% quickderivs.sty
%
% Copyright (C) 2026 James Petersen <m@jamespetersen.ca>
% Licensed under MIT. See LICENSE.

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{quickderivs}[2026-03-25 0.1.1 Derivatives and Differentials]

\newif\ifquickderivs@upright
\newif\ifquickderivs@defp
\quickderivs@defpfalse
\quickderivs@uprightfalse
\DeclareOption{upright}{\quickderivs@uprighttrue}
\DeclareOption{partial}{\quickderivs@defptrue}
\ProcessOptions\relax

\RequirePackage{xstring}
\RequirePackage{xparse}

\ifquickderivs@upright
	\newsavebox\quickderivs@unslant@box

	\NewDocumentCommand{\quickderivs@unslant}{som}{%
		{%
			\mbox{%
				\sbox{\quickderivs@unslant@box}{$#3$}%
				\hskip\wd\quickderivs@unslant@box%
				\pdfsave%
				\IfBooleanTF{#1}{%
					\IfValueTF{#2}{%
						\expandafter\pdfsetmatrix{1 #2 0 1}%
					}{%
						\pdfsetmatrix{1 -.25 0 1}%
					}%
				}{%
					\IfValueTF{#2}{%
						\expandafter\pdfsetmatrix{1 0 #2 1}%
					}{%
						\pdfsetmatrix{1 0 -.25 1}%
					}%
				}%
				\llap{\usebox{\quickderivs@unslant@box}}%
				\pdfrestore%
			}%
			\kern-0.8pt
		}%
	}
\fi

% Defines quickderivs@isinteger macro if it doesn't exist.
\newcommand*{\quickderivs@isinteger}[3]{%
	\edef\quickderivs@isinteger@tmp{\expandafter\detokenize\expandafter{#1}}%
	\IfStrEq{\quickderivs@isinteger@tmp}{ }{%
		#3%
	}{%
		\IfInteger{\quickderivs@isinteger@tmp}{#2}{#3}%
	}%
}

\ifcsname df\endcsname
  \PackageError{quickderivs}{Command df is already defined}
\fi
\ifcsname dv\endcsname
  \PackageError{quickderivs}{Command dv is already defined}
\fi

\ifquickderivs@upright
	\newcommand\dvsetupright{%
		\gdef \quickderivs@chrs@d {\mathrm{d}}%
		\gdef \quickderivs@chrs@D {\mathrm{D}}%
		\gdef \quickderivs@chrs@e {\quickderivs@unslant{\delta}}%
		\gdef \quickderivs@chrs@p {\quickderivs@unslant{\partial}}%
	}
\fi
\newcommand\dvsetnormal{%
  \gdef \quickderivs@chrs@d {d}%
  \gdef \quickderivs@chrs@D {D}%
  \gdef \quickderivs@chrs@e {\delta}%
  \gdef \quickderivs@chrs@p {\partial}%
}
\gdef \quickderivs@chrs@E {\Delta}

\gdef\quickderivs@chrsspac@d{}
\gdef\quickderivs@chrsspac@D{}
\gdef\quickderivs@chrsspac@e{}
\gdef\quickderivs@chrsspac@E{\!}
\gdef\quickderivs@chrsspac@p{}

\def\quickderivs@dfchar #1{%
  \edef\arg{\detokenize{#1}}%
  % If empty, do default
  \if\relax\arg\relax%
    % default is d.
		\ifquickderivs@defp%
			\quickderivs@chrs@p%
		\else%
			\quickderivs@chrs@d%
		\fi%
  \else%
    \ifcsname quickderivs@chrs@\arg\endcsname%
      \csname quickderivs@chrs@\arg\endcsname%
    \else%
      \PackageError{quickderivs}{Unknown Character Argument}%
    \fi%
  \fi%
}

\def\quickderivs@dfcharspac #1{%
  \edef\arg{\detokenize{#1}}%
  % If empty, do default
  \if\relax\arg\relax%
    % default is d.
		\ifquickderivs@defp%
			\quickderivs@chrsspac@p%
		\else%
			\quickderivs@chrsspac@d%
		\fi%
  \else%
    \ifcsname quickderivs@chrsspac@\arg\endcsname%
      \csname quickderivs@chrsspac@\arg\endcsname%
    \else%
      \PackageError{quickderivs}{Unknown Character Argument}%
    \fi%
  \fi%
}

\def\quickderivs@prt{%
	\advance\quickderivs@idx by -1\relax%
	\advance\quickderivs@leftidx by 1\relax%
	\ifquickderivs@didexp^\fi{\StrMid{\quickderivs@arg}{\the\quickderivs@leftidx}{\the\quickderivs@idx}}%
	\advance\quickderivs@idx by 1\relax%
	\quickderivs@leftidx=\quickderivs@idx%
}

\newcount\quickderivs@idx
\newcount\quickderivs@leftidx
\newif\ifquickderivs@didexp
\newif\ifquickderivs@wassemicol
\newif\ifquickderivs@indv
\quickderivs@indvfalse

\NewDocumentCommand{\df}{sm!o!s}{%
	\expandarg%
  \ensuremath{%
    % Get character argument
    \ifquickderivs@indv\else%
			\edef\quickderivs@chararg{\IfBooleanTF{#4}{\ifquickderivs@defp d\else p\fi}{\IfValueT{#3}{#3}}}%
      \def\quickderivs@char{%
        \expandafter\quickderivs@dfchar\quickderivs@chararg{}%
      }%
    \fi%
		\def\quickderivs@arg{#2}%
    % Get argument length
    \StrLen{#2}[\arglen]%
    \ifnum\arglen=0\relax%
      \PackageError{quickderivs}{Empty Argument in Differential}%
    \fi%
    \quickderivs@idx=1%
    \quickderivs@leftidx=0%
    \quickderivs@didexpfalse%
    \loop%
    \StrChar{#2}{\the\quickderivs@idx}[\thc]%
    \if\thc,%
			\ifquickderivs@didexp\else%
				\quickderivs@char%
			\fi%
			\quickderivs@prt%
			\IfBooleanF{#1}{\,}%
      \quickderivs@didexpfalse%
    \else%
      \if\thc;%
				\quickderivs@char%
				\quickderivs@prt%
        \quickderivs@didexptrue%
				\quickderivs@wassemicoltrue%
      \else%
				\quickderivs@isinteger{\thc}{%
					\ifquickderivs@didexp\else%
						\quickderivs@char%
						\quickderivs@prt%
						\advance\quickderivs@leftidx by -1\relax%
						\quickderivs@didexptrue%
						\quickderivs@wassemicolfalse%
					\fi%
				}{%
					\ifquickderivs@didexp%
						\ifquickderivs@wassemicol\else%
							\quickderivs@prt%
							\IfBooleanF{#1}{\,}%
							\advance\quickderivs@leftidx by -1\relax%
							\quickderivs@didexpfalse%
						\fi%
					\fi%
				}%
      \fi%
    \fi%
    \advance\quickderivs@idx by 1\relax%
    \ifnum\quickderivs@idx > \arglen\relax%
    \else \repeat%
		\ifquickderivs@didexp\else%
			\quickderivs@char%
		\fi%
		\quickderivs@prt%
  }%
}

\newif\ifquickderivs@nonint
\newif\ifquickderivs@endloop
\newcount\quickderivs@expcount

\newcommand{\quickderivs@countexp}[1]{%
	\expandarg%
  \StrLen{#1}[\arglen]%
  \ifnum\arglen=0\relax%
    \PackageError{quickderivs}{Empty Argument in Differential}%
  \fi%
  \quickderivs@idx=1%
  \quickderivs@expcount=0%
  \quickderivs@didexpfalse%
  \quickderivs@nonintfalse%
  \loop%
  \StrChar{#1}{\the\quickderivs@idx}[\thc]%
  \if\thc,%
    \ifquickderivs@didexp%
      \advance \quickderivs@leftidx by 1\relax%
      \advance \quickderivs@idx by -1\relax%
      \StrMid{#1}{\the\quickderivs@leftidx}{\the\quickderivs@idx}[\intstr]%
      \quickderivs@isinteger{\intstr}{%
        \advance \quickderivs@expcount by \intstr\relax%
      }%
      {%
        \quickderivs@noninttrue%
      }%
      \advance \quickderivs@idx by 1\relax%
    \else%
      \advance \quickderivs@expcount by 1\relax%
    \fi%
    \quickderivs@didexpfalse%
  \else%
    \if\thc;%
      \quickderivs@didexptrue%
      \quickderivs@leftidx=\quickderivs@idx%
			\quickderivs@wassemicoltrue%
    \else%
			\quickderivs@isinteger{\thc}{%
				\ifquickderivs@didexp\else%
					\quickderivs@leftidx=\quickderivs@idx%
					\advance\quickderivs@leftidx by -1\relax%
					\quickderivs@didexptrue%
					\quickderivs@wassemicolfalse%
				\fi%
			}{%
				\ifquickderivs@didexp%
					\ifquickderivs@wassemicol\else%
						\advance \quickderivs@leftidx by 1\relax%
						\advance\quickderivs@idx by -1\relax%
						\StrMid{#1}{\the\quickderivs@leftidx}{\the\quickderivs@idx}[\intstr]%
						\advance\quickderivs@idx by 1\relax%
						\quickderivs@leftidx=\quickderivs@idx%
						\advance\quickderivs@leftidx by -1\relax%
						\quickderivs@isinteger{\intstr}{%
							\advance \quickderivs@expcount by \intstr\relax%
						}%
						{%
							\quickderivs@noninttrue%
						}%
						\quickderivs@didexpfalse%
					\fi%
				\fi%
			}%
    \fi%
  \fi%
  \advance\quickderivs@idx by 1\relax%
  \ifnum\quickderivs@idx > \arglen\relax%
    \quickderivs@endlooptrue%
  \else%
    \quickderivs@endloopfalse%
  \fi%
  \ifquickderivs@nonint%
    \quickderivs@endlooptrue%
  \fi%
  \ifquickderivs@endloop \else \repeat%
  \ifquickderivs@nonint%
  \else%
    \ifquickderivs@didexp%
      \advance \quickderivs@leftidx by 1\relax%
      \advance \quickderivs@idx by -1\relax%
      \StrMid{#1}{\the\quickderivs@leftidx}{\the\quickderivs@idx}[\intstr]%
      \quickderivs@isinteger{\intstr}{%
        \advance \quickderivs@expcount by \intstr\relax%
      }%
      {%
        ^{\StrMid{#1}{\the\quickderivs@leftidx}{\the\quickderivs@idx}[\expstr]}%
        \quickderivs@noninttrue%
      }%
    \else%
      \advance \quickderivs@expcount by 1\relax%
    \fi%
  \fi%
  \ifquickderivs@nonint%
  \else%
    \ifnum\quickderivs@expcount > 1\relax%
      ^{\quickderivs@charspac\the\quickderivs@expcount}%
    \fi%
  \fi%
}

\NewDocumentCommand{\dv}{soom!o!s}{%
	\expandarg%
  \ensuremath{%
    \edef\quickderivs@chararg{\IfBooleanTF{#6}{\ifquickderivs@defp d\else p\fi}{\IfValueT{#5}{#5}}}%
    \def\quickderivs@char{%
      \expandafter\quickderivs@dfchar\quickderivs@chararg{}%
    }%
		\def\quickderivs@charspac{%
			\expandafter\quickderivs@dfcharspac\quickderivs@chararg{}%
		}%
    \IfBooleanF{#1}{\frac}%
    {%
      \quickderivs@char%
      \IfValueTF{#3}%
      {%
        ^{\quickderivs@charspac#3}%
      }%
      {%
        \quickderivs@countexp{#4}%
      }%
			{\IfValueT{#2}{#2}}%
      \IfBooleanT{#1}{/}%
    }%
    {%
			\quickderivs@indvtrue
      \df*{#4}%
			\quickderivs@indvfalse
    }%
  }%
}