package db

import (
	"context"
	"fmt"
	"io/fs"
	"sort"
	"strings"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgxpool"
)

// migrateLockID is held for the whole migration pass so two replicas cannot
// apply the same file at once. The lock lives on one pooled connection and
// is released when that session ends.
const migrateLockID int64 = 764727013

// Migrate runs all *.sql files in the migrations fs in lexicographic order.
// Tracks applied migrations in schema_migrations.
func Migrate(ctx context.Context, pool *pgxpool.Pool, migrations fs.FS) error {
	conn, err := pool.Acquire(ctx)
	if err != nil {
		return fmt.Errorf("migrate lock: %w", err)
	}
	defer conn.Release()
	if _, err := conn.Exec(ctx, `SELECT pg_advisory_lock($1)`, migrateLockID); err != nil {
		return fmt.Errorf("migrate lock: %w", err)
	}
	defer func() {
		_, _ = conn.Exec(context.Background(), `SELECT pg_advisory_unlock($1)`, migrateLockID)
	}()

	if _, err := pool.Exec(ctx, `CREATE TABLE IF NOT EXISTS schema_migrations (version TEXT PRIMARY KEY, applied_at TIMESTAMPTZ NOT NULL DEFAULT now())`); err != nil {
		return fmt.Errorf("create migrations table: %w", err)
	}
	entries, err := fs.ReadDir(migrations, ".")
	if err != nil {
		return err
	}
	var names []string
	for _, e := range entries {
		if !e.IsDir() && strings.HasSuffix(e.Name(), ".sql") {
			names = append(names, e.Name())
		}
	}
	sort.Strings(names)

	for _, name := range names {
		applied, err := pool.Exec(ctx, `SELECT 1 FROM schema_migrations WHERE version=$1`, name)
		if err != nil {
			return err
		}
		if applied.RowsAffected() > 0 {
			continue
		}
		body, err := fs.ReadFile(migrations, name)
		if err != nil {
			return err
		}
		tx, err := pool.BeginTx(ctx, pgx.TxOptions{})
		if err != nil {
			return err
		}
		if _, err := tx.Exec(ctx, string(body)); err != nil {
			_ = tx.Rollback(ctx)
			return fmt.Errorf("apply %s: %w", name, err)
		}
		if _, err := tx.Exec(ctx, `INSERT INTO schema_migrations (version) VALUES ($1)`, name); err != nil {
			_ = tx.Rollback(ctx)
			return err
		}
		if err := tx.Commit(ctx); err != nil {
			return err
		}
	}
	return nil
}