[SOLVED] Lua conditional evaluation of new language operators

Lua conditional evaluation of new language operators

I'm writing a Wireshark dissector in Lua for a custom packet type and Wireshark versions including Lua 5.2 and below (current release versions) have the BitOp library included, whereas upcoming releases will have Lua 5.3 or greater and will use builtin bitwise operations.

My goal is to have this changeover automatically handled by Lua version without having to update the dissector for each version. My current code is:

local bit = nil
if tonumber(string.sub(_VERSION, 5)) < 5.3 then
    bit = require("bit")
end

local version_byte = 0xaf
if bit then
    print("BitOp method")
else
    print((version_byte >> 4) * 10 + (version_byte & 0xf))
end

The issue is that Lua 5.2 (of course) doesn't recognize the bitwise shift and & operators and throws unexpected symbol near '>'. Is there a way to conditionally evaluate something like this?

Solution

This is conditional compilation. Of course, you can.

if bit then
    print("BitOp method")
else
    load[[
        -- arbitrary chunk of code
        print((version_byte >> 4) * 10 + (version_byte & 0xf))
    ]]()
end

However, this will cause problems with access to locals (e.g. local version_byte). So you'll have to rewrite your code accordingly.