package migrate

import (
	"context"
	"database/sql"
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"time"
)

// Dialect abstracts the few SQL differences between database engines.
type Dialect interface {
	// Placeholder returns the bind parameter for position n (1-indexed).
	// Postgres: "$1", "$2". MySQL/SQLite: "?".
	Placeholder(n int) string

	// TableExistsQuery returns SQL that checks whether a table exists.
	// The query must accept a single bind parameter (the table name) and
	// return a single boolean column.
	TableExistsQuery() string
}

// Options configures the migration runner.
type Options struct {
	// MigrationsDir is the directory containing *.up.sql migration files.
	// If empty, it defaults to "migrations" relative to the working directory.
	MigrationsDir string

	// BootstrapTable is the name of a table whose existence signals that the
	// database already has a schema applied before the schema_migrations
	// tracking table was introduced. When non-empty and this table exists but
	// schema_migrations does not, all discovered migrations are recorded as
	// already applied (bootstrapped). When empty, no bootstrapping is performed.
	BootstrapTable string
}

// Run executes forward migrations against db using the given dialect.
// It discovers *.up.sql files in the configured directory, tracks applied
// versions in a schema_migrations table, and applies any pending migrations
// in filename-sorted order.
func Run(ctx context.Context, db *sql.DB, dialect Dialect, opts *Options) error {
	if opts == nil {
		opts = &Options{}
	}

	migrationsTableExists := tableExists(ctx, db, dialect, "schema_migrations")

	if err := ensureMigrationsTable(ctx, db); err != nil {
		return fmt.Errorf("failed to create migrations table: %w", err)
	}

	migrationsPath := opts.MigrationsDir
	if migrationsPath == "" {
		wd, err := os.Getwd()
		if err != nil {
			return fmt.Errorf("failed to get working directory: %w", err)
		}
		migrationsPath = filepath.Join(wd, "migrations")
	}

	migrations, err := discoverMigrations(migrationsPath)
	if err != nil {
		return fmt.Errorf("failed to discover migrations: %w", err)
	}

	if !migrationsTableExists && opts.BootstrapTable != "" {
		if err := bootstrapMigrationState(ctx, db, dialect, migrations, opts.BootstrapTable); err != nil {
			return fmt.Errorf("failed to bootstrap migration state: %w", err)
		}
	}

	applied, err := getAppliedMigrations(ctx, db)
	if err != nil {
		return fmt.Errorf("failed to get applied migrations: %w", err)
	}

	for _, migration := range migrations {
		if applied[migration] {
			continue
		}

		migrationSQL, err := os.ReadFile(filepath.Join(migrationsPath, migration))
		if err != nil {
			return fmt.Errorf("failed to read migration file %s: %w", migration, err)
		}

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

		if err := recordMigration(ctx, db, dialect, migration); err != nil {
			return fmt.Errorf("failed to record migration %s: %w", migration, err)
		}

		fmt.Printf("Applied migration: %s\n", migration)
	}

	return nil
}

// ensureMigrationsTable creates the schema_migrations table if it doesn't exist.
func ensureMigrationsTable(ctx context.Context, db *sql.DB) error {
	_, err := db.ExecContext(ctx, `
		CREATE TABLE IF NOT EXISTS schema_migrations (
			version VARCHAR(255) PRIMARY KEY,
			applied_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
		)
	`)
	return err
}

// tableExists checks if a table exists in the database.
func tableExists(ctx context.Context, db *sql.DB, dialect Dialect, tableName string) bool {
	var exists bool
	err := db.QueryRowContext(ctx,
		dialect.TableExistsQuery(),
		tableName,
	).Scan(&exists)
	return err == nil && exists
}

// bootstrapMigrationState marks all discovered migrations as applied when the
// database already contains a schema (detected via sentinelTable) but has no
// schema_migrations table yet.
func bootstrapMigrationState(ctx context.Context, db *sql.DB, dialect Dialect, migrations []string, sentinelTable string) error {
	if !tableExists(ctx, db, dialect, sentinelTable) {
		return nil
	}

	for _, migration := range migrations {
		if err := recordMigration(ctx, db, dialect, migration); err != nil {
			return fmt.Errorf("failed to record bootstrapped migration %s: %w", migration, err)
		}
		fmt.Printf("Bootstrapped migration (already applied): %s\n", migration)
	}

	return nil
}

// getAppliedMigrations returns a set of already-applied migration filenames.
func getAppliedMigrations(ctx context.Context, db *sql.DB) (map[string]bool, error) {
	applied := make(map[string]bool)

	rows, err := db.QueryContext(ctx, "SELECT version FROM schema_migrations")
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	for rows.Next() {
		var version string
		if err := rows.Scan(&version); err != nil {
			return nil, err
		}
		applied[version] = true
	}

	return applied, rows.Err()
}

// recordMigration records a migration version as applied.
func recordMigration(ctx context.Context, db *sql.DB, dialect Dialect, version string) error {
	_, err := db.ExecContext(ctx,
		fmt.Sprintf("INSERT INTO schema_migrations (version, applied_at) VALUES (%s, %s)",
			dialect.Placeholder(1), dialect.Placeholder(2)),
		version, time.Now(),
	)
	return err
}

// discoverMigrations finds all *.up.sql files in the given directory and
// returns them sorted by filename (relies on numeric prefixes like
// 000001_, 000002_, etc.).
func discoverMigrations(migrationsPath string) ([]string, error) {
	entries, err := os.ReadDir(migrationsPath)
	if err != nil {
		return nil, err
	}

	var migrations []string
	for _, entry := range entries {
		if entry.IsDir() {
			continue
		}
		name := entry.Name()
		if strings.HasSuffix(name, ".up.sql") {
			migrations = append(migrations, name)
		}
	}

	sort.Strings(migrations)

	return migrations, nil
}