Swizzling in Julia: Basic Mechanism

julia_swizzling_hello_world.jl

# Basic mechanism for how to implement GLSL-like swizzling in Julia

mutable struct Vec2
    _private_00::Float32
    _private_01::Float32
    Vec2(x, y) = new(x, y)
end

function Base.getproperty(instance::Vec2, sym::Symbol)
		
    # within this function we still somehow need to get the values of instances field. Using regular Julia syntax `.` 
    # would cause a stackoverflow error as `.` calls `getproperty` which is this function
    # instead, we use the Julia C-Library to access the values 
    # at that point we simply need to parse the `sym` argument and return the correct value permutation
    # this function is currently not type stable but with some more effort I feel like this is doable for 
    # all the glm vectors types without sacrificing performance at all
    
    if sym == :_private_00 || sym == :_private_00
    	# prevent users from messing with actual values
        throw(ErrorException("type " * string(Vec2) * " has no field " * string(sym)))
    else
        x_ptr = ccall(:jl_get_nth_field, Ptr{Cvoid}, (Ptr{Cvoid}, Cint), pointer_from_objref(instance), 0)
        y_ptr = ccall(:jl_get_nth_field, Ptr{Cvoid}, (Ptr{Cvoid}, Cint), pointer_from_objref(instance), 1)
        x = ccall(:jl_unbox_float32, Float32, (Ptr{Cvoid},), x_ptr)
        y = ccall(:jl_unbox_float32, Float32, (Ptr{Cvoid},), y_ptr)
        
        if sym == :x return x
        elseif sym == :y return y
        elseif sym == :xy return Vec2(x, y)
        elseif sym == :yx return Vec2(y, x)
        elseif sym == :xx return Vec2(x, x)
        elseif sym == :yy return Vec2(y, y)
        else 
           # simulate the regular behavior of a non-overloaded Base.getproperty
           throw(ErrorException("type " * string(Vec2) * " has no field " * string(sym)))
        end
    end		
end

# usage
test = Vec2(1, 123)
println(test.yx)
# prints Vec2(123.0f0, 1.0f0)