-- filter to reference images/figures in text
-- inspired by pandoc-fignos (works with fignos @fig:<label> syntax)
-- currently only conversion to PDF & HTML
-- tested with Pandoc 3.1
-- written by Thomas Jurczyk

-- GLOBAL VARIABLES
-- table to store fig-refs found
Figref_Table = {}
-- counter for figure numbering
Counter = 1

-- HELPER FUNCTIONS

--- Function to add a Figure str pandoc.Inline Element to a pandoc.Caption
-- @param caption The pandoc.Plain block from the figure.caption.long element.
-- @return The new figure.caption.long.Plain block now starting with with 'Figure <NUMBER>: '.
function addFigureTagToCaption(plain)
    -- add a "Figure <NUMBER>:" tag at the beginning of a caption
    table.insert(plain.content, 1, pandoc.Str("Figure"))
    table.insert(plain.content, 2, pandoc.Space())
    table.insert(plain.content, 3, pandoc.Str(Counter .. ":"))
    table.insert(plain.content, 4 ,pandoc.Space())
    return plain
end

--- Function to find the index of an element in a table. This function is used in setRefsToFigure().
-- @param element The element searched for in the table.
-- @param table The table in which the element is searched for.
-- @return (str|nil) The index as a STRING. nil if element has not been found.
function findIndex(element, table)
    for index, entry in ipairs(table) do
        if entry == element then
            return tostring(index)
        end
    end
    return nil
end

---Function to replace a string that matches a figure id with HTML/LaTeX links/refs.
-- @param The plain string from Cite.citations[1].id
-- @return (pandoc.RawInline|nil) 
function stringReplacementRef(word)
    if tableContains(Figref_Table, word) then
        local referenceNumber = findIndex(word, Figref_Table)
        if FORMAT:match "html" then
            return pandoc.RawInline('html', '<a href="#'..word..'">'..referenceNumber..'</a>')
        --TODO: add LaTeX reference
        end
    else
        return nil
    end
end

--- Function to check if an element (here: the identifier used in the fig-reference) is in a table (here: the globale table with all found figure identifiers).
-- @param table The table (= global Figref_Table).
-- @param element The element to search for (=the identifier used in the in-text reference).
-- @return (bool) true or false
function tableContains(table, element)
    for _,value in ipairs(table) do
        if value == element then
            return true
        end
    end
    return false
end

-- MAIN FUNCTIONS

--- Checks every pandoc.Figure for identifiers; if one was found, add it to global Figref_Table
-- @param figure The pandoc.Figure element.
-- @return (nil | pandoc.Figure). New pandoc.Figure element if identifier was found, else nil.
function figureParser(figure)
    -- only exchange caption if output format is 'html' -> LaTeX automatically adds figure-tags like "Figure 1:" to captions
    if figure.identifier then
        -- add identifier to global id-collection (used to find in-text refs)
        table.insert(Figref_Table, Counter, figure.identifier)
        -- change figure caption in AST: add "Figure <NO>: " to caption
        if FORMAT:match "html" then 
            figure.caption.long = figure.caption.long:walk {
                Plain=addFigureTagToCaption
            }
        end
        -- increase global Counter
        Counter = Counter + 1
        return figure
    else
        return nil
    end
end

--- Function to replace the in-text figure reference (such as @fig:fig1), which is wrapped in a pandoc.Cite element, with the matching link syntax of the output format (HTML/LaTeX).
-- @param word The pandoc.Cite element.
-- @return (nil|pandoc.RawInline) pandoc.RawInline if reference was found. Else nil.
function setRefsToFigure(cite)
    return stringReplacementRef(cite.citations[1].id)
end

return {
    {Figure=figureParser},
    {Cite=setRefsToFigure}
  }