Customizing square brackets

Question

I am trying to modify LaTeX's brackets (\cases) to make something like the one on the left:

I could only find code for square box and brackets (\cases), but not for this particular variant on any forums. The problem is how to either how to add a "tip" on [ or straighten up the corners on \cases. Could anyone help me with this?

Thanks.

Answer 1

DISCLAIMER: This is almost entirely taken from Steven B. Segletes answer and what I made here is also more a matrix that can be abused for cases. But the upshot is that with Steven's code one can draw any bracket one wants after very simple adjustments. Of course, some basic knowledge of TikZ helps.

\documentclass{article} % https://tex.stackexchange.com/a/327584/121799
\usepackage{tikz,amsmath,environ}
\usetikzlibrary{decorations.pathreplacing,calc}
\tikzset{
    ncbar angle/.initial=90,
    ncbar/.style={
        to path=(\tikztostart)
        -- ($(\tikztostart)!#1!\pgfkeysvalueof{/tikz/ncbar angle}:(\tikztotarget)$)
        -- ($(\tikztotarget)!($(\tikztostart)!#1!\pgfkeysvalueof{/tikz/ncbar angle}:(\tikztotarget)$)!\pgfkeysvalueof{/tikz/ncbar angle}:(\tikztostart)$)
        -- (\tikztotarget)
    },
    ncbar/.default=0.5cm,
    freaky dim/.default=4pt,
    freaky/.style={
        to path={let \p1=(\tikztostart),\p2=(\tikztotarget) in (\tikztostart)
        -- ($(\tikztostart)!#1!\pgfkeysvalueof{/tikz/ncbar angle}:(\tikztotarget)$)
        -- ({\x1-#1},{(\y1+\y2)/2-abs(#1)})
        -- ({\x1-2*#1},{(\y1+\y2)/2})
        -- ({\x1-#1},{(\y1+\y2)/2+abs(#1)})
        -- ($(\tikztotarget)!($(\tikztostart)!#1!\pgfkeysvalueof{/tikz/ncbar angle}:(\tikztotarget)$)!\pgfkeysvalueof{/tikz/ncbar angle}:(\tikztostart)$)
        -- (\tikztotarget)}
    },
    freaky/.default=0.5cm,
}

\tikzset{freaky left brace/.style={freaky=0.5ex}}
\tikzset{freaky right brace/.style={freaky=-0.5ex}}

\tikzset{square left brace/.style={ncbar=0.5ex}}
\tikzset{square right brace/.style={ncbar=-0.5ex}}

\tikzset{round left paren/.style={ncbar=0.3cm,out=115,in=-115}}
\tikzset{round right paren/.style={ncbar=0.3cm,out=65,in=-65}}
%

\NewEnviron{flcases}{\setbox0=\hbox{$\,\begin{matrix}\BODY\end{matrix}\,$}%
  \setbox2=\hbox{\begin{tikzpicture}
    \draw [thick] (0,\botdim) to [freaky left brace] (0,\topdim);
    \copy0
  \end{tikzpicture}}
  \vcenter{\hbox{\copy2}}%
}


\def\topdim{\the\dimexpr+\ht0+.5\ht\strutbox-.5\dp\strutbox-3pt\relax}
\def\botdim{\the\dimexpr-\ht0+.5\ht\strutbox-.5\dp\strutbox+3pt\relax}
\begin{document}

\[ f(x)=
\begin{flcases}
   0 & \text{if}~x<0  \\ 
   \pi & \text{if}~x\ge0  
\end{flcases}%
\]

\end{document}