% ==============================================
%  linkedthm.sty — Linked Theorem ↔ Proof helpers
%  v1.0 — 2025/07/21 — Luis A. Ortega
%  License: LPPL 1.3c
% ==============================================
\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{linkedthm}[2025/07/21 v1.0 Linked theorem–proof hyperlinks]

% ==============================================
% Requirements
% ==============================================
\RequirePackage{amsthm}    % For theorem-like environments
\RequirePackage{xparse}    % For parsing environments
\RequirePackage{hyperref}  % For hyperlinks

% ==============================================
% Internal storage helpers
% ==============================================
\makeatletter

% Save a labeled statement's body and type
\newcommand{\lthm@savestmt}[3]{% #1=label, #2=env type, #3=body
  \expandafter\gdef\csname lthm@body@#1\endcsname{#3}%
  \expandafter\gdef\csname lthm@type@#1\endcsname{#2}%
}

% Retrieve the body of a saved statement
\newcommand{\lthm@getbody}[1]{%
  \@ifundefined{lthm@body@#1}{%
    \PackageWarning{linkedthm}{No body stored for #1}%
  }{%
    \csname lthm@body@#1\endcsname}}

% Retrieve the type of a saved statement
\newcommand{\lthm@gettype}[1]{%
  \@ifundefined{lthm@type@#1}{%
    \PackageWarning{linkedthm}{No type stored for #1}%
    ???%
  }{%
    \csname lthm@type@#1\endcsname}}

\makeatother

% ==============================================
% Define linked theorem environments
% \DeclareLinkedTheorem{linked-env}{base-env}{Printable Name}
% ==============================================
\makeatletter
\NewDocumentCommand{\DeclareLinkedTheorem}{mmm}{%
  % Store printable name for use in proof heading
  \@ifundefined{lthm@name@#2}{%
    \expandafter\newcommand\csname lthm@name@#2\endcsname{#3}%
  }{}%

  % Define new linked environment
  \NewDocumentEnvironment{#1}{m +b}{%
    \lthm@savestmt{##1}{#2}{##2}% store statement
    \begin{#2}\label{##1}%
      ##2\ %
      \textup{[\hyperref[proof:##1]{Proof}]}% backlink to proof
    \end{#2}%
  }{}%
}
\makeatother

% ==============================================
% Define linked proof environment
% \begin{linkedproof}{label} ... \end{linkedproof}
% ==============================================
\makeatletter
\NewDocumentEnvironment{linkedproof}{m +b}{%
  \def\lthm@thislabel{#1}%
  \edef\lthm@thistype{\lthm@gettype{#1}}%

  % Resolve printable name
  \@ifundefined{lthm@name@\lthm@thistype}{%
    \def\lthm@printname{\lthm@thistype}%
  }{%
    \edef\lthm@printname{\csname lthm@name@\lthm@thistype\endcsname}%
  }%

  % Hyperlink target and restated heading
  \par\medskip
  \phantomsection
  \label{proof:#1}%
  \noindent\textbf{\lthm@printname~\ref{#1}}%
  \ \textup{[\hyperref[\lthm@thislabel]{Return}]}\textbf{. }%
  \lthm@getbody{#1}\par\medskip

  % Begin actual proof
  \begin{proof}%
    #2%
}{%
  \end{proof}%
  \par\medskip
}
\makeatother

\endinput