tile-based pico8 collision i wrote in like an hour or two

collision.lua

platscene = {
	xaccel = 0.5,
	xmax = 2,
	gravity = 0.5,
	ymax = 16,
}
function platscene:switch()
	self.mapbbox = {0, 0, 16, 16}
	self.playerbbox = {
		0, 0,
		(critter.w or 1) * 8,
		(critter.h or 1) * 8,
	}
	self.vel = {x=0, y=0}
	self.ground = false
	self.walktimer = 0
end

function platscene:update()
	self.debug = {}
	
	if self.ground then
		self.walktimer += 1
	end

	-- collision
	-- tile-based
	-- one axis at a time
	-- only move by pixels
	local d = {
		x = int(self.vel.x),
		y = int(self.vel.y),
	}
	self.debugd = d
	local pbb = self.playerbbox
	
	local wasground = self.ground
	self.ground = false
	for axis in all{"x", "y"} do
		local move = abs(d[axis])
		local movedir = sgn(self.vel[axis])
		local idx = 1
		if axis == "y" then
			idx = 2
		end
		
		-- move to tile boundary
		local edge = pbb[idx]
		local mapedge = self.mapbbox[idx]
		if movedir > 0 then
			edge += pbb[idx + 2]
			mapedge += self.mapbbox[idx + 2]
		end
		mapedge *= movedir * 8
		local step = min(move, edge * -movedir % 8)
		move -= step
		pbb[idx] += movedir * step
		edge += movedir * step

		-- move one tile at a time
		-- always run this loop at
		-- least once, so we know if
		-- we're on the ground even
		-- if not moving
		local stopped
		while edge % 8 == 0 do
			if edge * movedir >= mapedge then
				stopped = true
				break
			end
			local blocked
			local mx = edge / 8
			if movedir < 0 then
				mx -= 1
			end
			local my = mx
			for other = flr(pbb[3 - idx] / 8), flr((pbb[3 - idx] + pbb[5 - idx] - 1) / 8) do
				if axis == "x" then
					my = other
				else
					mx = other
				end
				local s = mget(mx, my)
				add(self.debug, {mx, my, fget(s, 7)})
				if fget(s, 7) or (fget(s, 6) and axis == "y" and movedir > 0) then
					blocked = true
					break
				end
			end
			if blocked then
				stopped = true
				break
			end
			local step = min(8, move)
			move -= step
			pbb[idx] += movedir * step
			edge += movedir * step
			if move <= 0 then
				break
			end
		end
		
		-- handle solid collision
		if stopped then
			self.vel[axis] = 0
			if axis == "y" and movedir > 0 then
				self.ground = true
				if not wasground then
					self.walktimer = 0
					sfx(59)
				end
			end
		end
	end

	-- passive
	self.vel.y = min(self.ymax, self.vel.y + self.gravity)

	-- player movement
	-- yes, after the collision!
	-- i do this for reasons: for
	-- example, now i can tell if
	-- the player is trying to
	-- move via self.vel.x
	if btn(0) then  -- \139
		self.vel.x = max(-self.xmax, self.vel.x - self.xaccel)
	elseif btn(1) then  -- \145
		self.vel.x = min(self.xmax, self.vel.x + self.xaccel)
	else
		self.vel.x = self.vel.x * 3/4
		if abs(self.vel.x < self.xaccel) then
			self.vel.x = 0
		end
	end
	if btnp(4) then  -- \142
		if self.ground then
			self.vel.y = -sqrt(2 * self.gravity * 8 * 1.25)
			sfx(58)
		end
	end
end

function platscene:draw()
	cls()
	
	local bb = self.mapbbox
	map(bb[1], bb[2], 0, 0, bb[3] - bb[1], bb[4] - bb[2])
	critter:draw_tl(
		self.playerbbox[1],
		self.playerbbox[2],
		not self.ground or (self.vel.x ~= 0 and self.walktimer % 30 > 15))
	
	rect(self.playerbbox[1], self.playerbbox[2], self.playerbbox[1] + self.playerbbox[3] - 1, self.playerbbox[2] + self.playerbbox[4] - 1, 9)
	for d in all(self.debug) do
		rect(d[1] * 8, d[2] * 8, d[1] * 8 + 7, d[2] * 8 + 7, d[3] and 8 or 11)
	end
	print(""..self.playerbbox[1]..","..self.playerbbox[2].."/"..self.playerbbox[3]..","..self.playerbbox[4], 0, 0, 9)
	print(self.vel.x..","..self.vel.y, 0, 6, 8)
	if self.debugd then
		print(self.debugd.x..","..self.debugd.y, 0, 12, 7)
	end
end