package database

import (
	"context"
	"database/sql"
	"embed"
	"errors"
	"fmt"
	"io/fs"
	"log/slog"
	"sort"
	"strconv"
	"strings"
)

//go:embed migrations/*.sql
var migrationsFS embed.FS

// bootstrapVersion is the migration that creates the schema_migrations
// table itself. It is applied before the normal migration loop.
const bootstrapVersion = 0

// ErrInvalidMigrationFilename indicates a migration filename does not follow
// the expected "<version>.sql" or "<version>_<description>.sql" pattern.
var ErrInvalidMigrationFilename = errors.New("invalid migration filename")

// ParseMigrationVersion extracts the numeric version prefix from a migration
// filename. Filenames must follow the pattern "<version>.sql" or
// "<version>_<description>.sql", where version is a zero-padded numeric
// string (e.g. "001", "002"). Returns the version as an integer and an
// error if the filename does not match the expected pattern.
func ParseMigrationVersion(filename string) (int, error) {
	name := strings.TrimSuffix(filename, ".sql")
	if name == "" || name == filename {
		return 0, fmt.Errorf("%w: %q has no .sql extension or is empty", ErrInvalidMigrationFilename, filename)
	}

	// Split on underscore to separate version from description.
	// If there's no underscore, the entire stem is the version.
	versionStr, _, _ := strings.Cut(name, "_")

	if versionStr == "" {
		return 0, fmt.Errorf("%w: %q has empty version prefix", ErrInvalidMigrationFilename, filename)
	}

	// Validate the version is purely numeric.
	for _, ch := range versionStr {
		if ch < '0' || ch > '9' {
			return 0, fmt.Errorf(
				"%w: %q version %q contains non-numeric character %q",
				ErrInvalidMigrationFilename, filename, versionStr, string(ch),
			)
		}
	}

	version, err := strconv.Atoi(versionStr)
	if err != nil {
		return 0, fmt.Errorf("%w: %q: %w", ErrInvalidMigrationFilename, filename, err)
	}

	return version, nil
}

// collectMigrations reads the embedded migrations directory and returns
// migration filenames sorted lexicographically.
func collectMigrations() ([]string, error) {
	entries, err := fs.ReadDir(migrationsFS, "migrations")
	if err != nil {
		return nil, fmt.Errorf("failed to read migrations directory: %w", err)
	}

	var migrations []string

	for _, entry := range entries {
		if !entry.IsDir() && strings.HasSuffix(entry.Name(), ".sql") {
			migrations = append(migrations, entry.Name())
		}
	}

	sort.Strings(migrations)

	return migrations, nil
}

// bootstrapMigrationsTable ensures the schema_migrations table exists by
// applying 000_migration.sql if the table is missing.
func bootstrapMigrationsTable(ctx context.Context, db *sql.DB, log *slog.Logger) error {
	var tableExists int

	err := db.QueryRowContext(ctx,
		"SELECT COUNT(*) FROM sqlite_master WHERE type='table' AND name='schema_migrations'",
	).Scan(&tableExists)
	if err != nil {
		return fmt.Errorf("failed to check for migrations table: %w", err)
	}

	if tableExists == 0 {
		return applyBootstrapMigration(ctx, db, log)
	}

	return nil
}

// applyBootstrapMigration reads and executes 000_migration.sql to create the
// schema_migrations table on a fresh database.
func applyBootstrapMigration(ctx context.Context, db *sql.DB, log *slog.Logger) error {
	content, err := migrationsFS.ReadFile("migrations/000_migration.sql")
	if err != nil {
		return fmt.Errorf("failed to read bootstrap migration 000_migration.sql: %w", err)
	}

	if log != nil {
		log.Info("applying bootstrap migration", "version", bootstrapVersion)
	}

	_, err = db.ExecContext(ctx, string(content))
	if err != nil {
		return fmt.Errorf("failed to apply bootstrap migration: %w", err)
	}

	return nil
}

// ApplyMigrations applies all pending migrations to db. An optional logger
// may be provided for informational output; pass nil for silent operation.
// This is exported so tests can apply the real schema without the full fx
// lifecycle.
func ApplyMigrations(ctx context.Context, db *sql.DB, log *slog.Logger) error {
	bootstrapErr := bootstrapMigrationsTable(ctx, db, log)
	if bootstrapErr != nil {
		return bootstrapErr
	}

	migrations, err := collectMigrations()
	if err != nil {
		return err
	}

	for _, migration := range migrations {
		version, parseErr := ParseMigrationVersion(migration)
		if parseErr != nil {
			return parseErr
		}

		// Check if already applied.
		var count int

		err = db.QueryRowContext(ctx,
			"SELECT COUNT(*) FROM schema_migrations WHERE version = ?",
			version,
		).Scan(&count)
		if err != nil {
			return fmt.Errorf("failed to check migration %s: %w", migration, err)
		}

		if count > 0 {
			if log != nil {
				log.Debug("migration already applied", "version", version)
			}

			continue
		}

		// Apply migration in a transaction.
		applyErr := applyMigrationTx(ctx, db, migration, version)
		if applyErr != nil {
			return applyErr
		}

		if log != nil {
			log.Info("migration applied", "version", version)
		}
	}

	return nil
}

// applyMigrationTx reads and executes a migration file within a transaction,
// recording the version in schema_migrations on success.
func applyMigrationTx(ctx context.Context, db *sql.DB, filename string, version int) error {
	content, err := migrationsFS.ReadFile("migrations/" + filename)
	if err != nil {
		return fmt.Errorf("failed to read migration %s: %w", filename, err)
	}

	transaction, err := db.BeginTx(ctx, nil)
	if err != nil {
		return fmt.Errorf("failed to begin transaction for migration %s: %w", filename, err)
	}

	defer func() {
		if err != nil {
			_ = transaction.Rollback()
		}
	}()

	_, err = transaction.ExecContext(ctx, string(content))
	if err != nil {
		return fmt.Errorf("failed to execute migration %s: %w", filename, err)
	}

	_, err = transaction.ExecContext(ctx,
		"INSERT INTO schema_migrations (version) VALUES (?)",
		version,
	)
	if err != nil {
		return fmt.Errorf("failed to record migration %s: %w", filename, err)
	}

	err = transaction.Commit()
	if err != nil {
		return fmt.Errorf("failed to commit migration %s: %w", filename, err)
	}

	return nil
}

func (d *Database) migrate(ctx context.Context) error {
	return ApplyMigrations(ctx, d.database, d.log)
}