Prevent placing blocks inside a block

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#1

So I got a block placement system but one problem is that you can place a block inside a block.

local function canPlaceBlocks()
	local characterPrimary = character.PrimaryPart
	local rayOrigin = characterPrimary.Position
	local snapValue = 3 -- Grid size

	local rayDirection = (characterPrimary.CFrame.LookVector - characterPrimary.CFrame.UpVector * placeBlockAngle).Unit * 8
	local raycastResult = workspace:Raycast(rayOrigin, rayDirection)

	if raycastResult then
		local hitPosition = raycastResult.Position
		local snapPosition = Vector3.new(
			math.floor(hitPosition.X / snapValue + 0.5) * snapValue,
			math.floor(hitPosition.Y / snapValue + 0.5) * snapValue,
			math.floor(hitPosition.Z / snapValue + 0.5) * snapValue
		)
		placeBlock(snapPosition)
	end
end
#2

If the blocks are always going to be 3 x 3 x 3 studs in size, then you could check before placing the block if it will have the same position as any other block that has already been placed. Because you have a set grid size of 3 studs, then, given that the blocks have a size of 3 x 3 x 3 studs, blocks that intersect would overlap exactly.