is correct here: the token already has full entropy, so
// there is nothing to brute-force and lookups must stay cheap.
func newSessionToken() (token string, hash string, err error) {
	raw := make([]byte, 32)
	if _, err := rand.Read(raw); err != nil {
		return "", "", fmt.Errorf("auth: read session token: %w", err)
	}
	token = base64.RawURLEncoding.EncodeToString(raw)
	return token, hashToken(token), nil
}

// hashToken is the deterministic mapping from token to stored value.
func hashToken(token string) string {
	sum := sha256.Sum256([]byte(token))
	return base64.RawURLEncoding.EncodeToString(sum[:])
}

// CreateSession stores a new session for userID and returns the raw token.
// The raw token is returned exactly once and never persisted.
func CreateSession(ctx context.Context, db *sql.DB, userID int64, ttl time.Duration, deviceLabel, ip, userAgent string) (string, error) {
	token, hash, err := newSessionToken()
	if err != nil {
		return "", err
	}

	now := time.Now().UTC()
	_, err = db.ExecContext(ctx, `
		INSERT INTO sessions (id, user_id, token_hash, device_label, ip, user_agent, created_at, last_seen, expires_at)
		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)`,
		hash[:16], userID, hash, deviceLabel, ip, userAgent,
		core.Stamp(now), core.Stamp(now),
		core.Stamp(now.Add(ttl)),
	)
	if err != nil {
		return "", fmt.Errorf("auth: create session: %w", err)
	}
	return token, nil
}

// SessionByToken resolves a raw token to its session, or ErrNoSession if the
// token is unknown or expired.
func SessionByToken(ctx context.Context, db *sql.DB, token string) (Session, error) {
	var (
		s         Session
		created   string
		lastSeen  string
		expiresAt string
	)
	err := db.QueryRowContext(ctx, `
		SELECT id, user_id, device_label, ip, user_agent, created_at, last_seen, expires_at
		FROM sessions WHERE token_hash = ?`, hashToken(token),
	).Scan(&s.ID, &s.UserID, &s.DeviceLabel, &s.IP, &s.UserAgent, &created, &lastSeen, &expiresAt)
	if errors.Is(err, sql.ErrNoRows) {
		return Session{}, ErrNoSession
	}
	if err != nil {
		return Session{}, fmt.Errorf("auth: load session: %w", err)
	}

	if s.CreatedAt, err = time.Parse(time.RFC3339Nano, created); err != nil {
		return Session{}, fmt.Errorf("auth: parse created_at: %w", err)
	}
	if s.LastSeen, err = time.Parse(time.RFC3339Nano, lastSeen); err != nil {
		return Session{}, fmt.Errorf("auth: parse last_seen: %w", err)
	}
	if s.ExpiresAt, err = time.Parse(time.RFC3339Nano, expiresAt); err != nil {
		return Session{}, fmt.Errorf("auth: parse expires_at: %w", err)
	}
	if time.Now().UTC().After(s.ExpiresAt) {
		return Session{}, ErrNoSession
	}
	return s, nil
}

// TouchSession updates last_seen. Best-effort: a failure here must never fail
// the request that triggered it, so callers log rather than propagate.
func TouchSession(ctx context.Context, db *sql.DB, sessionID string) error {
	_, err := db.ExecContext(ctx,
		`UPDATE sessions SET last_seen = ? WHERE id = ?`,
		core.Now(), sessionID)
	return err
}

// DeleteSession logs out a single session by its raw token.
func DeleteSession(ctx context.Context, db *sql.DB, token string) error {
	_, err := db.ExecContext(ctx, `DELETE FROM sessions WHERE token_hash = ?`, hashToken(token))
	return err
}

// DeleteUserSessions logs a user out everywhere.
func DeleteUserSessions(ctx context.Context, db *sql.DB, userID int64) error {
	_, err := db.ExecContext(ctx, `DELETE FROM sessions WHERE user_id = ?`, userID)
	return err
}

// DeleteExpiredSessions is housekeeping, run periodically.
func DeleteExpiredSessions(ctx context.Context, db *sql.DB) (int64, error) {
	res, err := db.ExecContext(ctx, `DELETE FROM sessions WHERE expires_at < ?`,
		core.Now())
	if err != nil {
		return 0, err
	}
	return res.RowsAffected()
}