Documenting the CollisionGroupData property format

[phoriah]

If you wish to document the format behind the CollisionGroupData property - here's luau code that generates CollisionGroupData-like output using buffers.

CollisionGroupData = function()
	local collision_groups = game:GetService("PhysicsService"):GetRegisteredCollisionGroups()

	local col_groups_n = #collision_groups

	if col_groups_n == 0 then
		return "\1\0"
	end

	local buffer_size = 3 -- Initial size

	for _, group in collision_groups do
		buffer_size += 7 + #group.name
	end

	buffer_size -= 1 -- Except Default group

	local b = buffer.create(buffer_size)

	local offset = 0

	buffer.writeu8(b, offset, 1) -- ? [CONSTANT] Version byte (likely)
	offset += 1
	buffer.writeu16(b, offset, col_groups_n * 10) -- Group count (not sure about u16, it could be u8, because groups are limited to 32 afaik, and there could be another u8 as a 0 constant after it instead)
	offset += 2

	for i, group in collision_groups do
		local name, id, mask = group.name, i - 1, group.mask
		local nameLength = #name

		if id ~= 0 then
			buffer.writeu8(b, offset, id) -- ID
			offset += 1
		end

		buffer.writeu8(b, offset, 4) -- ? [CONSTANT] Not sure what this is (also not sure about u8, could be i8)
		offset += 1

		buffer.writei32(b, offset, mask) -- Mask value as signed 32-bit integer
		offset += 4

		-- It's weird that string length is u8 and not u32 like it usually is
		buffer.writeu8(b, offset, nameLength) -- Name length
		offset += 1
		buffer.writestring(b, offset, name) -- Name
		offset += nameLength
	end

	return buffer.tostring(b)
end

Hopefully it's enough. Feel free to correct the code if it's wrong!