Emulate XeTeX font primitives in LuaTeX

test.tex

\ifdefined\directlua
  \input XeTeXemulate.sty
  \input luaotfload.sty
  \XeTeXemulate
\fi

\font\1="[lmroman10-italic.otf]" at 10pt\1
\font\2=cmr10 at 10pt

\edef\x{\the\XeTeXfonttype\1}“\x”
\edef\x{\the\XeTeXfonttype\2}“\x”

\edef\x{\the\XeTeXversion}“\x”
\edef\x{\XeTeXrevision}“\x”

\edef\x{\char\XeTeXfirstfontchar\1}“\x”
\edef\x{\char\XeTeXfirstfontchar\2}“\x”

\edef\x{\char\XeTeXlastfontchar\1}“\x”
\edef\x{\char\XeTeXlastfontchar\2}“\x”

\edef\x{\XeTeXglyph150}“\x”

\edef\x{\the\XeTeXcountglyphs\1}“\x”
\edef\x{\the\XeTeXcountglyphs\2}“\x”

\edef\x{\XeTeXglyphname\1 150}“\x”

\edef\x{\the\XeTeXglyphindex"acircumflexgrave"\relax}“\x”
\edef\x{\the\XeTeXglyphindex"acircumflexgrave" }“\x”

\edef\x{\the\XeTeXcharglyph"00A5}“\x”

\edef\x{\the\XeTeXglyphbounds 1 \XeTeXcharglyph`f}“\x”
\edef\x{\the\XeTeXglyphbounds 2 \XeTeXcharglyph`f}“\x”
\edef\x{\the\XeTeXglyphbounds 3 \XeTeXcharglyph`f}“\x”
\edef\x{\the\XeTeXglyphbounds 4 \XeTeXcharglyph`f}“\x”

\edef\x{[\the\XeTeXglyphindex yen ] x}“\x”
\edef\x{[\the\XeTeXglyphindex "yen" ] x}“\x”
\edef\x{[\the\XeTeXglyphindex"yen"\relax] x }“\x”
\edef\x{[\the\XeTeXglyphindex"yen"] x }“\x”
\edef\x{[\the\XeTeXglyphindex "yen" *2]}“\x”
\edef\x{[\the\XeTeXglyphindex "yen"\relax*2]}“\x”

\edef\x{[\XeTeXglyphindex"yen"] }“\meaning\x”
\edef\x{[\XeTeXglyphindex] }“\meaning\x”

\bye

XeTeXemulate.sty

\ifcsname ProvidesPackage\endcsname
  \NeedsTeXFormat{LaTeX2e}
  \ProvidesPackage{XeTeXemulate}[2018/09/03 v0.0 Emulate the XeTeX font primitives in LuaTeX]
\fi

\def\XeTeXemulate{%
  \chardef\XeTeXversion=0
  \def\XeTeXrevision{.99999}%
  % Circumvent \outer error
  \csname newcount\endcsname\XeTeXtracingfonts \XeTeXtracingfonts=0
  \csname newcount\endcsname\XeTeXuseglyphmetrics \XeTeXuseglyphmetrics=1
  %
  \let\XeTeXmathcode\Umathcode
  % 
  \protected\def\XeTeXfonttype{%
    \directlua{
      local fid = token.scan_int()
      local tfmdata = font.getfont(fid)
      local fonttype = tfmdata.format == "opentype" and 2 or 0
      tex.print([[\numexpr]] .. fonttype .. [[\relax]])
    }\fontid
  }%
  %
  \protected\def\XeTeXfirstfontchar{%
    \directlua{
      local fid = token.scan_int()
      local tfmdata = font.getfont(fid)
      local min
      for slot in pairs(tfmdata.characters) do
          min = min and (slot < min and slot or min) or slot
      end
      tex.print([[\numexpr]] .. min .. [[\relax]])
    }\fontid
  }%
  %
  \protected\def\XeTeXlastfontchar{%
    \directlua{
      local fid = token.scan_int()
      local tfmdata = font.getfont(fid)
      local max
      for slot in pairs(tfmdata.characters) do
          max = max and (slot > max and slot < 2^16 and slot or max) or slot
      end
      tex.print([[\numexpr]] .. max .. [[\relax]])
    }\fontid
  }%
  %
  \def\XeTeXglyph{%
    \directlua{
      local index = token.scan_int()
      local tfmdata = font.getfont(font.current())
      if tfmdata.format \string~= "opentype" then
          tex.error([[Cannot use \string\XeTeXglyph]] .. [[ with ]] ..
                    tfmdata.name .. [[; not a native platform font]])
      end
      for slot, char in pairs(tfmdata.characters) do
          if char.index == index then
              tex.print(utf.char(char.unicode))
              return
          end
      end
    }}%
  %
  \protected\def\XeTeXcountglyphs{%
    \directlua{
      local fid = token.scan_int()
      local tfmdata = font.getfont(fid)
      local count = 0
      if tfmdata.format == "opentype" then
          for _ in pairs(tfmdata.characters) do
              count = count + 1
          end
      end
      tex.print([[\numexpr]] .. count .. [[\relax]])
    }\fontid
  }%
  %
  \def\XeTeXglyphname{%
    \directlua{
      local fid = token.scan_int()
      local index = token.scan_int()
      local tfmdata = font.getfont(fid)
      if tfmdata.format \string~= "opentype" then
          tex.error([[Cannot use \string\XeTeXglyphname]] .. [[ with ]] ..
                    tfmdata.name .. [[; not a native platform font]])
      end
      local count = 0
      for slot, char in pairs(tfmdata.shared.rawdata.descriptions) do
          if char.index == index then
              tex.print(char.name)
              return
          end
      end
    }\fontid
  }%
  %
  \protected\def\XeTeXglyphindex{%
    \directlua{
      local k = string.gsub(token.scan_string(),'"','')
      token.scan_keyword(' ') % remove optional space
      local tfmdata = font.getfont(font.current())
      if tfmdata.format \string~= "opentype" then
          tex.error([[Cannot use \string\XeTeXglyphindex]] .. [[ with ]] ..
                    tfmdata.name .. [[; not a native platform font]])
      end
      local index = 0
      for slot, char in pairs(tfmdata.shared.rawdata.descriptions) do
          if char.name == k then
              index = char.index
              break
          end
      end
      tex.print([[\numexpr]] .. index .. [[\relax]])
    }}%
  %
  \protected\def\XeTeXcharglyph{%
    \directlua{
      local id = token.scan_int()
      local tfmdata = font.getfont(font.current())
      if tfmdata.format \string~= "opentype" then
          tex.error([[Cannot use \string\XeTeXcharglyph]] .. [[ with ]] ..
                    tfmdata.name .. [[; not a native platform font]])
      end
      local index = tfmdata.characters[id].index
      tex.print([[\numexpr]] .. index .. [[\relax]])
    }}%
  %
  \protected\def\XeTeXglyphbounds{%
    \directlua{
      local edge = token.scan_int()
      local slot = token.scan_int()
      local tfmdata = font.getfont(font.current())
      if tfmdata.format \string~= "opentype" then
          tex.error([[Cannot use \string\XeTeXglyphname]] .. [[ with ]] ..
                    tfmdata.name .. [[; not a native platform font]])
      end
      for _,char in pairs(tfmdata.shared.rawdata.descriptions) do
          if char and char.index == slot then
              local bbox = char.boundingbox
              local dimen = 0
              if edge == 1 then
                  dimen = bbox and bbox[1] / 100 * 2^16 or 0
              elseif edge == 2 then
                  dimen = tfmdata.characters[char.unicode].height
              elseif edge == 3 then
                  dimen = bbox and (char.width - bbox[3]) / 100 * 2^16 or 0
              elseif edge == 4 then
                  dimen = tfmdata.characters[char.unicode].depth
              end
              tex.print([[\dimexpr]] .. tex.round(dimen) .. [[sp\relax]])
              break
          end
      end
    }}%
}

\endinput

currently

\typeout{YEN: \the\XeTeXglyphindex"yen" }

produces

YEN: 638\ignorespaces

in luatex as \ignorespaces isn't expandable, also it modifies the value of \count@ which may cause some surprises.

May I suggest

\documentclass{article}

\makeatletter
\ifx\directlua\@undefined\else

  \protected\def\XeTeXglyphindex"#1"{%
    \directlua{
      local tfmdata = font.getfont(font.current())
      if tfmdata.format \string~= "opentype" then
          tex.error([[Cannot use \string\XeTeXglyphindex]] .. [[ with ]] ..
                    tfmdata.name .. [[; not a native platform font]])
      end
      local index = 0
      for slot, char in pairs(tfmdata.shared.rawdata.descriptions) do
          if char.name == "\luaescapestring{#1}" then
              index = char.index
              break
          end
      end
      tex.print([[\numexpr(]] .. index .. ')')
    }}%
%

\fi

\begin{document}

\typeout{YEN 1: [\the\XeTeXglyphindex"yen" ] x}
\typeout{YEN 2: [\the\XeTeXglyphindex"yen"\relax] x }% a bit weird (so emulation drops \relax)
\typeout{YEN 3: [\the\XeTeXglyphindex"yen"] x }% too weird to contemplate emulating
\typeout{YEN 4: [\XeTeXglyphindex"yen"] }
\typeout{YEN 5: [\XeTeXglyphindex] }

\end{document}

It may be better to add the \relax in the tex.print to prevent following tokens being takes as the numexpr, I added (..) but that would still gobble *2 etc. Also the " in xetex are actually optional, this works

\typeout{YEN 1b: [\the\XeTeXglyphindex yen ] x}

better emulation using token scanner

\documentclass{article}

\makeatletter
\ifx\directlua\@undefined\else

  \protected\def\XeTeXglyphindex"#1"{%
    \directlua{
      local tfmdata = font.getfont(font.current())
      if tfmdata.format \string~= "opentype" then
          tex.error([[Cannot use \string\XeTeXglyphindex]] .. [[ with ]] ..
                    tfmdata.name .. [[; not a native platform font]])
      end
      local index = 0
      for slot, char in pairs(tfmdata.shared.rawdata.descriptions) do
          if char.name == "\luaescapestring{#1}" then
              index = char.index
              break
          end
      end
      tex.print([[\numexpr(]] .. index .. ')')
    }}%
%
  \protected\def\XeTeXglyphindex{%
    \directlua{
      local k = string.gsub(token.scan_string(),'"','')
      local sp = token.scan_keyword(' ')
      local tfmdata = font.getfont(font.current())
      if tfmdata.format \string~= "opentype" then
          tex.error([[Cannot use \string\XeTeXglyphindex]] .. [[ with ]] ..
                    tfmdata.name .. [[; not a native platform font]])
      end
      local index = 0
      for slot, char in pairs(tfmdata.shared.rawdata.descriptions) do
          if char.name == k then
              index = char.index
              break
          end
      end
      tex.print([[\numexpr]] .. index .. [[\relax]])
    }}%
%

\fi

\begin{document}

\typeout{YEN 0: [\the\XeTeXglyphindex yen ] x}
\typeout{YEN 1: [\the\XeTeXglyphindex "yen" ] x}
\typeout{YEN 2: [\the\XeTeXglyphindex"yen"\relax] x }
\typeout{YEN 3: [\the\XeTeXglyphindex"yen"] x }
\typeout{YEN 4: [\XeTeXglyphindex"yen"] }
\typeout{YEN 5: [\XeTeXglyphindex] }
\typeout{YEN 6: [\the\XeTeXglyphindex "yen" *2]}
\typeout{YEN 7: [\the\XeTeXglyphindex "yen"\relax*2]}
\end{document}

with xelatex

YEN 0: [638] x
YEN 1: [638] x
YEN 2: [638\relax ] x 
YEN 3: [0x 
YEN 4: [\XeTeXglyphindex "yen"] 
YEN 5: [\XeTeXglyphindex ] 
YEN 6: [638*2]
YEN 7: [638\relax *2]

with lualatex

YEN 0: [638] x
YEN 1: [638] x
YEN 2: [638\relax ] x 
YEN 3: [0x 
YEN 4: [\XeTeXglyphindex "yen"] 
YEN 5: [\XeTeXglyphindex ] 
YEN 6: [638*2]
YEN 7: [638\relax *2]

IMHO, it would be great to have this released ...

1. it seems it would be good to check for truetype as well as opentype in the tfmdata.format \string. at least, once I made that change, it all worked for me with a ttf.
2. it is possible to use ttf/otf which are not platform fonts, so the error message is misleading. also, it's helpful to show the actual string being tested.

I ended up with thefor \XeTeXglyph, used to typeset the visible glyph (a la Unicode font charts) for U+FE00 (variation selector-1). Using Base rendering mode was also necessary to stop the normal Unicode interpretation. I'll append my whole test document for the heck of it, but the relevant thing for your code is just the tweaks to the Lua condition and error string.

\documentclass{article}
\pagestyle{empty}
\usepackage{fontspec}

\ifx\undefined\XeTeXglyph
  \def\XeTeXglyph{%
    \directlua{
      local index = token.scan_int()
      local tfmdata = font.getfont(font.current())
      if tfmdata.format \string ~= "opentype"
         and tfmdata.format \string ~= "truetype" then
          tex.error([[Cannot use \string\XeTeXglyph]] .. [[ with ]] ..
                    tfmdata.name .. tfmdata.format \string ..
                    [[; not opentype or truetype]])
      end
      for slot, char in pairs(tfmdata.characters) do
          if char.index == index then
              tex.print(utf.char(char.unicode))
              return
          end
      end
    }}%
  %
\newfontfamily{\NSM}[Renderer=Base]{NotoSansManichaean-Regular.ttf}
\newcommand{\VSone}{{\NSM\XeTeXglyph58}} % FE00
\else
\newcommand{\VSone}{{\fontspec{NotoSansManichaean-Regular.ttf}\XeTeXglyph58}}
\fi
\begin{document}
\VSone
\end{document}

I'd actually rather reimplement the entire thing in pure Lua and expose the commands via token.set_lua. That makes them feel a lot more like actual primitives since they expand in a single step. That will need some expandafter magic for \fontid, but other than that should be straight-forward.

@kberry https://github.com/latex3/xxetex is in the works

Thanks!