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+                    GNU GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
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+
+                            Preamble
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+  16. Limitation of Liability.
+
+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program 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 General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+    <program>  Copyright (C) <year>  <name of author>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<https://www.gnu.org/licenses/>.
+
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.  But first, please read
+<https://www.gnu.org/licenses/why-not-lgpl.html>.
diff --git a/main.lua b/main.lua
new file mode 100644
index 0000000..267943b
--- /dev/null
+++ b/main.lua
@@ -0,0 +1,228 @@
+local quadtree = require("quadtree")
+local ok, bit = pcall(require, "bit")
+if not ok then
+    ok, bit = pcall(require, "bit32")
+    if not ok then error("no bit library") end
+end
+
+local state
+
+function love.load()
+    love.graphics.setBackgroundColor(1, 1, 1)
+    love.window.setTitle("Quadtree")
+    local w, h = love.window.getMode()
+    love.window.setMode(w, h, { resizable = true })
+
+    state = {}
+    state.map = quadtree.new(1, 1, 0.01)
+    state.pos = { x = state.map.width/2, y = state.map.height/2 }
+    state.zoom = 25
+    state.poly = {}
+    state.draft = {}
+    state.mode = "move"
+    local font = love.graphics.newFont(20)
+    state.text = {
+        move = love.graphics.newText(font, "Mode: Move\n\nControls:\nDrag: Move map"),
+        polygon = love.graphics.newText(font, "Mode: Polygon\n\nControls:\nClick: Add vertex\nP: Finish polygon\n"),
+        debug = love.graphics.newText(font, "Mode: Debug\n\nControls:\nClick: Print debug info about node"),
+        general = love.graphics.newText(font,
+[[
+M: Switch to Move Mode
+P: Switch to Polygon Mode
+D: Switch to Debug Mode
+Q: Quit
+Mouse Wheel: Zoom map
+]]
+        )
+    }
+    return ed
+end
+
+local function proj(x, y)
+    local w, h = love.graphics:getDimensions()
+    return w/2+(x-state.pos.x)*state.zoom, h/2-(y-state.pos.y)*state.zoom
+end
+
+local function unproj(x, y)
+    local w, h = love.graphics:getDimensions()
+    return state.pos.x+(x-w/2)/state.zoom, state.pos.y+(-y+h/2)/state.zoom
+end
+
+local function add_to_map(poly)
+    quadtree.insert(state.map, poly, 0.8)
+end
+
+local function poly_bounds(poly)
+    local x1, y1, x2, y2
+    for _, p in ipairs(poly) do
+        x1 = math.min(x1 or p.x, p.x)
+        x2 = math.max(x2 or p.x, p.x)
+        y1 = math.min(y1 or p.y, p.y)
+        y2 = math.max(y2 or p.y, p.y)
+    end
+    return x1, y1, x2, y2
+end
+
+local function shift_poly(poly, x, y)
+    local new = {}
+    for _, p in ipairs(poly) do
+        table.insert(new, { x = p.x + x, y = p.y + y})
+    end
+    return new
+end
+
+local function grow_map(poly)
+    local x1, y1, x2, y2 = poly_bounds(poly)
+    local x1, y1, x2, y2 = x1-2, y1-2, x2+2, y2+2
+    local w, h = state.map.width, state.map.height
+    if x1 < 0 or y1 < 0 then
+        local ox, oy = -math.min(0, x1), -math.min(0, y1)
+        x2, y2 = x2 + ox, y2 + oy
+        w, h = w + ox, h + oy
+        state.pos.x, state.pos.y = state.pos.x+ox, state.pos.y+oy
+        poly = shift_poly(poly, ox, oy)
+        for i, p in ipairs(state.poly) do
+            state.poly[i] = shift_poly(p, ox, oy)
+        end
+    end
+    w, h = math.max(w, x2), math.max(h, y2)
+    if w ~= state.map.width or h ~= state.map.height then
+        state.map = quadtree.new(w, h, 0.02)
+        for _, p in ipairs(state.poly) do
+            add_to_map(p)
+        end
+    end
+    return poly
+end
+
+local function add_poly(poly)
+    if not quadtree.poly_simple(poly) then
+        return
+    end
+
+    poly = grow_map(poly)
+
+    for _, tri in ipairs(quadtree.triangulate(poly)) do
+        add_to_map(tri)
+        table.insert(state.poly, tri)
+    end
+end
+
+local function bin(x)
+    local s = ""
+    for i = 1, 16 do
+        s = bit.band(x, 1) .. s
+        x = bit.rshift(x, 1)
+    end
+    return s
+end
+
+function love.mousepressed(x, y, button)
+    local x, y = unproj(x, y)
+    if button ~= 1 then
+        return
+    end
+    if state.mode == "polygon" then
+        table.insert(state.draft, { x = x, y = y })
+    elseif state.mode == "debug" then
+        print("---")
+        for _, d in ipairs(quadtree.debug) do
+            if x >= d.p1[1] and x < d.p2[1] and y >= d.p1[2] and y < d.p2[2] then
+                print(d.msg, "x="..bin(d.bits[1]), "y="..bin(d.bits[2]), "g="..bin(d.g))
+            end
+        end
+    end
+end
+
+function love.mousemoved(x, y, dx, dy, istouch)
+    if state.mode == "move" then
+        if love.mouse.isDown(1) then
+            state.pos.x = state.pos.x - dx/state.zoom
+            state.pos.y = state.pos.y + dy/state.zoom
+        end
+    end
+end
+
+function love.wheelmoved(x, y)
+    local cx, cy = love.mouse.getPosition()
+    local ox, oy = unproj(cx, cy)
+
+    local factor = y < 0 and 0.9 or 1.1
+    state.zoom = state.zoom * factor
+
+    local nx, ny = unproj(cx, cy)    
+    state.pos.x = state.pos.x+ox-nx
+    state.pos.y = state.pos.y+oy-ny
+end
+
+function love.keypressed(key)
+    if key == "p" then
+        if state.mode == "polygon" then
+            if #state.draft >= 3 then
+                add_poly(state.draft)
+            end
+            state.draft = {}
+            state.mode = "move"
+        else
+            state.mode = "polygon"
+        end
+    elseif key == "d" then
+        state.mode = "debug"
+    elseif key == "m" then
+        state.mode = "move"
+    elseif key == "q" then
+        love.event.quit()
+    end
+end
+
+local function draw_node(node, w, h, x, y)
+    if node == true then
+        love.graphics.setColor(0.7, 0.7, 0.7)
+        love.graphics.rectangle("fill", x, y, w, h)
+    elseif node then
+        local w, h = w/2, h/2
+        draw_node(node[1], w, h, x,   y)
+        draw_node(node[2], w, h, x,   y+h)
+        draw_node(node[3], w, h, x+w, y)
+        draw_node(node[4], w, h, x+w, y+h)
+        return
+    end
+
+    love.graphics.setColor(0, 0, 0)
+    love.graphics.rectangle("line", x, y, w, h)
+end
+
+local function make_verts(poly)
+    local verts = {}
+    for _, p in ipairs(poly) do
+        local x, y = proj(p.x, p.y)
+        table.insert(verts, x)
+        table.insert(verts, y)
+    end
+    return verts
+end
+
+function love.draw()
+    love.graphics.setLineWidth(1)
+    draw_node(state.map.root, state.map.width*state.zoom, -state.map.height*state.zoom, proj(0, 0))
+
+    love.graphics.setColor(0, 0, 1)
+    love.graphics.setLineWidth(3)
+    for _, pl in ipairs(state.poly) do
+        love.graphics.polygon("line", make_verts(pl))
+    end
+
+    love.graphics.setColor(0, 1, 1)
+    if #state.draft == 1 then
+        -- love.graphics.setPointWidth(3)
+        love.graphics.points(proj(state.draft[1].x, state.draft[1].y))
+    elseif #state.draft >= 2 then
+        love.graphics.line(make_verts(state.draft))
+    end
+
+    love.graphics.setColor(0, 0, 0)
+    love.graphics.draw(state.text[state.mode])
+    love.graphics.draw(state.text.general, 0, state.text[state.mode]:getHeight())
+end
+
+return editor_new
diff --git a/quadtree.lua b/quadtree.lua
new file mode 100644
index 0000000..01c6018
--- /dev/null
+++ b/quadtree.lua
@@ -0,0 +1,349 @@
+local ok, bit = pcall(require, "bit")
+if not ok then
+    ok, bit = pcall(require, "bit32")
+    if not ok then error("no bit library") end
+end
+
+local function clamp(x, a, b)
+    if x < a then
+        return a
+    elseif x > b then
+        return b
+    else
+        return x
+    end
+end
+
+local function dot(x1, y1, x2, y2)
+    return x1*x2 + y1*y2
+end
+local function sq_len(x, y)
+    return x*x + y*y
+end
+local function sq_dist(x1, y1, x2, y2)
+    return sq_len(x2-x1, y2-y1)
+end
+
+-- distance
+
+local function closest_point_aabb(x, y, x1, y1, x2, y2)
+    return clamp(x, x1, x2), clamp(y, y1, y2)
+end
+local function sq_dist_point_aabb(x, y, x1, y1, x2, y2)
+    return sq_dist(x, y, closest_point_aabb(x, y, x1, y1, x2, y2))
+end
+
+local function closest_point_edge(x, y, x1, y1, x2, y2)
+    local l = sq_dist(x1, y1, x2, y2)
+    if l == 0 then
+        return x1, y1
+    end
+
+    local t = clamp(dot(x-x1, y-y1, x2-x1, y2-y1) / l, 0, 1)
+    return x1 + t * (x2 - x1), y1 + t * (y2 - y1)
+end
+local function sq_dist_point_edge(x, y, x1, y1, x2, y2)
+    return sq_dist(x, y, closest_point_edge(x, y, x1, y1, x2, y2))
+end
+
+local function signed_triangle_area(x1, y1, x2, y2, x3, y3)
+    return (x2 - x1) * (y3 - y1) - (y2 - y1) * (x3 - x1)
+end
+local function point_edge_left(x, y, x1, y1, x2, y2)
+    return signed_triangle_area(x1, y1, x2, y2, x, y) > 0
+end
+
+local function edge_intersect(ax, ay, bx, by, cx, cy, dx, dy)
+    local oa = signed_triangle_area(cx, cy, dx, dy, ax, ay)
+    local ob = signed_triangle_area(cx, cy, dx, dy, bx, by)
+    local oc = signed_triangle_area(ax, ay, bx, by, cx, cy)
+    local od = signed_triangle_area(ax, ay, bx, by, dx, dy)
+    return oa*ob < 0 and oc*od < 0
+end
+
+local function point_in_triangle(x, y, x1, y1, x2, y2, x3, y3)
+    return
+        not point_edge_left(x, y, x1, y1, x2, y2) and
+        not point_edge_left(x, y, x2, y2, x3, y3) and
+        not point_edge_left(x, y, x3, y3, x1, y1)
+end
+
+local function signed_poly_area(poly)
+    local area = 0
+    for i, a in ipairs(poly) do
+        local b = poly[i % #poly + 1]
+        area = area + (b.x - a.x) * (b.y - a.y)
+    end
+    return area
+end
+
+local function point_in_poly(x, y, poly)
+    for i, a in ipairs(poly) do
+        local b = poly[i % #poly + 1]
+        if not point_edge_left(x, y, a.x, a.y, b.x, b.y) then
+            return false
+        end
+    end
+    return true
+end
+
+local function dist_point_poly_lt(sq_v, poly, x, y)
+    for i, a in ipairs(poly) do
+        local b = poly[i % #poly + 1]
+        if sq_dist_point_edge(x, y, a.x, a.y, b.x, b.y) < sq_v then
+            return true
+        end
+    end
+    return point_in_poly(x, y, poly)
+end
+
+-- triangulate
+
+local function orient_poly(poly)
+    if signed_poly_area(poly) > 0 then
+        return poly
+    end
+    local rev = {}
+    for i = #poly, 1, -1 do
+        table.insert(rev, poly[i])
+    end
+    return rev
+end
+
+local function triangulate(poly)
+    --[[
+    poly = orient_poly(poly)
+    local tri = {}
+    local i, stop = 1, 1
+    while #poly > 3 do
+        local a, b, c = poly[i], poly[i % #poly + 1], poly[(i+1) % #poly + 1]
+        local is_ear
+        if not point_edge_left(c.x, c.y, a.x, a.y, b.x, b.y) then
+            is_ear = true
+            for j = 3, #poly-1 do
+                local pa, pb, pc = poly[(i+j-2)%#poly+1], poly[(i+j-1)%#poly+1], poly[(i+j)%#poly+1]
+                -- point edge left check???
+                if point_in_triangle(pb.x, pb.y, a.x, a.y, b.x, b.y, c.x, c.y) and
+                    point_edge_left(pa.x, pa.y, pb.x, pb.y, pc.x, pc.y) then
+                    is_ear = false
+                    break
+                end
+            end
+        end
+        if is_ear then
+            table.insert(tri, { a, b, c })
+            table.remove(poly, i % #poly + 1)
+            i = i % #poly + 1
+            stop = i
+        else
+            i = i % #poly + 1
+            if i == stop then
+                error("triangulation fail")
+            end
+        end
+    end
+    table.insert(tri, poly)
+    return tri]]
+    local verts = {}
+    for _, p in ipairs(poly) do
+        table.insert(verts, p.x)
+        table.insert(verts, p.y)
+    end
+    local tris = {}
+    for _, t in ipairs(love.math.triangulate(verts)) do
+        table.insert(tris, orient_poly({
+            { x = t[1], y = t[2] },
+            { x = t[3], y = t[4] },
+            { x = t[5], y = t[6] },
+        }))
+    end
+    return tris
+end
+
+local function poly_simple(poly)
+    for i, a in ipairs(orient_poly(poly)) do
+        local b = poly[i+1]
+        for j = i+2, #poly do
+            local c = poly[j]
+            local d = poly[j % #poly + 1]
+            if edge_intersect(a.x, a.y, b.x, b.y, c.x, c.y, d.x, d.y) then
+                return false
+            end
+        end
+    end
+    return true
+end
+
+-- intersect
+
+local function project_aabb(x, y, x1, y1, x2, y2)
+    local d1 = dot(x, y, x1, y1)
+    local d2 = dot(x, y, x1, y2)
+    local d3 = dot(x, y, x2, y1)
+    local d4 = dot(x, y, x2, y2)
+    return math.min(d1, d2, d3, d4), math.max(d1, d2, d3, d4)
+end
+
+local function project_poly(x, y, poly)
+    local p1, p2
+    for _, p in ipairs(poly) do
+        local d = dot(x, y, p.x, p.y)
+        if not p1 then
+            p1 = d
+            p2 = d
+        else
+            p1 = math.min(p1, d)
+            p2 = math.max(p2, d)
+        end
+    end
+    return p1, p2
+end
+
+local function collect_axes(poly)
+    local axes = { { x = 0, y = 1 }, { x = 1, y = 0 } }
+    for i, a in ipairs(poly) do
+        local b = poly[i % #poly + 1]
+        local x, y = (a.y - b.y), -(a.x - b.x)
+        local l = math.sqrt(sq_len(x, y))
+        table.insert(axes, { x = x/l, y = y/l })
+    end
+    for _, ax in ipairs(axes) do
+        ax.p1, ax.p2 = project_poly(ax.x, ax.y, poly)
+    end
+    return axes
+end
+
+local function overlap(a1, a2, b1, b2)
+    return a2 >= b1 and b2 >= a1
+end
+
+local function intersect_aabb_poly(x1, y1, x2, y2, axes)
+    for _, ax in ipairs(axes) do
+        if not overlap(ax.p1, ax.p2, project_aabb(ax.x, ax.y, x1, y1, x2, y2)) then
+            return false
+        end
+    end
+    return true
+end
+
+local debug = {}
+
+local function to_float(map, x, y)
+    return x*map.width/0x8000, y*map.height/0x8000
+end
+
+local function cached_dist_point_poly_lt(map, query, x, y)
+    local key = bit.bor(x, bit.lshift(y, 16))
+    if query.cache[key] == nil then
+        query.cache[key] = dist_point_poly_lt(query.sq_dist, query.poly, to_float(map, x, y))
+    end
+    return query.cache[key]
+end
+
+local function classify(map, query, g, x, y)
+    local p1 = cached_dist_point_poly_lt(map, query, x,   y)
+    local p2 = cached_dist_point_poly_lt(map, query, x,   y+g)
+    local p3 = cached_dist_point_poly_lt(map, query, x+g, y)
+    local p4 = cached_dist_point_poly_lt(map, query, x+g, y+g)
+
+    -- all corners are too close to wall (<=> the entire square is)
+    if p1 and p2 and p3 and p4 then
+        table.insert(debug, { msg = "full cover", p1 = { to_float(map, x, y) }, p2 = { to_float(map, x+g, y+g) }, bits = { x, y }, g = g })
+        return true
+    end
+
+    -- at least one corner is too close to wall
+    if p1 or p2 or p3 or p4 then
+        table.insert(debug, { msg = "close to corner", p1 = { to_float(map, x, y) }, p2 = { to_float(map, x+g, y+g) }, bits = { x, y }, g = g })
+        return
+    end
+
+    local x1, y1 = to_float(map, x, y)
+    local x2, y2 = to_float(map, x+g, y+g)
+
+    -- cell intersects wall
+    if intersect_aabb_poly(x1, y1, x2, y2, query.axes) then
+        table.insert(debug, { msg = "intersect", p1 = { to_float(map, x, y) }, p2 = { to_float(map, x+g, y+g) }, bits = { x, y }, g = g })
+        return
+    end
+
+    -- wall is too close to an edge of the cell
+    for _, p in ipairs(query.poly) do
+        if sq_dist_point_aabb(p.x, p.y, x1, y1, x2, y2) < query.sq_dist then
+            table.insert(debug, { msg = "close to edge", p1 = { to_float(map, x, y) }, p2 = { to_float(map, x+g, y+g) }, bits = { x, y }, g = g })
+            return
+        end
+    end
+
+    table.insert(debug, { msg = "free", p1 = { to_float(map, x, y) }, p2 = { to_float(map, x+g, y+g) }, bits = { x, y }, g = g })
+    return false
+end
+
+local function node_insert(map, query, node, g, x, y)
+    if node == true then
+        return node
+    end
+
+    local class = classify(map, query, g, x, y)
+    if class == false then
+        return node
+    elseif class == true then
+        return true
+    end
+
+    g = bit.rshift(g, 1)
+    if g < map.limit then
+        return true
+    end
+
+    local node = node or { false, false, false, false }
+    node[1] = node_insert(map, query, node[1], g, x,   y)
+    node[2] = node_insert(map, query, node[2], g, x,   y+g)
+    node[3] = node_insert(map, query, node[3], g, x+g, y)
+    node[4] = node_insert(map, query, node[4], g, x+g, y+g)
+    if node[1] == true and node[2] == true and node[3] == true and node[4] == true then
+        return true
+    end
+    return node
+end
+
+local function node_merge(a, b)
+    if a == true or b == true then
+        return true
+    end
+    if a == false then
+        return b
+    end
+    if b == false then
+        return a
+    end
+    local x = {}
+    for i = 1, 4 do
+        x[i] = merge(a[i], b[i])
+    end
+    if x[1] == true and x[2] == true and x[3] == true and x[4] == true then
+        return true
+    end
+    return x
+end
+
+local function insert(map, poly, dist)
+    map.root = node_insert(map, { cache = {}, poly = poly, axes = collect_axes(poly), sq_dist = dist*dist }, map.root, 0x8000, 0, 0)
+end
+
+local function new(width, height, precision)
+    return {
+        width = width,
+        height = height,
+        limit = bit.lshift(1, math.max(0, 15-math.ceil(math.log(math.max(width, height)/precision, 2)))),
+        root = false,
+    }
+end
+
+return {
+    new = new,
+    insert = insert,
+    triangulate = triangulate,
+    poly_simple = poly_simple,
+    debug = debug,
+}