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+%% This is part of OpTeX project, see http://petr.olsak.net/optex
+
+\_codedecl \outlines {PDF outlines <2020-03-12>} % preloaded in format
+
+\_def\_outlines#1{\_pdfcatalog{/PageMode/UseOutlines}\_openref
+ \_ifx\_toclist\_empty
+ \_opwarning{\_noexpand\outlines -- data unavailable. TeX me again}%
+ \_else
+ \_ifx\_dest\_destactive \_else
+ \_opwarning{\_noexpand\outlines doesn't work when \_noexpand\hyperlinks isn't declared}\_fi
+ {\_let\_tocline=\_outlinesA
+ \_count0=0 \_count1=0 \_count2=0 \_count3=0 \_toclist % calculate numbers o childs
+ \_def\_outlinelevel{#1}\_let\_tocline=\_outlinesB
+ \_tocrefnum=0 \_count0=0 \_count1=0 \_count2=0 \_count3=0
+ \_toclist}% create outlines
+ \_fi
+}
+\_def\_outlinesA#1#2#3#4#5#6{%
+ \_advance\_count#1 by1
+ \_ifcase#1\_or
+ \_addoneol{_ol:\_the\_count0}\_or
+ \_addoneol{_ol:\_the\_count0:\_the\_count1}\_or
+ \_addoneol{_ol:\_the\_count0:\_the\_count1:\_the\_count2}\_or
+ \_addoneol{_ol:\_the\_count0:\_the\_count1:\_the\_count2:\_the\_count3}\_fi
+}
+\_def\_addoneol#1{%
+ \_ifcsname #1\_endcsname
+ \_tmpnum=\_csname#1\_endcsname\_relax
+ \_advance\_tmpnum by1 \_sxdef{#1}{\_the\_tmpnum}%
+ \_else \_sxdef{#1}{1}%
+ \_fi
+}
+\_def\_outlinesB#1#2#3#4#5#6{%
+ \_advance\_count#1 by1
+ \_ifcase#1%
+ \_tmpnum=\_trycs{_ol:\_the\_count0}{0}\_or
+ \_tmpnum=\_trycs{_ol:\_the\_count0:\_the\_count1}{0}\_relax\_or
+ \_tmpnum=\_trycs{_ol:\_the\_count0:\_the\_count1:\_the\_count2}{0}\_relax\_or
+ \_tmpnum=\_trycs{_ol:\_the\_count0:\_the\_count1:\_the\_count2:\_the\_count3}{0}\_relax\_or
+ \_tmpnum = 0\_relax\_fi
+ \_pdfunidef\_tmp{#4}%
+ \_advance\_tocrefnum by1
+ \_outlinesC{#1}{toc:\_the\_tocrefnum}{\_ifnum#1<\_outlinelevel\_space\_else-\_fi}{\_tmpnum}{\_tmp}%
+}
+\_def\_outlinesC#1#2#3#4#5{\_pdfoutline goto name{#2} count #3#4{#5}\_relax}
+
+\_newcount\_oulnum
+\_def\_insertoutline#1{\_global\_advance\_oulnum by1
+ \_pdfdest name{oul:\_the\_oulnum} xyz\_relax
+ \_pdfunidef\_tmp{#1}%
+ \_pdfoutline goto name{oul:\_the\_oulnum} count0 {\_tmp}\_relax
+}
+\_public \outlines \insertoutline ;
+
+\_endcode % ----------------------------
+
+The problem is that PDF format needs to know the number of direct
+descendants of each outline if we need to create the tree of structured
+outlines. But we know only the level of each outline. The required data
+should be calculated from TOC data. We use two steps over TOC data saved in
+the `\_toclist` where each record is represented by one `\_tocline`.
+
+First step, the \`\outlines` macro sets `\_tocline` to \`\_outlinesA` and
+calculates the number of direct descendants of each record. Second step,
+the `\outlines` macro sets `\_tocline` to \`\_outlinesB` and it uses prepared
+data and create outlines.
+
+Each ouline is mapped to the control sequence of the type
+`\_ol:<num>` or `\_ol:<num>:<num>` or `\_ol:<num>:<num>:<num>` or etc.
+The first one is reserved for level 0, the second one for level 1 (chapters), third
+one for level 2 (sections) etc. The number of direct descentants will be stored
+in these macros after first step is finished. Each new outline of given level
+increases the `<num>` at given level. When the first step is processed then
+(above that) the `\_ol:..` sequence of the parent inceases its value too. The
+`_ol:...` sequences are implemented by `\_ol:\_count0:\_count1:\count2` etc.
+For example, when section (level 2) is processed in the first step then we do:
+
+\begtt
+ \advance \count2 by 1
+ % increases the mapping pointer of the type
+ % \_ol:<\_count0:\_count1:\_count2> of this section
+ \advance \<_ol:\_count0:\_count1> by 1
+ % increases the number of descendants connected
+ % to the parent of this section.
+\endtt
+
+When second step is processed, then we only read the stored data about the
+number of descentants. Ad we use it in `count` parameter of `\_pdfoutline`
+primitive.
+
+For linking, we use the same links as in TOC, i.e. the `toc:\_the\_tocrefnum`
+labels are used.
+
+\`\insertoutline` `{<text>}` inserts one outline with zero direct descendants.
+It creates link destination of the type `oul:<num>` into the document
+(where \^`\insertoutline` is used) and the link itself is created too in the
+outline.