Add support for indent conditionals (#1078)

autoload/vimtex.vim

@@ -170,6 +170,12 @@ function! vimtex#init_options() " {{{1
         \ { 'lhs' : 'vr', 'rhs' : '\varrho' },
         \])
 
+  call s:init_option('vimtex_indent_conditionals', {
+        \ 'open': '\v\\if%(num|false)?>',
+        \ 'else': '\\else\>',
+        \ 'close': '\\fi\>',
+        \})
+
   call s:init_option('vimtex_index_hide_line_numbers', 1)
   call s:init_option('vimtex_index_resize', 0)
   call s:init_option('vimtex_index_show_help', 1)

doc/vimtex.txt

@@ -1232,6 +1232,17 @@ Options~
 
   Default value: 1
 
+*g:vimtex_indent_conditionals*
+  This is a dictionary that defines regexes for indenting conditionals. Set it
+  to an empty dictionary to disable this type of indentation.
+
+  Default value: >
+    let g:vimtex_indent_conditionals = {
+          \ 'open': '\v\\if%(num|false)?>',
+          \ 'else': '\\else\>',
+          \ 'close': '\\fi\>',
+          \}
+
 *g:vimtex_indent_delims*
   A dictionary that defines the list of opening and closing delimiters (in
   regex form) that should add/reduce indents.

indent/tex.vim

@@ -61,6 +61,7 @@ function! VimtexIndent(lnum) abort " {{{1
   let l:ind = indent(l:prev_lnum)
   let l:ind += s:indent_envs(l:line, l:prev_line)
   let l:ind += s:indent_delims(l:line, a:lnum, l:prev_line, l:prev_lnum)
+  let l:ind += s:indent_conditionals(l:line, a:lnum, l:prev_line, l:prev_lnum)
   let l:ind += s:indent_tikz(l:prev_lnum, l:prev_line)
   return l:ind
 endfunction
@@ -167,6 +168,25 @@ if s:re_opt.include_modified_math
   let s:re_close .= '\|' . g:vimtex#delim#re.delim_mod_math.close
 endif
 
+" }}}1
+function! s:indent_conditionals(line, lnum, prev_line, prev_lnum) abort " {{{1
+  if empty(g:vimtex_indent_conditionals) | return 0 | endif
+
+  if a:line =~# g:vimtex_indent_conditionals.close
+    silent! unlet s:conditional_opened
+    return -s:sw
+  elseif get(s:, 'conditional_opened')
+        \ && a:line =~# g:vimtex_indent_conditionals.else
+    return -s:sw
+  elseif get(s:, 'conditional_opened')
+        \ && a:prev_line =~# g:vimtex_indent_conditionals.else
+    return s:sw
+  elseif a:prev_line =~# g:vimtex_indent_conditionals.open
+    let s:conditional_opened = 1
+    return s:sw
+  endif
+endfunction
+
 " }}}1
 function! s:indent_tikz(lnum, prev) abort " {{{1
   if !has_key(b:vimtex.packages, 'tikz') | return 0 | endif