Script used to reposition GUI elements when window resizes

anchor.lua

local floor = math.floor

local point = {
	Center      = 0,
	North       = 1,
	NorthWest   = 2,
	West        = 3,
	SouthWest   = 4,
	South       = 5,
	SouthEast   = 6,
	East        = 7,
	NorthEast   = 8
}

local anchor = {}
anchor.__index = Anchor

-- MVP anchor.new("Center", 0,0)
function anchor.new(anchorPoint, x, y, width, height, padding)
	local self = {}
	setmetatable(self, anchor)
	
	if width then
		if type(width) == "number" then
			width = {min=width,max=width}
		else
			width.min = width.min or 0
			width.max = width.max or 0
		end
	else
		width = {min=0,max=0}
	end
	if height then
		if type(height) == "number" then
			height = {min=height,max=height}
		else
			height.min = height.min or 0
			height.max = height.max or 0
		end
	else
		height = {min=0,max=0}
	end
	
	padding = padding or {}
	padding.top     = padding.top       or 0
	padding.bottom  = padding.bottom    or 0
	padding.left    = padding.left      or 0
	padding.right   = padding.right     or 0
	
	self.anchorPoint = point[anchorPoint or error("Anchor requires AnchorPoint")] or error("Invalid AnchorPoint")
	self.anchorPointStr = anchorPoint
	self.x = x or error("Anchor requires X")
	self.y = y or error("Anchor requires Y")
	self.width = width
	self.height = height
	self.padding = padding
	
	return self
end

local function CalculatePosition(anchorPoint, x, y, w, h, padding, windowWidth, windowHeight)
	local centerX, centerY = floor(windowWidth / 2) - floor(w / 2), floor(windowHeight / 2) - floor(h / 2)
	x = x + (padding.right - padding.left)
	y = y + (padding.bottom - padding.top)
	if anchorPoint == point.Center then
		return centerX + x, centerY + y
	elseif anchorPoint == point.North then
		return centerX + x, y
	elseif anchorPoint == point.NorthWest then
		return x, y
	elseif anchorPoint == point.West then
		return x, centerY + y
	elseif anchorPoint == point.SouthWest then
		return x, windowHeight - y
	elseif anchorPoint == point.South then
		return centerX + x, windowHeight - y
	elseif anchorPoint == point.SouthEast then
		return windowWidth - x, windowHeight - y
	elseif anchorPoint == point.East then
		return windowWidth - x, centerY + y
	elseif anchorPoint == point.NorthEast then
		return windowWidth - x, y
	end
	error("Unknown anchor point")
end

local function CalculateLength(min, max, padding, windowLength)
	if max + padding <= windowLength then
		return max
	elseif min + padding <= windowLength then
		return windowLength - padding
	else
		return min
	end
end

function anchor:getRect(windowWidth, windowHeight)
	local w = CalculateLength(self.width.min, self.width.max, self.padding.left + self.padding.right, windowWidth)
	local h = CalculateLength(self.height.min, self.height.max, self.padding.top + self.padding.bottom, windowHeight)
	
	local x, y = CalculatePosition(self.anchorPoint, self.x, self.y, w, h, self.padding, windowWidth, windowHeight)
	
	return x, y, w, h
end

return setmetatable(anchor, {__call=function(_, ...) return anchor.new(...) end})