hyperref and bibunit: Keep the destination of the cite-link in the same bibunit

Question

I have a rather big and complex document with several bibunits and recurring citations. By default, all citations target the first appearance of a reference in a bibliography, leading to awkward jumps in the flow of the document and in irritating potential readers.

Unfortunately, the solution proposed in this question and its answer does not work for me because the bibunits are essentially section*, so there is not standard LaTeX counter available to disambiguate them. The bibunits are \phantomsections but I don't know how to make use of this fact. The bibunits are also in subfiles if this helps to set something up.

Also, the solution proposed in this older question and its answer is not enough for me, because natbib's \citep is used a lot and is not covered by the macros there.

What else can I do?

Answer 1

This is a complete rewrite of the answer. I found a solution to the problem that works at least for my project. It is definitely not a general solution (it does not handle compatibility with revtex, harvard, or chicago) and was achieved by deep surgery in the guts of hyperref.sty. For disambiguation of the bibunits the value of the counter \the\@bibunitauxcnt is inserted in the destination of the reference.

%hyperbu.sty
\ProvidesPackage{hyperbu}[2021/10/28 1.0 (JKn)]
%
% Makes Hyperlinks of citations and bibliographies local to bibunits
%
\RequirePackage{natbib}
\RequirePackage{bibunits}
\RequirePackage{hyperref}
\def\hyper@natlinkstart#1{%
  \Hy@backout{#1}%
  \hyper@linkstart{cite}{cite.\the\@bibunitauxcnt.#1}%
  \def\hyper@nat@current{#1}%
}
\def\hyper@natlinkend{%
  \hyper@linkend
}
\def\hyper@natlinkbreak#1#2{%
  \hyper@linkend#1\hyper@linkstart{cite}{cite.\the\@bibunitauxcnt.#2}%
}
\def\hyper@natanchorstart#1{%
  \Hy@raisedlink{\hyper@anchorstart{cite.\the\@bibunitauxcnt.#1}}%
}\def\hyper@natanchorstart#1{%
  \Hy@raisedlink{\hyper@anchorstart{cite.\the\@bibunitauxcnt.#1}}%
}
\def\hyper@natanchorend{\hyper@anchorend}
\ltx@IfUndefined{NAT@parse}{%
  \providecommand*\@extra@binfo{}%
  \providecommand*\@extra@b@citeb{}%
  \def\bibcite#1#2{%
    \@newl@bel{b}{#1\@extra@binfo}{%
      \hyper@@link[cite]{}{cite.\the\@bibunitauxcnt.#1    \@extra@b@citeb}{#2}%
    }%
  }%
  \gdef\@extra@binfo{}%
  \let\Hy@bibcite\bibcite
  \begingroup
    \@ifundefined{bbl@cite@choice}{}{%
      \g@addto@macro\bbl@cite@choice{%
        \let\bibcite\Hy@bibcite
      }%
    }%
  \endgroup
  \providecommand*{\@BIBLABEL}{\@biblabel}%
  \def\@lbibitem[#1]#2{%
    \@skiphyperreftrue
    \H@item[%
      \ifx\Hy@raisedlink\@empty
        \hyper@anchorstart{cite.\the\@bibunitauxcnt.#2\@extra@b@citeb}%
          \@BIBLABEL{#1}%
        \hyper@anchorend
      \else
        \Hy@raisedlink{%
          \hyper@anchorstart{cite.\the\@bibunitauxcnt.#2\@extra@b@citeb}\hyper@anchorend
        }%
        \@BIBLABEL{#1}%
      \fi
      \hfill
    ]%
\@skiphyperreffalse
\if@filesw
  \begingroup
    \let\protect\noexpand
    \immediate\write\@auxout{%
      \string\bibcite{#2}{#1}%
    }%
  \endgroup
\fi
\ignorespaces
}%
\def\@bibitem#1{%
\@skiphyperreftrue\H@item\@skiphyperreffalse
\Hy@raisedlink{%
  \hyper@anchorstart{cite.\the\@bibunitauxcnt.#1\@extra@b@citeb}\relax\hyper@anchorend
    }%
    \if@filesw
      \begingroup
        \let\protect\noexpand
        \immediate\write\@auxout{%
          \string\bibcite{#1}{\the\value{\@listctr}}%
        }%
      \endgroup
    \fi
    \ignorespaces
  }%
}{}
\endinput