User:Monster Iestyn/Lua

Lua is a scripting language that can be used by SRB2. It allows users to write custom game logic without editing the game's source code, in addition to many of the capabilities SOC also has. For these reasons, Lua scripting is considered a more powerful method of modification available to SRB2 compared to SOC. However, Lua does not supercede SOC completely – there are still a number of aspects of modding that can only be configured through SOC. A file created in the Lua language is commonly known as a "Lua script".

The name "Lua" itself is from the Portuguese for moon; it should not be referred to as "LUA", as it is not an acronym.

About

The version of Lua used by SRB2 is Lua 5.1. The reference manual for Lua 5.1 can be found here: [1]

For users new to the Lua language, it is highly recommended to read the Lua Tutorial on the lua-users wiki to learn the basics and syntax of Lua.

Changes from vanilla Lua

SRB2's implementation of Lua has a few changes from most implementations of the language. Some of the most important changes are listed and detailed below:

Numbers

While vanilla Lua uses floating-point doubles for all of its math, SRB2's implementation uses purely signed integers. Variables that require greater precision than whole numbers are usually dealt with in multiples of FRACUNIT to accommodate. Fixed math functions such as FixedMul() and FixedDiv() exist to assist with math in these situations.

Vanilla Lua can interpret nil as false in some situations; SRB2's implementation additionally allows 0 to be interpreted as the same thing.

Standard Libraries

The only standard libraries loaded are:

• The basic library (including the coroutine manipulation sub-library)
• The string manipulation library
• The table manipulation library

The remaining libraries have not been included due to safety concerns, or are redundant/unneeded for SRB2 Lua scripting purposes.

Bitwise operations

Lua in SRB2 supports all standard bitwise operations: AND, OR, XOR, NOT, and left/right bit-shifts. Note that, since Lua already uses the ^ (caret) symbol for exponentation, XOR is represented as two caret symbols in succession (^^).

Operation	Symbol(s)
AND	&
OR	|
XOR	^^
NOT	~ !
Left Shift	<<
Right Shift	>>

Alternative syntax

Lua in SRB2 supports alternative syntax for several operations, some of which exist to more closely resemble the syntax of the C programming language:

Syntax	Lua syntax	Alt syntax
Bitwise NOT	~	!
Not equals	~=	!=
Short comment	-- comment	// comment
Long comment	--[[ comment ]] --[=[ comment ]=] --[==[ comment ]==] etc	/* comment */
String concatenation	"string"..var.."string"	"string"+var+"string" "string\$var\string"

Pseudo-numbers

Pseudo-numbers are a new concept added to Lua in SRB2; these are used to simplify assigning new values to a variable (or variables) relatively to its old value. The old value for a variable is represented by the dollar symbol ($). An example of a pseudo-number in use is shown below:

// without pseudo-numbers
variable = variable + 1
// with pseudo-numbers
variable = $ + 1

This concept also extends to multiple variable assignment:

// without pseudo-numbers
var1, var2 = var1+5, var2-10
// with pseudo-numbers
var1, var2 = $+5, $-10

Note that, in the above example, $ is the value of var1 for var1's new value, and then var2 for var2's own new value.

Numbers can be placed after the $ symbol if needed, for identification of the individual variables – $1, $2, $3 and so on then represent the first, second, third and further variables in an assignment. This is particularly useful for the ability to swap the values of two variables, for example:

var1, var2 = $2, $1

Keywords

New keywords have been added, to support a more C-like syntax in Lua scripts:

• "continue" and "break N" keywords are supported in loops .
• do ... end blocks are converted into function() ... end blocks when passed as an argument or assigned as a variable.
• "do" after for and while, and "then" after if have been rendered optional.

String literals

Other types of string literals besides decimal code input (\nnn) have been added:

• Hexadecimal: \xnn or \xnnnn (2 or 4 digits must be used)
• Unicode (UTF-8-encoded): \unnnn or \unnnnnn (4 or 6 digits must be used)

Metatables

Several new metatable hook types have been added:

• __usedindex metatable hook, for when an existing index is assigned a new value.
• Bitwise operations:
  • __and (&)
  • __or (|)
  • __xor (^^)
  • __shl (<<)
  • __shr (>>)
  • __not (~)

Special characters

Certain ASCII characters have special effects for text printed using the functions print or CONS_Printf:

Chat text

Decimal	Hexadecimal	Usage
3	0x03	Plays a chat beep sound
4	0x04	Plays a chat beep sound and colors text yellow

Text colors

See also: SOC > Custom colors

Decimal	Hexadecimal	Usage
128	0x80	White/Reset
129	0x81	Purple
130	0x82	Yellow
131	0x83	Green
132	0x84	Blue
133	0x85	Red
134	0x86	Gray
135	0x87	Orange

Script execution

The most common way to load Lua scripts is to add them to a WAD file as a text lump. Lumps with the name prefix LUA_ are recognized as Lua scripts by SRB2 and are loaded automatically upon loading the WAD file, in the order that they are found in the file. Lua scripts with other names are not loaded automatically, and there is no method of loading them after the WAD file containing them has been added. It is important to note that all Lua scripts in a WAD file will be loaded before any SOC lumps also within the file, regardless of lump ordering. This means that any changes to the game in the WAD file's SOC lumps will not be recognised in the file's Lua scripts.

Alternatively, Lua scripts can be stored as standalone files with the file ending .lua. The console command ADDFILE can be used to load standalone Lua script files.

Global variables in SRB2

As well as the global variables that come with Lua (e.g.: _G and _VERSION), SRB2 allows access to a number of its own pre-defined global variables for use in Lua scripting.

Types of userdata in SRB2

SRB2 allows for userdata variables of many types in Lua scripts. The recognised types of userdata are listed below for reference – note that the type names have no actual relevance to Lua scripts themselves.

General

Type name	Accessibility	Allows custom variables	Description
mobj_t	Read + Write	Yes	This userdata type represents an Object. This is not to be confused with an Object type (e.g. MT_PLAYER) – these are just constants representing Object type numbers, and will be not be accepted in functions requiring mobj_t variables (e.g. P_SpawnGhostMobj(MT_PLAYER) will cause Lua to give an error).
player_t	Read + Write	Yes	This userdata type represents the properties of a player that are independent of the player Object, which may carry over between maps. This is not to be confused with mobj_t or vice versa.
ticcmd_t	Read + Write	No	This userdata type represents a player's current button information, i.e. what buttons are being pressed currently.
skin_t	Read-only	No	This userdata type represents a skin – i.e. the properties for a character set in the S_SKIN.

SOC

Type name	Accessibility	Allows custom variables	Description
mobjinfo_t	Read + Write	Yes	This userdata type represents the SOC properties of an Object type definition.
state_t	Read + Write	No	This userdata type represents the SOC properties of a State.
sfxinfo_t	Read + Write	No	This userdata type represents the SOC properties of a Sound.
hudinfo_t	Read + Write	No	This userdata type represents the SOC properties of a HUD item.

Map

Type name	Accessibility	Allows custom variables	Description
mapthing_t	Read + Write	No	This userdata type represents a Thing.
sector_t	Read + Write	No	This userdata type represents a sector.
subsector_t	Read-only	No	This userdata type represents a sub-sector.
line_t	Read-only	No	This userdata type represents a linedef.
side_t	Read-only	No	This userdata type represents a sidedef.
vertex_t	Read-only	No	This userdata type represents a vertex.
ffloor_t	Read + Write	No	This userdata type represents an FOF.

Miscellaneous

Type name	Accessibility	Allows custom variables	Description
mapheader_t	Read-only	SOC-only	This userdata type represents the properties of a level header.
camera_t	Read-only	No	This userdata type represents a player's camera.
consvar_t	Read-only	No	This userdata type represents a console variable.
patch_t	Read-only	No	This userdata type allows access to the properties of a graphics patch (see the HUD library).

See also

• Constants: A list of constants which can be used in Lua.