Create reciprocal hyperlinks between glossary and index

Question

In my textbook, I want to include both an index and a glossary. For the glossary, for which I'm currently using the glossaries package, I would prefer to have pointers (and hyperlinks) that don't go straight to one or more occurrences in the main text, but instead point to a single index entry (typically for the same term).

Maybe a sketch is in order.

Conventional scenario:

Index 
    Combustion engine -> page 23, page 56, ...

Glossary 
    Combustion engine: an endangered species -> page 23, page 56, ...

What I would like instead:

Index 
    Combustion engine -> page 23, page 56, ...

Glossary 
    Combustion engine: an endangered species. -> (page of) index entry 
                                                 on combustion engine

And while we are at it, we might as well also generate a pointer to the glossary entry within the index item.

Motivation:

1. No cluttering up the glossary with potentially long lists of page references that are already maintained in the index.

2. I usually give fairly extensive explanations in the glossary entries themselves, and the main text typically does not add much to those; thus, I don't want to frustrate the reader by inviting him to sift through the main text in search of detail that isn't there.

3. While writing, I don't want to think about questions like: should this occurrence go into the index only, the glossary only, or both? Thinking about what to include in the index is distraction enough.

PS: Before commenting on the above sample text, note that the book is about neither engines nor endangered species.

Answer 1

Here is what I came up with: I ditched the glossaries package and instead implemented a couple of macros based directly on hyperref. The index is generated as usual by makeindex, but I use a few tricks in order to let it generate the hyperlinks and -targets to match the ones created in the glossary. The glossary can simply be written as a description environment (although in real life I would use the enumitem package to customize it).

% arara: pdflatex
% arara: makeindex
% arara: pdflatex
% arara: pdflatex

% we need two pdflatex runs after makeindex for this to work. 
% arara is great for this kind of per-document build configuration. 

\documentclass[12pt]{article}

\usepackage[margin=1in,papersize={6in,8in}]{geometry}

% create a custom style file for makeindex that wraps the standard 
% index item commands such that we can get hold of their text contents
% 
% e.g:
%
% \inx{drugs}
%     \subinx{lithium}\hyperpage{2}, \hyperindexformat{\glsitem}{3}
%     \subinx{molecular size}\hyperpage{1}

\begin{filecontents*}{\jobname.mst}
line_max        2048

item_0 "\n\\inx{"
item_1 "\n    \\subinx{"
item_01 "\n    \\subinx{"
item_x1 "}\n    \\subinx{"
item_2 "\n        \\subsubinx{"
item_12 "\n        \\subsubinx{"
item_x2 "}\n        \\subsubinx{"

delim_0 "}"
delim_1 "}"
delim_2 "}"
\end{filecontents*}

% load and run makeindex
\usepackage{makeidx}
\makeindex

% we use hyperref to implement the reciprocal links between glossary and index
\usepackage[colorlinks,linkcolor=blue]{hyperref}

% implement the commands

% raise hypertargets by one line to get them into the viewport
\newcommand*{\raisedtarget}[1]{\raisebox{\ht\strutbox}{\hypertarget{#1}{}}}

% a glossary entry. Mandatory arguments are term and definition. 
% The optional argument becomes a prefix for the index key. 
\newcommand{\glsentry}[3][]{%
    \edef\glskey{#1#2|glsitem}%
    \expandafter\index\expandafter{\glskey}%
    \item [#2] \raisedtarget{gls-#2} #3 \hyperlink{inx-#2}{\pageref*{inx-#2}\emph{i}}
}

% wrap the index \item and \subitem commands to store the current index term
\newcommand*{\inx}[1]{%
    \global\edef\currentinx{#1}%
    \item #1 
}

\newcommand*{\subinx}[1]{%
    \global\edef\currentinx{#1}%
    \subitem #1 
}

\newcommand*{\subsubinx}[1]{%
    \global\edef\currentinx{#1}%
    \subsubitem #1 
}

% format the index page references generated by glossary entries  
\newcommand{\glsitem}[1]{%
    \label{inx-\currentinx}% we need the label to typeset the index page in the glossary ...
    \raisedtarget{inx-\currentinx}% ... and the explicit hypertarget to make the link work
    \hyperlink{gls-\currentinx}{#1\emph{g}}%
}


\begin{document}

\section{Introduction to pharmacology}

... document text snipped ...    

\section*{Glossary}

\begin{description}

% the [drugs!] prefix makes the link go to a subitem in the index
\glsentry[drugs!]{lithium}{An alkali metal used in batteries that moonlights as an antidepressant}

% no prefix will point the link to a main index item
\glsentry{Paracelsus}{A famous physician from the 16th century who investigated the relationship of drugs and poisons by way of self-experimentation}

\end{description}

\printindex

\end{document}

The glossary looks like this:

The index like this: