Lua x C

Workshop's Difficulty
▰▰▰▱ Hard 🔥

Important

This workshop is about the lua programming language. A little lua knowledge is requiered to do this workshop.
You will also need C or C++ knowledges.

If you never touched lua in your life, you should follow the negative tasks before. Else, go directly to ground zero;

Note

This part of lua are advanced mechanics, so feel free to ask me if you need help !

Negative tasks

Click here !

-1 | A lua world

Lua is an interpretted language created in 1993. It kinda looks like Ruby.

Here is what creating a variable looks like:

local num = 3
local str = "hello"

Lua is not strict typed, however, it is not recommanded to change types and you can get a warning.

local a = 42
a = "foo" -- warning

Without the local keyword, your variable is global:

a = 42

And now you also learnt above how to do a comment !

Lua have 8 (or 9 it depends how you count) types:

• nil
  local a = nil; local a;
• number (and integer)
  local a = 42; local a = 3.14
• string
  local a = "hello"
• boolean
  local a = true
• thread
  local a = coroutine.create(...)
• function
  function a() ... end
• table
  local a = { ... }
• userdata
  can't be created, it's a C pointer

So normally you should understand the types from nil to thread.

The bottom ones are the types we are mostly gonna use in this workshop.

However, we have to see more basics before.

-2 | Basic lua knowledge

Now we are gonna see some lua basics.

Here is three ways to do looping in lua:

local a = 0
while a < 10 do
    print(a)
    a = a + 1
end

for b = 0, 10 do
    print(b)
end

local c = 0
repeat
    print(c)
    c = c + 1
until c >= 10

Here is how to do conditions:

if you == "league player" then
    print("stinky")
elseif you == "at epitech" then
    print("stinky too")
else
    print("sticky again")
end

The if statements checks for a non-nil and not false value.

if nil then -- wont do
    ... 
end
if 1 == 2 then -- 1 == 2 is false, so wont do
    ...
end

-3 | or functions, and

And here is a function (example for factorial recursive):

local function factorial(n)
    if (n == 1) or (n == 0) then
        return 1
    end
    return n * factorial(n)
end

Lua can have ; but it is for psychopaths. You can also write lua on a single line, but that's also for psychopaths.

function f(n)if n<=0 then return 1 end return n*f(n-1)end

And yes this works.

You can also do ternary using only or and and. These opperators are really usefull since its not like C.

Here, the or will chose the first non-nil and non-false value, else it takes the last one:

false or 3 -- 3
42 or 53 -- 42
nil or false -- false
false or nil -- nil

and will do the opposite and return the first nil or false value he found, else it returns the last value:

false and 3 -- false
42 and 53 -- 53
nil and false -- nil
false and nil -- false

So using this, you can do ternary like that:

(condition and first) or second

Now we can improve our psychopath's factorial:

f=function(n)return n<=0 and 1 or n*f(n-1)end

-4 | Tables and chairs

Tables are the most important mechanics in lua. It allows user to store any value at a given key. The key can be litterally anything.

To create a table, you just need to use {}. By default, if you don't give a key, it acts like an array, and the key are 1, 2, 3...

local fruits = { "banana", "apple", "mango" }
print(fruits[1]) -- banana (not apple)

If your key is a string, you can set/get it with a dot. By default, it's set as nil:

local me = {
    money = 0,
    bitches = 0,
    int32limit = 2147483647
}
print(me.bitches) -- 0
print(me.charism) -- nil
me.charism = 0
print(me.charism) -- 0
print(me["charism"]) -- 0, its the same

You can also loop through the tables using the for loop:

for key, value in pairs(me) do
    print(key, value) -- money 0, bitches 0, ...
end

Tables can have functions in it, which is interesting in OOP mechanics.

So to call them, you can just call them with ., but if you need to actually get the table used to call the function, you can use : and you will get the self variable.

local me = {
    age = 19,
    name = "pol"
}

function me.sayHi()
    print("Hi")
end

function me:birthday()
    self.age = self.age + 1
end

me.sayHi() -- Hi
print(me.age) -- 19
me:birthday()
print(me.age) -- 20

You can also use something that is called metatables to actually give tables funny attributes. But I let you search by your own.

Finally ! Now you can start the workshop !

0 | Prerequisites met

1. Install lua using the following table:

	installation command
dnf	sudo dnf install lua-devel -y
apt	sudo apt install liblua5.4-dev
pacman	sudo pacman -Syu lua
brew	brew install lua@5.4

2. Export the lua cpath like so

export LUA_CPATH="$(pwd)/?.so;;"

3. Write your code. A starter function should look like this:

#include <lua.h>
#include <lauxlib.h>
#include <lualib.h>

int luaopen_coolmodule(lua_State *L) // 'coolmodule' is the name of your module !
{
    // your code
    return 0; // this number is for the number of variables you return
}

4. Compile the lua using the following table:

	compilation command
apt	gcc -shared -fpic module.c -llua5.4 -lm -o module.so
brew	gcc -I/opt/homebrew/Cellar/lua/5.4.6/include/lua -bundle -undefined dynamic_lookup module.c -o module.so
others	gcc -shared -fpic module.c -llua -lm -o module.so

Important

This workshop is made with 2 STEPS. It's hard to do both, so you can chose which one you want to do first:

• C in Lua
• Lua in C

Step 1 - C in your Lua

1 | My First Module

${\textsf{\color{#f00}╸}}$━━━━━━━━━━━━━━━━━━━ 0xp

See this little bar ? This is your lua skills that are upgrading ! Each steps

1. You will first have to create a module that returns a number:

local foo = require("coolmodule")

print(foo) -- 42

2. You will then return a little C function you can call in lua:

local foo = require("coolmodule")

foo() -- Hello world !

3. Perfect ! Now return a table

Note

(if you don't know what a table is, you should definitely look back at negative tasks)

You table should contains 1 little method and 1 little variable:

local foo = require("coolmodule")
print(foo.number) -- 42
foo:get() -- My number is 42
foo.number = 13
foo:get() -- My number is 13

2 | Your own adder !

${\textsf{\color{#c04}━╸}}$━━━━━━━━━━━━━━━━━━ 10xp

Hey see your lua skills ! They upgraded ! Now let's create a simple adder.

So your goal is to create a adder, which is how I want to store my number in the super game i'm developping.

I should be able to increment my variable stored in my adder object. This is how you would do it !

1. Create a new userdata type:

local adder = require("coolmodule")

print(type(adder)) -- userdata

2. Make your userdata store a int variable and display it:

adder:display() -- 0

Tip

Did you know you can apply a metatable to a light user data ? It will then act like a table, but self will be you pointer.

The __index metamethod is definetely usefull !

3. Make a function that increments your adder value:

adder:display() -- 0
adder:increment()
adder:display() -- 1

4. Make your function take an optionnal parametter that is the number it can holds:

adder:display()     -- 0
adder:increment()
adder:display()     -- 1
adder:increment(3)
adder:display()     -- 4

Tip

Did you know that passing 0 parametters is like giving nil as parametter ?

3 | An Adder with a capital 'A'

${\textsf{\color{#A2A}━━━━━╸}}$━━━━━━━━━━━━━━ 50xp

Wow, ok you are definetely getting great at lua, however, would you pass this one ?

Ok so I want to have multiple and individuals Adder in my game, so can you make me a Adder class that allows me to create multiple adders !

local Adder = require("coolmodule")

local adder1 = Adder.new()
adder1:increment(42)

local adder2 = Adder.new()
adder2:increment(69)

adder2:display() -- 69
adder1:display() -- 42

Hmm ok this looks kind of what I want, can you now (please) make it printable ?

local adder = require("coolmodule").new()
adder:increment(12)
print(adder) -- 12

Tip

I heard about a super metamethod called __tostring, it's an absolute banger what can it do, I think you should check for it 👀

4 | My Window

${\textsf{\color{#62f}━━━━━━━╸}}$━━━━━━━━━━━━ 70xp

Wait, I just realised something... it's useless to do an adder, since you already have the lua variables to do it !

Instead I will do something funnier !

Do you remember you Tek1 bsq/setting up ? Do you still have it ? Perfect! Because we are not gonna use it.

Instead, you will encapsulate any graphical C/C++ library of your choice.

Don't worry you don't have to create everything, just 2 simple things:

1. A createWindow method to create a new window, which takes the title and the size.

2. An implicite destructor that clears your window, which is called at the end of the file.

Tip

Destructor ? Does it exist a metamethod for destruction in lua ?

Walkthrough:

1. You should create a simple function that returns a new light user data.

2. Then you can set the userdata to a new window.

3. Then you can try to apply metamethod to the userdata with the destructor.

4. And then you are done !

Here is a code snippet that you should base your work on:

local Graphics = require("coolmodule")

local window = Graphics.createWindow("foo", 800, 600)
print(type(window))
os.execute("sleep 3")
print("foo")

It should outputs:

userdata
// waits for 3 seconds
foo
destructor

Here is a little video

Step 2 - Lua in your C

5 | The role changed...

${\textsf{\color{#08f}━━━━━━━━━━━━╸}}$━━━━━━━━━━ 120xp

OK so you learnt how to do C in your lua, (or not) but do you know you can do the opposite ?

It's like a huge bonus to include some lua in your code, like in RPG or Minishell. And when you will see how to do that, you will be surprised to see how it is simple.

Now here is your starter C code:

#include <lua.h>
#include <lauxlib.h>
#include <lualib.h>

int main(void)
{
    return 0;
}

Then compile it like this:

$ gcc foo.c -llua -lm

Walkthrough

1. First, you will create a new lua_State *.
   Don't forget to close it at the end, and open the libs in your state !

Tip

The name of the functions you will have to use are really intuitive.
There are 3 functions to use in the first exercise, and don't forget to check lua_ AND luaL_ functions.

2. Now that you have you lua state, you will have to use it to do execute lua code. Find which function to use to execute the following code in your state:

print(1 + 1)

It should output 2 in the standard output.

6 | Closuring and Tableing

${\textsf{\color{#0A8}━━━━━━━━━━━━━━━╸}}$━━━━━━━ 150xp

OK so you learnt how to create state and execute lua inside, that's great !

Now let's understand more how it works by pushing stuff in your stack:

1. So first, you will have to push a string in your stack and get it back.

The following code should work fine (of course with the right functions/code):

int main(void)
{
    char const *src = "hello !";
    /* use a code to push your 'src' string in your state. */;

    char const *dst = /* use a code to get the string from the first state variable. */;
    printf("%s\n", dst);
}

Should output "hello !".

3. So you will have to push an integer in your stack and then get it back, but as a number (not an integer).

4. Now you will have to push a c function in the stack, then execute it. The function will just output ducks everywhere !.

Just a hint for you, lua C functions works like this:

int myfunc(lua_State *L)
{
    // your code
    return 0; // the number of variable you return.
}

5. Ok so you have a function, now create a function that will add two number togeter, and return the result.

Your C function needs to have the same behavior as this one:

local function add(a, b)
    return a + b
end

6. Whoa well done for the functions, I'm glad you're here so far ! OK OK last one for the tables and then its another task

Push a table in your lua state with 3 fields inside, like this one:

local t = {
  cat = "hoppy",
  number = 42,
  ape = "Alexandre"
}

It needs to have the same keys and same values.

If you have finished this exercise, you should see me to show me your code.

7 | Do you wanna build a snowman ?

${\textsf{\color{#0C4}━━━━━━━━━━━━━━━━━━╸}}$━━━━ 180xp

To be honnest I don't want either Elza, I just said this so I can steal your quote to write a workshop.

Anyways, you will have to create a snowman in your lua state.

1. My snowman is a light user data.

2. It is also really cold so when I print my snowman it should display A really cold snowman.

Tip

If you skipped the preceding tasks, know that you will need to use metatables for this task.
Go see what it is and how to use it, but you should watch one in particulary named __tostring.

3. My snowman is also a global variable, so i can access it everywhere. By the end, I should be able to run print(snowman) and it should always say A really cold snowman..