// Package db wraps sqlx and runs embedded SQL migrations on startup.
//
// Migrations are flat files in migrations/, named NNN_*.sql. They run in
// lexical order. Each is executed in its own transaction; a missing
// schema_migrations row indicates "not yet applied". This is a
// deliberately simple migration runner — for this project's size + team
// size, pulling in golang-migrate or atlas adds complexity without
// payback. If migration count grows past ~20, revisit.
package db

import (
	"context"
	"database/sql"
	"embed"
	"fmt"
	"sort"
	"strings"
	"time"

	_ "github.com/go-sql-driver/mysql"
	"github.com/jmoiron/sqlx"
)

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

func Open(ctx context.Context, dsn string) (*sqlx.DB, error) {
	d, err := sqlx.ConnectContext(ctx, "mysql", dsn)
	if err != nil {
		return nil, fmt.Errorf("connect mysql: %w", err)
	}
	d.SetMaxOpenConns(25)
	d.SetMaxIdleConns(5)
	d.SetConnMaxLifetime(5 * time.Minute)
	return d, nil
}

// RunMigrations applies any migrations that aren't yet present in the
// schema_migrations table. Idempotent — safe to call on every startup.
func RunMigrations(ctx context.Context, d *sqlx.DB) error {
	// Bootstrap the tracker table itself.
	if _, err := d.ExecContext(ctx, `
		CREATE TABLE IF NOT EXISTS schema_migrations (
			name VARCHAR(255) PRIMARY KEY,
			applied_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
		) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4`); err != nil {
		return fmt.Errorf("ensure schema_migrations: %w", err)
	}

	entries, err := migrationsFS.ReadDir("migrations")
	if err != nil {
		return fmt.Errorf("list migrations: %w", err)
	}
	var files []string
	for _, e := range entries {
		if !e.IsDir() && strings.HasSuffix(e.Name(), ".sql") {
			files = append(files, e.Name())
		}
	}
	sort.Strings(files)

	for _, name := range files {
		var found string
		err := d.GetContext(ctx, &found, `SELECT name FROM schema_migrations WHERE name = ?`, name)
		if err == nil {
			continue // already applied
		}
		if err != sql.ErrNoRows {
			return fmt.Errorf("check migration %s: %w", name, err)
		}

		content, err := migrationsFS.ReadFile("migrations/" + name)
		if err != nil {
			return fmt.Errorf("read migration %s: %w", name, err)
		}

		// MySQL doesn't support multi-statement in a single Exec by default
		// — split on ';' boundaries and run each individually. Comments are
		// passed through (server-side parser handles).
		statements := splitSQL(string(content))

		tx, err := d.BeginTxx(ctx, nil)
		if err != nil {
			return fmt.Errorf("tx for %s: %w", name, err)
		}
		for _, stmt := range statements {
			stmt = strings.TrimSpace(stmt)
			if stmt == "" {
				continue
			}
			if _, err := tx.ExecContext(ctx, stmt); err != nil {
				_ = tx.Rollback()
				return fmt.Errorf("apply %s [statement: %q]: %w", name, firstLine(stmt), err)
			}
		}
		if _, err := tx.ExecContext(ctx, `INSERT INTO schema_migrations(name) VALUES (?)`, name); err != nil {
			_ = tx.Rollback()
			return fmt.Errorf("record %s: %w", name, err)
		}
		if err := tx.Commit(); err != nil {
			return fmt.Errorf("commit %s: %w", name, err)
		}
	}
	return nil
}

func splitSQL(s string) []string {
	// Crude but adequate for our migrations (no string-literal semicolons).
	// If we ever need to embed semicolons inside strings, switch to a
	// proper SQL tokenizer.
	return strings.Split(s, ";")
}

func firstLine(s string) string {
	if i := strings.IndexByte(s, '\n'); i >= 0 {
		return s[:i]
	}
	return s
}