In a Lua filter, how to access previous (or next) element?

[pagiraud]

I’m writing a Lua filter, where I use the Figure function to change the figure appearance if it has a given class. But I also would like to change appearance of the previous element in the AST if it’s a header. How can I achieve that? And, reversely, if I select a header if it has a certain class, can I modify the next element in the AST if it’s a figure?

Excerpt of my AST:

, Header 2 ( "section" , [ "plain" ] , [] ) []
, Figure
    ( "" , [] , [] )
    (Caption
       Nothing
       [ Plain
           [ Str "Le"
           , Space
           , Str "petit"
           , Space
           , Str "\226ne"
           , Space
           , Str "gris"
           ]
       ])
    [ Plain
        [ Image
            ( "" , [ "fullframegraphicnoadjust" ] , [] )
            [ Str "Le"
            , Space
            , Str "petit"
            , Space
            , Str "\226ne"
            , Space
            , Str "gris"
            ]
            ( "images/somepicture.png" , "" )
        ]
    ]

Thanks for your time.

[kysko]

In this precise case, it is possible, if you hierarchicalize the doc using make_sections, which creates sections Divs out of Headers.

In the first problem, let's say the target Figure class is foo. You then ask the question:

Is there a section Div (so a Div with class section) for which the second Block (in its content) is a Figure with class foo?

If so, then modify the first Block of the Div, which by construction must be the Header associated with the section.

In the second problem, let's say the target Header class is bar. You then ask the question:

Is there a section Div so that the first Block has class bar, and which has a Figure as second Block?

If so, then modify that second Block.

After this, you can undo the make_sections if you must.

Of course, by "ask the question" above, I mean a well-crafted filter.

/Edit: hmm... i see you already have class section for the header, but this is not why I mentioned class section for section Div above, which is created by make_sections.
/Edit 2: oops again... misread the Native format: 'section' is your Header identifier. That shouldn't matter.

[pagiraud]

Thanks for your reply. It may be a solution. Actually, this may be a bit overkill but at least I realized that there are two different make_sections : pandoc.utils.make_sections (that I knew of) and pandoc.structure.make_sections (that I didn’t know).

[massifrg]

You may search that combination of Header+Figure with Blocks.

[pagiraud]

Thanks a lot. That is exactly what I needed. I had already read that and the example about removing spaces before citations but didn’t see I was trying to achieve something similar.

This is the code I used to test this solution:

local figuretype
local function is_figure_in_given_frame(frame, figure)
  if frame and frame.t == 'Header'
    and figure and (figure.t == 'Figure' or figure.t == 'Para')
    and frame.classes[1] == "given" then
    local figtype
    figtype = figure.t
    return figtype
  end
end

function Blocks (blocks)
  -- Go from end to start to avoid problems with shifting indices.
  for i = #blocks-1, 1, -1 do
    figuretype = is_figure_in_given_frame(blocks[i], blocks[i+1])
    if figuretype == "Figure" then --if there is a caption
      print(blocks[i+1].t)
    elseif figuretype == "Para" then --if there is no caption
      print(blocks[i+1].t)
    end
  end
  return blocks
end

Thanks to you I finished my filter!