package infra

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

const createMigrationsTable = `
CREATE TABLE IF NOT EXISTS _migrations (
    version    INTEGER PRIMARY KEY,
    name       TEXT    NOT NULL,
    up_sql     TEXT    NOT NULL,
    down_sql   TEXT    NOT NULL,
    applied_at TEXT    NOT NULL DEFAULT (datetime('now'))
)`

// MigrationUp reads all .sql migration files from dir, creates the _migrations
// table if it does not exist, and applies any pending migrations in version order.
// Each migration runs in its own transaction. Returns the list of applied migrations.
// If a migration fails, execution stops and the error is returned along with any
// migrations that were successfully applied before the failure.
func MigrationUp(db *sql.DB, dir string) ([]Migration, error) {
	// Ensure _migrations table exists
	if _, err := db.Exec(createMigrationsTable); err != nil {
		return nil, fmt.Errorf("migration_up: cannot create _migrations table: %w", err)
	}

	// Load files from directory
	allMigrations, err := loadMigrationsFromDir(dir)
	if err != nil {
		return nil, fmt.Errorf("migration_up: %w", err)
	}

	// Fetch already-applied versions
	applied, err := appliedVersions(db)
	if err != nil {
		return nil, fmt.Errorf("migration_up: %w", err)
	}

	// Filter pending migrations
	var pending []Migration
	for _, m := range allMigrations {
		if !applied[m.Version] {
			pending = append(pending, m)
		}
	}

	// Apply each pending migration in its own transaction
	var result []Migration
	for _, m := range pending {
		if err := applyMigration(db, m); err != nil {
			return result, fmt.Errorf("migration_up: applying version %d (%s): %w", m.Version, m.Name, err)
		}
		result = append(result, m)
	}

	return result, nil
}

// loadMigrationsFromDir reads and parses all .sql migration files from dir,
// returning them sorted by version ascending.
func loadMigrationsFromDir(dir string) ([]Migration, error) {
	entries, err := os.ReadDir(dir)
	if err != nil {
		return nil, fmt.Errorf("cannot read migrations directory %q: %w", dir, err)
	}

	var migrations []Migration
	for _, e := range entries {
		if e.IsDir() {
			continue
		}
		name := e.Name()
		if !strings.HasSuffix(strings.ToLower(name), ".sql") {
			continue
		}

		path := filepath.Join(dir, name)
		content, err := os.ReadFile(path)
		if err != nil {
			return nil, fmt.Errorf("cannot read %q: %w", path, err)
		}

		m, err := MigrationParse(name, string(content))
		if err != nil {
			return nil, fmt.Errorf("parse error in %q: %w", name, err)
		}
		migrations = append(migrations, m)
	}

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

	return migrations, nil
}

// appliedVersions returns a set of version numbers already recorded in _migrations.
func appliedVersions(db *sql.DB) (map[int]bool, error) {
	rows, err := db.Query("SELECT version FROM _migrations")
	if err != nil {
		return nil, fmt.Errorf("cannot query _migrations: %w", err)
	}
	defer rows.Close()

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

// applyMigration executes a migration's UpSQL within a transaction and records it
// in _migrations. If UpSQL contains multiple statements, they are executed sequentially
// using db.Exec (SQLite supports multiple statements via the C driver).
func applyMigration(db *sql.DB, m Migration) error {
	tx, err := db.Begin()
	if err != nil {
		return fmt.Errorf("begin transaction: %w", err)
	}
	defer tx.Rollback() //nolint:errcheck

	// Execute the up SQL (may contain multiple statements)
	if _, err := tx.Exec(m.UpSQL); err != nil {
		return fmt.Errorf("exec up_sql: %w", err)
	}

	// Record the migration
	const insertSQL = `INSERT INTO _migrations (version, name, up_sql, down_sql) VALUES (?, ?, ?, ?)`
	if _, err := tx.Exec(insertSQL, m.Version, m.Name, m.UpSQL, m.DownSQL); err != nil {
		return fmt.Errorf("record migration: %w", err)
	}

	return tx.Commit()
}