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diff --git a/src/constrain.lua b/src/constrain.lua
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+--[[
+A function that allows adding constraints on tables.
+This works by makeing a shallow copy of the table,
+and setting __index and __newindex metamethods of the (now empty) table,
+that access the appropriate values of the shallow copy after doing checks.
+
+constrain(tbl, ("get " | "set ") + "field name", function(tbl, value)
+	--this function is called with the table ("self") and the
+	--value that is being set or retrived
+end)
+
+This module is just a function that can add constraints on a table. Use
+it like this:
+
+	local constrain = require("constrain")
+	local my_tbl = {}
+	constrain(my_tbl,"get my_field",function(self,value)
+		--Here, self is my_tbl, value will be
+		--"my_field"
+		assert(value > 0 and value < 20, "my_value must be between 0 and 20")
+	end)
+	--From now on, if my_tbl.my_field gets set to anything outside of (0:20),
+	--an error will be thrown.
+
+This function should be totally transparent to the outside.
+]]
+
+local constrained_tables = {}
+
+return function(tbl,trigger,func)
+	local is_empty = true
+	for k,v in pairs(tbl) do
+		is_empty = false
+		break;
+	end
+	local meta = getmetatable(tbl)
+
+	--This table has never had constrain() called on it before,
+	--make the shallow copy with hooks and stuff
+	if constrained_tables[tbl] == nil then
+		--Copy all the variables into a shallow copy
+		local shallow_copy = {}
+		for k,v in pairs(tbl) do
+			shallow_copy[k] = v
+			tbl[k] = nil
+		end
+
+		--Set the shallow copy's metatable to the original table's
+		--metatable
+		setmetatable(shallow_copy,meta)
+
+		--Set the original table's metatable to the hookable thing
+		local t_meta = {}
+		t_meta.get_hooks = {}
+		t_meta.set_hooks = {}
+		t_meta.__index = function(self,key)
+			local ret = shallow_copy[key]
+			for _,hook in pairs(t_meta.get_hooks[key] or {}) do
+				hook(self,ret)
+			end
+			return ret
+		end
+		t_meta.__newindex = function(self,key,value)
+			for _,hook in pairs(t_meta.set_hooks[key] or {}) do
+				hook(self,value)
+			end
+			shallow_copy[key] = value
+		end
+		t_meta.__pairs = function(self)
+			return pairs(shallow_copy)
+		end
+		t_meta.__len = function(self)
+			return #shallow_copy
+		end
+		t_meta.__call = function(self,...)
+			return shallow_copy(...)
+		end
+		t_meta.__mode = meta and meta.__mode or ""
+		t_meta.__gc = meta and meta.__gc
+		t_meta.__metatable = meta
+
+		setmetatable(tbl,t_meta)
+		constrained_tables[tbl] = t_meta
+	end
+
+	--By this point, tbl is a "constrainable" table. we can just
+	--add functios to it's get_hooks and set_hooks to do whatever checking we need
+	--functions added to get_hooks should be
+	--	function(self,value) ... end
+	--functions added to set_hooks should be
+	--	function(self,value) ... end
+	--
+	local getset,field = string.match(trigger,"(get) (.+)")
+	if not getset then
+		getset,field = string.match(trigger,"(set) (.+)")
+	end
+	--local getset,field = string.match(trigger,"(get|set) (.+)")
+	assert(getset,"constrain() must be called with \"get\" or \"set\" as the first word in the pattern")
+	assert(field,"constrain() must specify a field to trigger on")
+	if getset == "get" then
+		local gh = constrained_tables[tbl].get_hooks
+		gh[field] = gh[field] or {}
+		table.insert(gh[field],func)
+	elseif getset == "set" then
+		local sh = constrained_tables[tbl].set_hooks
+		sh[field] = sh[field] or {}
+		table.insert(sh[field],func)
+	end
+end