\newcounter{subfigure}

\newcommand{\subfiglabelformat}[1]{\raggedright{#1}}
\newcommand{\subfiglabelstyle}[1]{\alph{#1}}
\newcommand{\subfiglabelsize}{\small}
\newenvironment{multifig}%
{\addtocounter{figure}{1}%
  \setcounter{subfigure}{0}%
  \renewcommand\thesubfigure{\thefigure~(\subfiglabelstyle{subfigure})}%
}
{\addtocounter{figure}{-1}}

\makeatletter
\ExplSyntaxOn
\newif\if@insubfig
\NewDocumentEnvironment{subfigure}{mO{}}{%
  \@insubfigtrue%
  \refstepcounter{subfigure}%
  \begin{minipage}{#1}%
    \subfiglabelformat{{\subfiglabelsize (\subfiglabelstyle{subfigure})~#2}}%
    }
    {\end{minipage}%
  \@insubfigfalse%
}
\ExplSyntaxOff

\let\multifig@old@label\label%
\def\label#1{%
  \multifig@old@label{#1}%
  \if@insubfig%
    \@bsphack%
    \protected@write\@auxout{}%
    {\string\newlabel{#1@multifig}{{(\subfiglabelstyle{subfigure})}%
        {\thepage}%
        {\@currentlabelname}%
        {\@currentHref}{}%
      }}%
    \@esphack%
  \fi
}%
\def\subref#1{\ref{#1@multifig}}
\makeatother