5.6 KiB
Vector library
Summary
Implement a standard library that provides functionality for the vector type.
Motivation
Currently, vectors are a primitive type implemented internally. All of the heavy work to implement vectors is done, but there is no runtime-agnostic way to use vectors in Luau code. Individual Luau runtimes, such as Lune or Roblox, must provide their own distinct libraries to work with native vectors which may be inconsistent. In cross-runtime code, this results in performance drawbacks & difficulty utilizing native vectors.
Design
The default metatable for vectors should now be the vector library. While buffers and coroutines do not have this, vectors likely should for ergonomics.
Library functions & constants
This RFC proposes the following basic functions & constants as a starting point, but as with all builtin libraries, future RFCs can propose additional functions.
vector(x: number?, y: number?, z: number?)
Creates a vector with 3 components: x, y, z. If the feature flag for wide vectors is enabled, a fourth argument w: number? will be introduced.
Due to the common usage of vectors, vector creation should be ergonomic. Therefore, it is probably worth breaking the create() naming standard.
vector.magnitude(vecA: vector): number
Calculates the magnitude of a given vector.
vector.normalized(vec: vector): vector
Returns the normalized version (aka unit vector) of a given vector. If a zero vector is passed, return zero. For further clarification, vector.normalized(vector(0, 0, 0)) should return zero.
vector.cross(vecA: vector, vecB: vector): vector
Returns the cross product of two vectors. If 4-wide vectors are enabled, this function will ignore the fourth component, and return the 3-dimensional cross product.
vector.dot(vecA: vector, vecB: vector): vector
Returns the dot product of two vectors.
vector.floor(vec: vector): vector
Equivalent of math.floor for vectors.
vector.ceil(vec: vector): vector
Equivalent of math.ceil for vectors.
vector.angle(vecA: vector, vecB: vector): vector
Returns the angle between two vectors.
vector.max(input: vector, maximum: vector): vector
Applies math.max component-wise for each vector. Equivalent of: vector(math.max(a.x, b.x), etc)
vector.min(input: vector, minimum: vector): vector
Same as vector.max. Equivalent of: vector(math.min(a.x, b.x), etc)
vector.zero
Vector where x=0, y=0, z=0, w?=0
vector.one
Vector where x=1, y=1, z=1, w?=1
Arithmetic operations
Primitive operators for vectors are already implemented, so this RFC doesn't concern vector arithmetic.
Native codegen
In the future, vectors will have special treatment in native codegen. This creates an expectation that the vector library will also have special treatment in codegen, but it isn't clear what that will look like, or if the performance benefits make sense.
Compiler options
Currently, there are 2 compiler options relating to vectors, vectorLib & vectorCtor. This poses an interesting problem: a builtin library would remove the requirement for such compiler options, however these options still need to be supported. The intuitive solution to this is to leave both compiler options working and maintained, but provide the built-in vector library by default, allowing two vector libraries and two vector constructors.
Typechecking
A new primitive vector type should be added to the type system.
Drawbacks
As per all additional globals, this creates a new global vector. This is a commonly used variable name, and many style guides will advise to avoid using vector as a variable name due to the global being added.
Introducing a vector library means introducing more globals. Due to the prescence of vectors in performance-intensive code, it can be assumed that the built-in functions will have fastcalls. This means more fastcall slots will be used.
Existing code won't get any improvements from a built-in library. Developers will have to switch over to the built-in library in order to gain any benefit.
Alternatives
Do nothing; vectors have an internal constructor, which lets the runtime implement vectors, and subsequently a library for vector math.
Alternative implementations
A more standard alternative to vector(...) could be something like vector.create or vector.new. This follows the standard set by buffer.create, coroutine.create, and table.create, and doesn't involve calling a table, which requires a metatable with __call to be set on the library. This breaks the standard previously set by Lua, as there is currently no library with a metatable set by default.
Another alternative to vector creation is special syntax for creating buffers, such as <x, y, z>, or [x, y, z]. This would require a lot more complexity & discussion, however it would fall more in line with the other primitive types.
Instead of vector.magnitude, the magnitude could be derived from the vector's coordinates themselves, and be accessed by a property instead of a function. There is a downside to this: all current properties of vectors are hard-coded to the VM, so any new property to vectors requires a lot of additional complexity & changes to the VM to allow for this. A library function, however, would be trivial. An easy and quick workaround to the verbosity would be at the runtime/C API level. It's trivial to set the metatable of vectors to be the vector library: this allows for vec:magnitude() without much issue.
Instead of ignoring the 4th dimension in vector.cross, the function could be disabled when four-dimensional vectors are enabled.