insidethebox/mods/boxes/valloc.lua

--[[

  ITB (insidethebox) minetest game - Copyright (C) 2017-2018 sofar & nore

  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public License
  as published by the Free Software Foundation; either version 2.1
  of the License, or (at your option) any later version.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public
  License along with this library; if not, write to the Free
  Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
  MA 02111-1307 USA

]]--

--[[
	Box allocation.

	To keep things simple, we will always allocate boxes with same width and
	depth, and with a sizee that is a multiple of boxes_resolution. Height is
	assumed to be unlimited above box_alloc_miny.
	In order to do that, we keep a quadtree of allocatable positions; an allocated
	box will always be a leaf of that tree. We also keep for each subtree the max
	size that can be allocated in it.

	Thus, this is https://en.wikipedia.org/wiki/Buddy_memory_allocation in 2D.
]]

local boxes_resolution = 256
local box_alloc_miny = 50
local boxes_tree = {
	minp = {x = 1024, z = 1024},
	edge_size = 8192,
	max_alloc_size = 8192,
}

local function split_leaf(node)
	assert (node.edge_size >= 2 * boxes_resolution)
	assert (node.children == nil)
	local new_size = node.edge_size / 2
	local x = node.minp.x
	local z = node.minp.z
	node.children = {
		{
			minp = {x = x, z = z},
			edge_size = new_size,
			max_alloc_size = new_size,
			parent = node
		},
		{
			minp = {x = x + new_size, z = z},
			edge_size = new_size,
			max_alloc_size = new_size,
			parent = node,
		},
		{
			minp = {x = x, z = z + new_size},
			edge_size = new_size,
			max_alloc_size = new_size,
			parent = node,
		},
		{
			minp = {x = x + new_size, z = z + new_size},
			edge_size = new_size,
			max_alloc_size = new_size,
			parent = node,
		},
	}
end

-- Update `max_alloc_size` for node and all its parents.
-- Also, merge subtrees if possible.
local function update_alloc_sizes(node)
	while node ~= nil do
		local max_size = 0
		local el = node.edge_size / 2
		local can_merge = true
		-- a crash here with the following error msg:
		-- `bad argument #1 to 'ipairs' (table expected, got nil)`
		-- means that there is an unknown node on the map
		-- hence, this assert:
		assert(node.children)
		for _, child in ipairs(node.children) do
			max_size = math.max(max_size, child.max_alloc_size)
			if child.max_alloc_size < el then
				can_merge = false
			end
		end
		if can_merge then
			node.children = nil
			node.max_alloc_size = node.edge_size
		else
			node.max_alloc_size = max_size
		end
		node = node.parent
	end
end

-- Function to know how close to zero a node is.
-- Used to keep allocations close to (0, box_alloc_miny, 0).
local function zero_close(node)
	local x = node.minp.x
	local z = node.minp.z
	local size = node.edge_size
	if x < 0 then x = x + size end
	if z < 0 then z = z + size end
	return math.abs(x) + math.abs(z)
end

-- Allocated a box of size `size` horizontally, and unbounded vertically
-- Returns box minimum position. The minimum position must be given to
-- boxes.vfree to free the corresponding area.
function boxes.valloc(size)
	assert (size > 0)
	if boxes_tree.max_alloc_size < size then
		return nil
	end

	-- Find leaf with enough room, splitting it if big enough
	local node = boxes_tree
	while node.edge_size / 2 >= size and node.edge_size / 2 >= boxes_resolution do
		if node.children == nil then
			split_leaf(node)
		end

		local best = nil
		local best_distance = 1e10 -- infinity
		for _, child in ipairs(node.children) do
			if child.max_alloc_size >= size and zero_close(child) < best_distance then
				best_distance = zero_close(child)
				best = child
			end
		end
		assert (best ~= nil)
		node = best
	end

	local result = {x = node.minp.x, y = box_alloc_miny, z = node.minp.z}
	node.max_alloc_size = 0
	update_alloc_sizes(node.parent)
	return result
end

function boxes.vfree(minp)
	assert (minp.y == box_alloc_miny)
	assert (boxes_tree.minp.x <= minp.x)
	assert (minp.x < boxes_tree.minp.x + boxes_tree.edge_size)
	assert (boxes_tree.minp.z <= minp.z)
	assert (minp.z < boxes_tree.minp.z + boxes_tree.edge_size)

	-- Find leaf containing minp
	local node = boxes_tree
	while node.children ~= nil do
		local cld = nil
		local el = node.edge_size / 2
		for _, child in ipairs(node.children) do
			if child.minp.x <= minp.x and minp.x < child.minp.x + el
					and child.minp.z <= minp.z and minp.z < child.minp.z + el then
				cld = child
				break
			end
		end
		assert (cld ~= nil)
		node = cld
	end

	assert (node.max_alloc_size == 0)
	node.max_alloc_size = node.edge_size
	update_alloc_sizes(node.parent)
end