exercise.sty

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{exercise}[2025-03-26 Typeset exercises]

\RequirePackage{xkeyval}
\RequirePackage{ifthen}

\makeatletter
% declare exercise styles
\define@key{ex@style}{headfont}{\def\currentHeadFont{#1}}
\define@key{ex@style}{bodyfont}{\def\currentBodyFont{#1}}

\setkeys{ex@style}{headfont=\sffamily\bfseries, bodyfont=\normalfont}

\newcommand{\NewExerciseStyle}[2]{
    \setkeys{ex@style}{#2}
    \def\styleName{#1}
    \expandafter\def\csname \styleName.headfont\endcsname{\currentHeadFont}
    \expandafter\def\csname \styleName.bodyfont\endcsname{\currentBodyFont}
}

% default exercise style
\NewExerciseStyle{ex@style@default}{headfont=\sffamily\bfseries, bodyfont=\normalfont}

% declare exercise environments
\define@key{ex@definition}{style}{\def\currentStyle{#1}}
\define@key{ex@definition}{numbered}{\def\currentNumbered{#1}}
\define@key{ex@definition}{numberWithin}{\def\currentNumberWithin{#1}}

\setkeys{ex@definition}{style=ex@style@default, numbered=false, numberWithin=none}

\newcommand{\NewExerciseDefinition}[3][style=ex@style@default]{%
    \setkeys{ex@definition}{#1}%
    \ifthenelse{\equal{\currentNumbered}{true}}{%
        \@ifundefined{ex@counter@#3}{%
            \ifthenelse{\equal{\currentNumberWithin}{none}}{%
                \newcounter{ex@counter@#3}%
            }{%
                \newcounter{ex@counter@#3}[\currentNumberWithin]%
                \expandafter\renewcommand\csname theex@counter@#3\endcsname{\csname the\currentNumberWithin\endcsname.\arabic{ex@counter@#3}}%
            }%
        }{}%
        \setcounter{ex@counter@#3}{0}%
    }{}%
    \newenvironment{#3}{%
        \par\noindent%
        {%
            \csname \currentStyle.headfont\endcsname #2%
            \ifthenelse{\equal{\currentNumbered}{true}}{%
                \refstepcounter{ex@counter@#3}%
                ~\csname theex@counter@#3\endcsname.%
            }{}%
        }%
        \csname \currentStyle.bodyfont\endcsname%
    }{\normalfont\par}%
}
\makeatother