Changing the font for a script

Typically, fonts contain only a selection of scripts, and changing them automatically (as well as the writing direction) to make sure the document is properly typeset would be a nice feature. Here is a mock up.

It is based on the pre_linebreak_filter callback, which contains the horizontal list of glyphs, kerns, whatsits, etc., before building the paragraph. This callback is used by the font loader, too. It works with LaTeX 2016 and LuaTeX 0.95, but it should work with previous versions with minimal changes.

It is intended just for small chunks of text (a proper noun, a few words, etc.). For proper bidi writing, see typo-dha.lua, typo-dua.lua, typo-dub.lua and typo-duc.lua in context.

You may also want to readjust the vertical positioning (the baselines don't always match). This part resorts to a pdf literal and it's commented out.

\documentclass{article}

\directlua{
local DIR = node.id("dir")
local prevglyf = nil
function arabic_text (head)
  local isArab = false
  for item in node.traverse_id(node.id"glyph", head) do
    if item.char > 1536 and item.char < 1792 then
      item.font = arabicfont % Defined below
      if isArab == false then 
        isArab = true
        local d = node.new(DIR)
        d.dir = '+TRT'
        node.insert_before(head, item, d)
        % local r = node.new(node.id'whatsit', node.subtype'pdf_literal')
        % r.data = '2 Ts'
        % node.insert_before(head, item, r)   
      end
    else
      if isArab == true then
        isArab = false
        local d = node.new(DIR)
        d.dir = '-TRT'
        node.insert_before(head, prevglyf, d)
        % local r = node.new(node.id'whatsit', node.subtype'pdf_literal')
        % r.data = '0 Ts'
        % node.insert_before(head, item, r)   
      end
    end
    prevglyf = item % A bit dangerous, but usually OK.
  end
  return head
end
luatexbase.add_to_callback("pre_linebreak_filter", arabic_text, "Arabic text")
luatexbase.add_to_callback("hpack_filter", arabic_text, "Arabic text")
}

% The contextual analysis for arabic must be after the font
% is changed. If fontspec is loaded before, then the callbacks must be
% reorderer, which is feasible.

\usepackage{fontspec}
\setmainfont{Iwona}
\newfontfamily\amiri[Script=Arabic]{Amiri}

\begin{document}

{\amiri (نص قصير)\directlua{ arabicfont = font.current() }} Some text

Some text نص قصير Some text

\begin{tabular}{rr}
One & Two \\
Three & أربعة % :-/ Extra space
\end{tabular}

Some text.

\end{document}