Adding test case and fix for #26

[gnois]

Added test case for gnois/luajit-lang-toolkit#2

For #26, the string.format("%q", ...) seems not able to print escapes such as \n, \r verbatim no matter how I try.
Anyhow I am not sure if the fix is the best way.

[franko]

Sorry, I was not clear. I mean:

function escape(val)
    local q = string.format("%q", val)
    return string.gsub(q, "\\\n", "\\n")
end

As for the implementation differences of "%q" I don't think that's a real issue. For me luajit's "%q" implementation is fine, except the problem with "\n" you was pointing out.
Re-implementing such a function in Lua can be very costly and should be avoided IMHO.

[gnois]

What about other escapes like \r \f \t \\ \' \" ?

[franko]

Ok, I see your point. I think it is ok to merge your proposition.

Now I'm just wondering why in your "escape" function ' and " are not in the list of all the other escape sequences. You are calling gsub for them outside of the for loop but I don't see why they are not treated like the others, in the for loop.

[franko]

Ok, I found something cool with just one gsub call:

local function replace(c)
    local esc = {
        ['\a'] = [[\a]], ['\b'] = [[\b]], ['\f'] = [[\f]], ['\n'] = [[\n]], ['\r'] = [[\r]], ['\t'] = [[\t]], ['\v'] = [[\v]], ['\''] = [[\']], ['\"'] = [[\"]]
    }
    return esc[c] and esc[c] or ('\\' .. string.format("%d", string.byte(c)))
end

local function escape(s)
    return string.gsub(s, "%c", replace)
end

I didn't make any measurement but I think it should be more fast to execute. With this function the string is scanned just one time and no temporary strings are created except the bit to represent a char in decimal notation.

[gnois]

Yeah, that sounds better! I am not aware that there's a pattern that match control chars. :)
Allow me to add some changes still, because the "%c" does not catch \\ \" which are non-controls.

local function replace(c)
    local esc = {
        ['\\'] = [[\\]], ['"'] = [[\"]], ['\a'] = [[\a]], ['\b'] = [[\b]], ['\f'] = [[\f]], ['\n'] = [[\n]], ['\r'] = [[\r]], ['\t'] = [[\t]], ['\v'] = [[\v]]
    }
    return esc[c] or string.format("\\%d", string.byte(c))
end

local function escape(s)
    -- \' is not needed since we quote with \"
    return string.gsub(s, '[%c\\"]', replace)
end

Without the change, the following would not pass.

print("\\\n\r\"'\'\0")
print('\v\\\t\"\'')

Forget about my previous implementation. The \\ is out of the loop because I think it has to be replaced earlier than the control chars, as pairs() does not observe ordering. The \' and \" are outside without good reason, other than they are punctuations like \\, which is non sense.

[franko]

Ok, I found a good solution and I made the change directly in the master branch. Here the commit 738f577.

[gnois]

Alright, thanks.