package store

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

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

// migration is one numbered SQL file: version parsed from the filename prefix.
type migration struct {
	version int
	name    string
	sql     string
}

// Migrate applies every embedded migration whose version has not yet been
// recorded, in ascending order, each in its own transaction. It is idempotent:
// a second run with no new files is a no-op. Migration files are named
// NNNN_description.sql (e.g. 0001_init.sql); NNNN is the version.
func (d *DB) Migrate(ctx context.Context) error {
	if _, err := d.sql.ExecContext(ctx,
		`CREATE TABLE IF NOT EXISTS schema_migrations (version INTEGER PRIMARY KEY)`,
	); err != nil {
		return fmt.Errorf("store: create schema_migrations: %w", err)
	}

	applied, err := d.appliedVersions(ctx)
	if err != nil {
		return err
	}

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

	for _, m := range migrations {
		if applied[m.version] {
			continue
		}
		if err := d.applyMigration(ctx, m); err != nil {
			return err
		}
		slog.Info("store: applied migration", "version", m.version, "name", m.name)
	}
	return nil
}

func (d *DB) appliedVersions(ctx context.Context) (map[int]bool, error) {
	rows, err := d.sql.QueryContext(ctx, `SELECT version FROM schema_migrations`)
	if err != nil {
		return nil, fmt.Errorf("store: read schema_migrations: %w", err)
	}
	defer rows.Close()

	applied := map[int]bool{}
	for rows.Next() {
		var v int
		if err := rows.Scan(&v); err != nil {
			return nil, fmt.Errorf("store: scan schema_migrations: %w", err)
		}
		applied[v] = true
	}
	return applied, rows.Err()
}

func (d *DB) applyMigration(ctx context.Context, m migration) error {
	tx, err := d.sql.BeginTx(ctx, nil)
	if err != nil {
		return fmt.Errorf("store: begin migration %d: %w", m.version, err)
	}
	defer tx.Rollback() //nolint:errcheck // no-op after a successful commit

	if _, err := tx.ExecContext(ctx, m.sql); err != nil {
		return fmt.Errorf("store: apply migration %d (%s): %w", m.version, m.name, err)
	}
	if _, err := tx.ExecContext(ctx,
		`INSERT INTO schema_migrations (version) VALUES (?)`, m.version,
	); err != nil {
		return fmt.Errorf("store: record migration %d: %w", m.version, err)
	}
	return tx.Commit()
}

// loadMigrations reads and parses every embedded migration file, sorted by
// version ascending.
func loadMigrations() ([]migration, error) {
	entries, err := fs.ReadDir(migrationsFS, "migrations")
	if err != nil {
		return nil, fmt.Errorf("store: read migrations dir: %w", err)
	}

	var migrations []migration
	for _, e := range entries {
		if e.IsDir() || !strings.HasSuffix(e.Name(), ".sql") {
			continue
		}
		version, err := parseVersion(e.Name())
		if err != nil {
			return nil, err
		}
		body, err := fs.ReadFile(migrationsFS, "migrations/"+e.Name())
		if err != nil {
			return nil, fmt.Errorf("store: read migration %s: %w", e.Name(), err)
		}
		migrations = append(migrations, migration{version: version, name: e.Name(), sql: string(body)})
	}

	sort.Slice(migrations, func(i, j int) bool {
		return migrations[i].version < migrations[j].version
	})
	return migrations, nil
}

// parseVersion extracts the leading integer from a migration filename such as
// "0001_init.sql".
func parseVersion(name string) (int, error) {
	prefix, _, ok := strings.Cut(name, "_")
	if !ok {
		return 0, fmt.Errorf("store: migration %q missing NNNN_ prefix", name)
	}
	v, err := strconv.Atoi(prefix)
	if err != nil {
		return 0, fmt.Errorf("store: migration %q has non-numeric version: %w", name, err)
	}
	return v, nil
}