ensure_registry_exists_in_dir Subroutine

    subroutine ensure_registry_exists_in_dir(config_dir)
        character(len=*), intent(in) :: config_dir
        character(len=512) :: registry_path
        logical :: registry_exists, success
        integer :: unit

        ! Get registry path in custom config directory
        registry_path = trim(config_dir)//'/registry.toml'

        ! Ensure config directory exists
        call ensure_config_dir(config_dir, success)
        if (.not. success) then
            print *, 'Error: Cannot create config directory: ', trim(config_dir)
            return
        end if

        ! Check if registry already exists
        inquire (file=registry_path, exist=registry_exists)
        if (registry_exists) return

        ! Create default registry
        open (newunit=unit, file=registry_path, status='replace')
        write (unit, '(a)') '# Fortran Package Registry'
    write(unit, '(a)') '# Maps module names to their packages for automatic dependency resolution'
        write (unit, '(a)') '#'
        write (unit, '(a)') '# Module resolution rules:'
        write (unit, '(a)') '# 1. Check explicit modules list'
        write (unit, '(a)') '# 2. Check if module starts with a custom prefix'
    write(unit, '(a)') '# 3. Use default behavior: package name = module name before first underscore'
        write (unit, '(a)') '# 4. If no underscore, package name = module name itself'
        write (unit, '(a)') ''
        write (unit, '(a)') '[packages]'
        write (unit, '(a)') ''
        write (unit, '(a)') '[packages.fortplot]'
        write (unit, '(a)') 'git = "https://github.com/krystophny/fortplot"'
    write(unit, '(a)') 'prefix = "fortplot"  # Any module starting with "fortplot" belongs to this package'
        write (unit, '(a)') ''
        write (unit, '(a)') '[packages.pyplot-fortran]'
        write (unit, '(a)') 'git = "https://github.com/jacobwilliams/pyplot-fortran"'
        write (unit, '(a)') '# No prefix specified, will use default behavior:'
    write (unit, '(a)') '# pyplot_module -> package name = "pyplot" (before underscore)'
        close (unit)

        print *, 'Created default registry at: ', trim(registry_path)

    end subroutine ensure_registry_exists_in_dir