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

	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()
	local length = 0
	if self.str then
		length = #self.str
	else
		length = -1
	end
	if not self.str then
		self.str = self.refill()
		self.offset = 1
	end
	if self.offset > #self.str then
		self.str = self.refill()
		self.offset = 1
	end
end

local function initial_stream(): string
	return ""
end


-- constructors
function InputStream:new(refill: function(): string): InputStream
	refill = refill or initial_stream
	local istream = setmetatable(
		{
			string = "",
			offset = 1,
		} as InputStream,
		istream_mt)
	istream:__setRefill(refill)
	return istream

end



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

-- 
function InputStream:curr(): string | nil
	if self.offset > #self.str then
		self:_manageBuffer()
	end
	if #self.str > 0 and self.offset < #self.str + 1 then
		return self.str:sub(self.offset, self.offset)
	end
	return nil
end

function InputStream:next(): string | nil
	if self.offset > #self.str then
		self:_manageBuffer()
	end

	local current_char: string | nil = nil
	if #self.str > 0 then
		current_char = self.str:sub(self.offset, self.offset)
		self.offset = self.offset + 1
	end
	return current_char
end

return InputStream