Lua Confusing Table Reference

Question

I'm trying to understand what's going on in the following function but I'm having trouble tracing through it.

function whatsthis(str)
  local val = ""
  local vtable = "?\029#sMb5\rY\041>hS\005Bm~2\021\016\024\023xN\019gRK\\;p!\002jJ_\t4:-\000vLiA]I=6l\018ec\0248\030w%7\b\0151,\003\004Z)9`{*kP\028F}o\v\n.[EfV\006^\001nC<U\027WQ&\022(\fTD\026Guz/+aO'q3d|H\025 \ay$\"X\017\0200r@t"
  for i = 1, #str do
    local b = str:byte(i)
    if b > 0 and b <= 127 then
      val = val .. string.char(vtable:byte(b))
    else
      val = val .. string.char(b)
    end
  end
  return val
end

Part of the problem, I suppose, is that I've been given this exercise and I don't even know if vtable is a table or a string!

Example call:

string1 = whatsthis(",5MX")

I assume str:byte(i) converts ascii to bytes. ie. ",5MX" to 2c 35 4d 58 but I have no idea how it references vtable or why a variable that is already a byte needs to be re-cast in vtable:byte(b) again. It's clear ",5MX" references vtable through the intermediary byte but then I get lost and have no idea what gets returned to string1 or why.

To make matters worse....

string2 = whatsthis("\f4kp,X\026")

... is that parameter treated like a single string or two values, and how are the \ escapes handled in the function if not as just another character.

I hope I've been clear. I'm new to lua so I hope someone can shed some light here. Thanks in advance.

Piglet · Accepted Answer · 2021-09-14 15:04:27Z

2

Part of the problem, I suppose, is that I've been given this exercise and I don't even know if vtable is a table or a string!

You assign something in double quotes to vtable, so it must be a string value.

local vtable = "?\029#sMb5\rY\..."

See https://www.lua.org/manual/5.4/manual.html#3

str:byte(i) is syntactic sugar for string.byte(str, i)

See https://www.lua.org/manual/5.4/manual.html#3.4.10

string.byte(str, i) will return the internal numeric code for character number i in the string str. So it gives you the byte value that represents that character.

See https://www.lua.org/manual/5.4/manual.html#pdf-string.byte

I assume str:byte(i) converts ascii to bytes. ie. ",5MX" to 2c 35 4d 58

Yes.

This code will check wether a character is part of the ASCII standard. If that's the case it will map it to the vtable string.

If it is part of the extended ASCII table it will use it as is

Compare print(whatsthis("ö")) vs print(whatsthis("a"))

edited Sep 14, 2021 at 15:04

answered Sep 14, 2021 at 14:47

Piglet

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1 Comment

Bobby Over a year ago

Excellent. I'll play around with it. I'm sure it'll help. At least, I feel like I've got direction now. Thank you.

Paul Kulchenko · Accepted Answer · 2021-09-15 04:12:12Z

1

I don't even know if vtable is a table or a string!

vtable is always a string.

why a variable that is already a byte needs to be re-cast in vtable:byte(b) again

It's because it gets the numerical value of a character and then finds a character at a position with that index. On a quick glance it looks like it implements some sort of substitution cipher, where it replaces one character with a particular index (<=127) with a different one (taken from the table).

edited Sep 15, 2021 at 4:12

answered Sep 14, 2021 at 14:45

Paul Kulchenko

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1 Comment

Bobby Over a year ago

Good good. Yes, it's definitely cipher-like. Just string manipulation to get a better handle on an unfamiliar language. Thanks, mate.

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