local type InputStream = record
	str: string
	offset: number
	refill: function(): string

	new: function(InputStream): InputStream
	new: function(InputStream, function(): string): InputStream

	setRefill: function(InputStream, function(): string)

	readCurrentCharacter: function(): string
	advanceOffset: function(): string
end

local istream_mt = {__index = InputStream}

function InputStream:_manageBuffer()
	if not self.str then
		self.str = self.refill()
		self.offset = 1
	end
	if self.offset > #self.str + 1 then
		self.str = self.refill()
		self.offset = 1
	end
end



-- constructors
function InputStream:new(): InputStream
	return setmetatable(
		{
			string = "",
			offset = 1
		} as InputStream,
		istream_mt)
end
function InputStream:new(refill: function(): string): InputStream
	local mt = setmetatable(
		{
			string = "",
			offset = 1
		} as InputStream,
		istream_mt)
	mt:setRefill(refill)
	return mt

end



-- setters/getters
function InputStream:setRefill(func: function(): string)
	self.refill = func
end

-- 
function InputStream:readCurrentCharacter(): string
	self:_manageBuffer()
	return self.str:sub(self.offset, self.offset)
end

function InputStream:advanceOffset(): string
	self.offset = self.offset + 1
	self:_manageBuffer()
	return self.str:sub(self.offset, self.offset)
end

return InputStream