pandoc_insert.lua

local function length(elem)
    return pandoc.text.len(pandoc.utils.stringify(elem))
end

function insert(doc, inserted, idx, before)
    idx = idx or -1
    local total_length = length(doc)
    if(idx < 0) then
        idx = idx + 1 + total_length
    end
    if(idx < 1 or idx > total_length) then
        error("Index out of bounds")
    end

    local function insert_string(string, inserted, idx, before)
        if(before) then idx = idx - 1 end
        local prefix = pandoc.text.sub(string, 1, idx)
        local suffix = pandoc.text.sub(string, idx + 1)
        return prefix..inserted..suffix
    end

    local function insert_inlines(elem, inserted, before)
        elem = pandoc.Inlines(elem)
        inserted = pandoc.Inlines(inserted)
        if(before) then
            return inserted..elem
        else
            return elem..inserted
        end
    end

    local function inline(elem)
        if(idx == 0) then
            return nil, false
        end

        local elem_length = length(elem)
        if(elem_length < idx) then
            idx = idx - elem_length
            return nil, false
        end

        if(elem.t == "Str") then
            elem = insert_string(elem.text, inserted, idx, before)
        elseif(elem.t == "Code" or elem.t == "Math") then
            elem.text = insert_string(elem.text, inserted, idx, before)
        elseif(elem.t == "Space" or elem.t == "LineBreak" or elem.t == "SoftBreak")
                or (elem.t == "RawInline" and elem.format == "html" and string.find(elem.text, "^<br")) then
            elem = insert_inlines(elem, inserted, before)
        elseif(elem.t == "Quoted" and (elem.quotetype == "SingleQuote" or elem.quotetype == "DoubleQuote")) then
            if((idx == 1 and before) or (idx == length and not before)) then
                elem = insert_inlines(elem, inserted, before)
            elseif(idx == 1 or idx == length) then
                elem.content = insert_inlines(elem.content, inserted, not before)
            else
                idx = idx - 1
                return nil, true
            end
        else
            return nil, true
        end
        idx = 0
        return elem, false
    end

    return doc:walk({traverse = 'topdown', Inline = inline})
end

function testInsert(markup, inserted, idx, before, expected)
    local doc = pandoc.read(markup, "markdown")
    doc = insert(doc, inserted, idx, before)
    local actual = pandoc.write(doc, "markdown")
    actual = string.gsub(actual, "\n$", "")
    if(actual == expected) then
        print("Test successful")
    else
        print(">>>>>>>>>>>>>>>>>>>>>>>>>>>>")
        print("Test failed!")
        print("Invalid result of inserting '"..inserted.."' "
            ..(before and "before" or "after").." index "..idx.." into:")
        print(markup)
        print("Actual:")
        print(actual)
        print("Expected:")
        print(expected)
        print(pandoc.json.encode(doc))
        print("<<<<<<<<<<<<<<<<<<<<<<<<<<<<")
    end
end

-- Str
testInsert("abc", "X", 2, true, "aXbc")
testInsert("abc", "X", 2, false, "abXc")
-- Code
testInsert("`abc`", "X", 2, true, "`aXbc`")
testInsert("`abc`", "X", 2, false, "`abXc`")
-- Math
testInsert("$abc$", "X", 2, true, "$aXbc$")
testInsert("$abc$", "X", 2, false, "$abXc$")
-- Space
testInsert("*a* *b*", "X", 2, true, "*a*X *b*")
testInsert("*a* *b*", "X", 2, false, "*a* X*b*")
-- LineBreak
testInsert("*a*\\\n*b*", "X", 2, true, "*a*X\\\n*b*")
testInsert("*a*\\\n*b*", "X", 2, false, "*a*\\\nX*b*")
-- SoftBreak
testInsert("*a*\n*b*", "X", 2, true, "*a*X *b*")
testInsert("*a*\n*b*", "X", 2, false, "*a* X*b*")
-- RawInline
testInsert("*a*`<br>`{=html}*b*", "X", 2, true, "*a*X`<br>`{=html}*b*")
testInsert("*a*`<br>`{=html}*b*", "X", 2, false, "*a*`<br>`{=html}X*b*")
-- SingleQuote
testInsert("*a*'*b*'*c*", "X", 2, true, "*a*X'*b*'*c*")
testInsert("*a*'*b*'*c*", "X", 2, false, "*a*'X*b*'*c*")
testInsert("*a*'*b*'*c*", "X", 3, true, "*a*'*Xb*'*c*")
testInsert("*a*'*b*'*c*", "X", 3, false, "*a*'*bX*'*c*")
testInsert("*a*'*b*'*c*", "X", 4, true, "*a*'*b*X'*c*")
testInsert("*a*'*b*'*c*", "X", 4, false, "*a*'*b*'X*c*")
-- DoubleQuote
testInsert('*a*"*b*"*c*', "X", 2, true, '*a*X"*b*"*c*')
testInsert('*a*"*b*"*c*', "X", 2, false, '*a*"X*b*"*c*')
testInsert('*a*"*b*"*c*', "X", 3, true, '*a*"*Xb*"*c*')
testInsert('*a*"*b*"*c*', "X", 3, false, '*a*"*bX*"*c*')
testInsert('*a*"*b*"*c*', "X", 4, true, '*a*"*b*X"*c*')
testInsert('*a*"*b*"*c*', "X", 4, false, '*a*"*b*"X*c*')