package migrate

import (
	"context"
	"fmt"
	"io/fs"
	"log"
	"net/url"
	"sort"
	"strings"
	"time"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgconn"

	mig "watch-party-backend/db/migration"
	"watch-party-backend/internal/config"
)

// Run applies all pending migrations.
// Uses cfg.DB.DSN() for the app database and derives an admin DSN (db=postgres) from it.
func Run(ctx context.Context, cfg config.Config) error {
	appDSN := cfg.DB.DSN()
	adminDSN := dsnWithDB(appDSN, "postgres")
	dbName := cfg.DB.Name

	// 1) Connect to admin DB to ensure target DB exists
	adminCtx, cancel := context.WithTimeout(ctx, 10*time.Second)
	defer cancel()

	adminConn, err := pgx.Connect(adminCtx, adminDSN)
	if err != nil {
		return fmt.Errorf("connect admin DB: %w", err)
	}
	defer adminConn.Close(ctx)

	if err := createDatabaseIfNotExists(ctx, adminConn, dbName); err != nil {
		return fmt.Errorf("create database: %w", err)
	}
	log.Printf("database %q is present", dbName)

	// 2) Connect to target DB and run migrations
	appCtx, cancel2 := context.WithTimeout(ctx, 10*time.Second)
	defer cancel2()

	appConn, err := pgx.Connect(appCtx, appDSN)
	if err != nil {
		return fmt.Errorf("connect app DB: %w", err)
	}
	defer appConn.Close(ctx)

	if err := ensureSchemaMigrations(ctx, appConn); err != nil {
		return fmt.Errorf("ensure schema_migrations: %w", err)
	}

	applied, err := fetchAppliedVersions(ctx, appConn)
	if err != nil {
		return fmt.Errorf("read applied versions: %w", err)
	}

	files, err := fs.Glob(mig.FS, "*.sql")
	if err != nil {
		return fmt.Errorf("glob migrations: %w", err)
	}
	sort.Strings(files)

	for _, name := range files {
		version := versionOf(name)
		if applied[version] {
			log.Printf("skip %s (already applied)", version)
			continue
		}

		sqlBytes, err := mig.FS.ReadFile(name)
		if err != nil {
			return fmt.Errorf("read %s: %w", name, err)
		}

		log.Printf("applying %s ...", version)
		if err := applyMigration(ctx, appConn, version, string(sqlBytes)); err != nil {
			return fmt.Errorf("apply %s failed: %w", version, err)
		}
		log.Printf("applied %s", version)
	}

	return nil
}

// dsnWithDB parses a Postgres URL DSN and replaces the DB path segment.
// Example: postgres://user:pass@host:5432/appdb?sslmode=disable -> .../postgres?sslmode=disable
func dsnWithDB(raw string, dbName string) string {
	u, err := url.Parse(raw)
	if err != nil {
		// if malformed, just return raw; pgx will error at connect time
		return raw
	}
	u.Path = "/" + dbName
	return u.String()
}

func createDatabaseIfNotExists(ctx context.Context, admin *pgx.Conn, dbName string) error {
	var exists bool
	if err := admin.QueryRow(ctx,
		"SELECT EXISTS(SELECT 1 FROM pg_database WHERE datname=$1)", dbName).
		Scan(&exists); err != nil {
		return fmt.Errorf("check db exists: %w", err)
	}
	if exists {
		return nil
	}
	// Quote identifier safely via quote_ident
	_, err := admin.Exec(ctx, `
DO $$
BEGIN
    IF NOT EXISTS (SELECT FROM pg_database WHERE datname = $1) THEN
        EXECUTE 'CREATE DATABASE ' || quote_ident($1);
    END IF;
END$$;`, dbName)
	if err != nil {
		// 42P04: duplicate_database
		if pgErr, ok := err.(*pgconn.PgError); ok && pgErr.Code == "42P04" {
			return nil
		}
		return fmt.Errorf("create database: %w", err)
	}
	return nil
}

func ensureSchemaMigrations(ctx context.Context, conn *pgx.Conn) error {
	_, err := conn.Exec(ctx, `
CREATE TABLE IF NOT EXISTS schema_migrations (
    version    TEXT PRIMARY KEY,
    applied_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
)`)
	return err
}

func fetchAppliedVersions(ctx context.Context, conn *pgx.Conn) (map[string]bool, error) {
	rows, err := conn.Query(ctx, `SELECT version FROM schema_migrations`)
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	out := make(map[string]bool)
	for rows.Next() {
		var v string
		if err := rows.Scan(&v); err != nil {
			return nil, err
		}
		out[v] = true
	}
	return out, rows.Err()
}

func applyMigration(ctx context.Context, conn *pgx.Conn, version, sqlText string) error {
	tx, err := conn.Begin(ctx)
	if err != nil {
		return err
	}
	defer func() { _ = tx.Rollback(ctx) }()

	if _, err := tx.Exec(ctx, sqlText); err != nil {
		return err
	}
	if _, err := tx.Exec(ctx, `INSERT INTO schema_migrations(version) VALUES ($1)`, version); err != nil {
		return err
	}
	return tx.Commit(ctx)
}

func versionOf(path string) string {
	base := path[strings.LastIndex(path, "/")+1:]
	// "0001_init.sql" -> "0001"
	dot := strings.Index(base, ".")
	if underscore := strings.Index(base, "_"); underscore > 0 && underscore < dot {
		return base[:underscore]
	}
	if dot > 0 {
		return base[:dot]
	}
	return base
}