local function set(self, r, c, v)
	if r < self.top or r > self.bottom or c < self.left or c > self.right then
		error(string.format("index (%d %d) out of bounds (%d..%d %d..%d)",
		                    r, c, self.top, self.bottom, self.left, self.right))
	end
	self.grid[r * self.width + c] = v
end

local function get(self, r, c)
	if r < self.top or r > self.bottom or c < self.left or c > self.right then
		if self.default then
			return self.default
		else
			error(string.format("index (%d %d) out of bounds (%d..%d %d..%d)",
			                    r, c, self.top, self.bottom, self.left, self.right))
		end
	end
	local item = self.grid[r * self.width + c]
	if item == nil and self.default then
		return self.default
	else
		return item
	end
end

local function iter(self, r, c, h, w)
	if r == nil then r = self.top end
	if c == nil then c = self.left end
	if h == nil then h = self.height end
	if w == nil then w = self.width end
	local r0, c0 = r, c
	return function()
		if r0 > r + h - 1 then return nil end
		local rb, cb = r0, c0
		c0 = c0 + 1
		if c0 > c + w - 1 then
			c0 = c
			r0 = r0 + 1
		end
		return self:get(rb, cb), rb, cb
	end
end

local function neigh9(self, r, c, exclude)
	local iter = self:iter(r - 1, c - 1, 3, 3)
	return function()
		local i, ri, ci = iter()
		if exclude and ri == r and ci == c then
			local i, ri, ci = iter()
		end
		return i, ri, ci
	end
end

local function write(self, format)
	if not format then format = function(i)
		return string.format("%3s", i)
	end end
	for r = self.top, self.bottom do
		for c = self.left, self.right do
			io.write(format(self:get(r, c)))
		end
		io.write("\n")
	end
end

local function grow(self)
	self.height = self.height + 1
	self.bottom = self.bottom + 1
end

function grid(top, left, width, height, default)
	return {
		top = top,
		left = left,
		height = height,
		width = width,
		bottom = top + height - 1,
		right = left + width - 1,
		default = default,
		grid = {},
		get = get,
		set = set,
		iter = iter,
		neigh9 = neigh9,
		write = write,
		grow = grow,
	}
end