// Package db provides SQLite database setup, migrations, and snapshots
// for Metacircular services.
//
// All databases are opened with the standard Metacircular pragmas (WAL mode,
// foreign keys, busy timeout) and restrictive file permissions (0600).
package db

import (
	"database/sql"
	"fmt"
	"os"
	"path/filepath"
	"time"

	_ "modernc.org/sqlite" // SQLite driver (pure Go, no CGo).
)

// Open opens or creates a SQLite database at path with the standard
// Metacircular pragmas:
//
//	PRAGMA journal_mode = WAL;
//	PRAGMA foreign_keys = ON;
//	PRAGMA busy_timeout = 5000;
//
// The file is created with 0600 permissions (owner read/write only).
// The parent directory is created if it does not exist.
//
// Open returns a standard [*sql.DB] — no wrapper types. Services use it
// directly with database/sql.
func Open(path string) (*sql.DB, error) {
	dir := filepath.Dir(path)
	if err := os.MkdirAll(dir, 0700); err != nil {
		return nil, fmt.Errorf("db: create directory %s: %w", dir, err)
	}

	// Pre-create the file with restrictive permissions if it does not exist.
	if _, err := os.Stat(path); os.IsNotExist(err) {
		f, createErr := os.OpenFile(path, os.O_CREATE|os.O_WRONLY, 0600) //nolint:gosec // path is caller-provided config, not user input
		if createErr != nil {
			return nil, fmt.Errorf("db: create file %s: %w", path, createErr)
		}
		_ = f.Close()
	}

	database, err := sql.Open("sqlite", path)
	if err != nil {
		return nil, fmt.Errorf("db: open %s: %w", path, err)
	}

	pragmas := []string{
		"PRAGMA journal_mode = WAL",
		"PRAGMA foreign_keys = ON",
		"PRAGMA busy_timeout = 5000",
	}
	for _, p := range pragmas {
		if _, execErr := database.Exec(p); execErr != nil {
			_ = database.Close()
			return nil, fmt.Errorf("db: %s: %w", p, execErr)
		}
	}

	// Ensure permissions are correct even if the file already existed.
	if err := os.Chmod(path, 0600); err != nil {
		_ = database.Close()
		return nil, fmt.Errorf("db: chmod %s: %w", path, err)
	}

	return database, nil
}

// Migration is a numbered, named schema change. Services define their
// migrations as a []Migration slice — the slice is the schema history.
type Migration struct {
	// Version is the migration number. Must be unique and should be
	// sequential starting from 1.
	Version int

	// Name is a short human-readable description (e.g., "initial schema").
	Name string

	// SQL is the DDL/DML to execute. Multiple statements are allowed
	// (separated by semicolons). Each migration runs in a transaction.
	SQL string
}

// Migrate applies all pending migrations from the given slice. It creates
// the schema_migrations tracking table if it does not exist.
//
// Each migration runs in its own transaction. Already-applied migrations
// (identified by version number) are skipped. Timestamps are stored as
// RFC 3339 UTC.
func Migrate(database *sql.DB, migrations []Migration) error {
	_, err := database.Exec(`CREATE TABLE IF NOT EXISTS schema_migrations (
		version    INTEGER PRIMARY KEY,
		name       TEXT NOT NULL DEFAULT '',
		applied_at TEXT NOT NULL DEFAULT ''
	)`)
	if err != nil {
		return fmt.Errorf("db: create schema_migrations: %w", err)
	}

	for _, m := range migrations {
		applied, checkErr := migrationApplied(database, m.Version)
		if checkErr != nil {
			return checkErr
		}
		if applied {
			continue
		}

		tx, txErr := database.Begin()
		if txErr != nil {
			return fmt.Errorf("db: begin migration %d (%s): %w", m.Version, m.Name, txErr)
		}

		if _, execErr := tx.Exec(m.SQL); execErr != nil {
			_ = tx.Rollback()
			return fmt.Errorf("db: migration %d (%s): %w", m.Version, m.Name, execErr)
		}

		now := time.Now().UTC().Format(time.RFC3339)
		if _, execErr := tx.Exec(
			`INSERT INTO schema_migrations (version, name, applied_at) VALUES (?, ?, ?)`,
			m.Version, m.Name, now,
		); execErr != nil {
			_ = tx.Rollback()
			return fmt.Errorf("db: record migration %d: %w", m.Version, execErr)
		}

		if commitErr := tx.Commit(); commitErr != nil {
			return fmt.Errorf("db: commit migration %d: %w", m.Version, commitErr)
		}
	}

	return nil
}

// SchemaVersion returns the highest applied migration version, or 0 if
// no migrations have been applied.
func SchemaVersion(database *sql.DB) (int, error) {
	var version sql.NullInt64
	err := database.QueryRow(`SELECT MAX(version) FROM schema_migrations`).Scan(&version)
	if err != nil {
		return 0, fmt.Errorf("db: schema version: %w", err)
	}
	if !version.Valid {
		return 0, nil
	}
	return int(version.Int64), nil
}

// Snapshot creates a consistent backup of the database at destPath using
// SQLite's VACUUM INTO. The destination file is created with 0600
// permissions.
func Snapshot(database *sql.DB, destPath string) error {
	dir := filepath.Dir(destPath)
	if err := os.MkdirAll(dir, 0700); err != nil {
		return fmt.Errorf("db: create snapshot directory %s: %w", dir, err)
	}

	if _, err := database.Exec("VACUUM INTO ?", destPath); err != nil {
		return fmt.Errorf("db: snapshot: %w", err)
	}

	if err := os.Chmod(destPath, 0600); err != nil {
		return fmt.Errorf("db: chmod snapshot %s: %w", destPath, err)
	}

	return nil
}

func migrationApplied(database *sql.DB, version int) (bool, error) {
	var count int
	err := database.QueryRow(
		`SELECT COUNT(*) FROM schema_migrations WHERE version = ?`, version,
	).Scan(&count)
	if err != nil {
		return false, fmt.Errorf("db: check migration %d: %w", version, err)
	}
	return count > 0, nil
}