alignment of subnumcases in the page

Question

I want to label each case in cases so I used subnumcases environment. However, it seems like what is centered is the first letter, not the entire chunk.

I made a MWE with the packages I need for my work, and by toggling off the packages one by one, I found that mathtools cellspace empheq are causing the ugly alignment. (for empheq, maybe I can go without it.)

This is the MWE script:

\documentclass[12pt]{article}
\usepackage[top=2.5cm, left=2.5cm, right=2.5cm, bottom=3cm]{geometry}
\usepackage{amsmath,amsfonts}
\usepackage{showframe}
\usepackage{cases}
% when any of the packagages below(mathtools, cellspace, empheq) is used, it gives the ugly output like in the screenshot. 
\usepackage{mathtools}
%\usepackage{cellspace}
%\usepackage{empheq}

\begin{document}
    %%%%%%%%%%%%%%%%%%%%%%% 
    When using {\bfseries subnumcases environment} with mathtools, cellspace, or empheq, it seemas like its starting point is centered
    \begin{subnumcases}{f(x)=}
        x & $0<x<1$ \\
        1000000000000000000000000000x  & $1\leq x$
    \end{subnumcases}
    
    But {\bfseries cases environment} is okay and and I want the alignment of subnumcases also to be like this.
    \begin{equation}
        f(x)=
        \begin{cases}
            x & 0<x<1 \\
            1000000000000000000000000000x  & 1\leq x
        \end{cases}
    \end{equation}
    
    %%%%%%%%%%%%%%%%%%%%%%% 
\end{document}

And this is my ugly output with mathtools:

How can I have center-aligned subnumcases environment while using mathtools cellspace empheq packages?

Thanks!

P.S. if you have any other way to have sub-numbers for cases, it's also welcome. I tried subequations, but found difficulty to put it in case for like "f(x) = { " part at the left of the subequations chunk.

+ I'm using texstudio and I'm adding the screenshot of the compiled output in texstudio.

Answer 1

I can't reproduce your issue, but you don't need cases if you load empheq.

\documentclass[12pt]{article}
\usepackage[
  top=2.5cm,
  left=2.5cm,
  right=2.5cm,
  bottom=3cm
]{geometry}
\usepackage{amsmath,amsfonts}

\usepackage{cellspace}
\usepackage{mathtools}
\usepackage{empheq}

% for debugging
\usepackage{showframe}

\newcommand{\topstrut}{\smash[b]{\vphantom{\big|}}}
\newcommand{\botstrut}{\smash[t]{\vphantom{\big|}}}

\begin{document}

When using the \textbf{subnumcases environment} with mathtools, cellspace, or empheq, 
it seems like its starting point is centered
\begin{subequations}
\begin{empheq}[left={f(x)=\empheqlbrace}]{alignat=2}
  & x                              &\quad& 0<x<1 \topstrut \\
  & 1000000000000000000000000000x  &\quad& 1\leq x \botstrut
\end{empheq}
\end{subequations}
But the \textbf{cases environment} is okay and and I want the alignment of subnumcases 
also to be like this.
\begin{equation}
  f(x)=
  \begin{cases}
    x & 0<x<1 \\
    1000000000000000000000000000x  & 1\leq x
  \end{cases}
\end{equation}
    
\end{document}

I added \topstrut and \botstrut just to make the two cases the same.