dodge-bolt/src/systems/physics.lua

local Physics = {}
local bump = require("lib.bump")
local pprint = require("lib.pprint")
local Vec = require("lib.brinevector")

-- TODO: Tweak bump world `cellSize` at runtime, when the map switches

Physics.required_groups = {"physical", "collider"}

function Physics:init()
	self.bump = bump.newWorld()
	self.boundCollisionFilter = function(...)
		return self:collisionFilter(...)
	end
end

function Physics:onMapSwitch(map)
	map:bump_init(self.bump)
end

local function getColliderBounds(e)
	local x = e.collider[1]
	local y = e.collider[2]
	local w = math.abs(e.collider[3] - e.collider[1])
	local h = math.abs(e.collider[4] - e.collider[2])
	if e.pos then
		x = x + e.pos.x
		y = y + e.pos.y
	end
	return x, y, w, h
end

function Physics:addToGroup(group, e)
	if group == "collider" then
		self.bump:add(e, getColliderBounds(e))
	end
end

function Physics:removeFromGroup(group, e)
	if group == "collider" then
		self.bump:remove(e)
	end
end

function Physics:collisionFilter(entity, other)
	if entity.hidden then return end
	if other.hidden then return end

	local bolt = entity.bolt or other.bolt
	local vel = entity.vel or other.vel
	local player = entity.bolts or other.bolts

	if bolt and player then
		return "cross"
	elseif bolt then
		return "bounce"
	elseif (vel or vel) and not (entity.bolts and other.bolts) then
		return "slide"
	end
end

function Physics:resolveCollisions(e, dt)
	local targetPos = e.pos + e.vel * dt
	local ox = e.collider[1]
	local oy = e.collider[2]
	self.bump:update(e, e.pos.x+ox, e.pos.y+oy)
	local x, y, cols = self.bump:move(e, targetPos.x+ox, targetPos.y+oy, self.boundCollisionFilter)

	local skip_moving = false
	if #cols > 0 then
		e.pos.x = x - ox
		e.pos.y = y - oy
		skip_moving = true
	end

	for _, col in ipairs(cols) do
		if col.type == "cross" then
			local player = col.other.bolts and col.other or e
			local bolt = col.other.bolt and col.other or e
			if player and bolt and bolt.owner ~= player then
				self.pool:emit("hitPlayer", player, bolt)
			end
		elseif col.type == "bounce" then
			-- sx and sy and just number for flipped the direction when something
			-- bounces.
			-- When `normal.x` is zero, sx = 1, otherwise sx = -1. Same with sy
			local sx = 1 - 2*math.abs(col.normal.x)
			local sy = 1 - 2*math.abs(col.normal.y)

			e.vel.x = e.vel.x * sx
			e.vel.y = e.vel.y * sy
			if e.acc then
				e.acc.x = e.acc.x * sx
				e.acc.y = e.acc.y * sy
			end
			skip_moving = true
		end
	end

	return skip_moving
end

function Physics:update(dt)
	for _, e in ipairs(self.pool.groups.physical.entities) do
		if e.acc then
			e.vel = e.vel + e.acc * dt
		end
		if e.friction then
			e.vel = e.vel * (1 - math.min(e.friction, 1)) ^ dt
		end

		local skip_moving = false
		if self.pool.groups.collider.hasEntity[e] then
			skip_moving = self:resolveCollisions(e, dt)
		end

		if not skip_moving then
			if e.max_speed then
				e.pos = e.pos + e.vel:trim(e.max_speed) * dt
			else
				e.pos = e.pos + e.vel * dt
			end
		end
	end
end

function Physics:castRay(from, to)
	local items = self.bump:querySegmentWithCoords(from.x, from.y, to.x, to.y, function(item)
		return not item.vel
	end)
	for _, item in ipairs(items) do
		if item.nx ~= 0 or item.ny ~= 0 then
			local pos = Vec(item.x1, item.y1)
			local normal = Vec(item.nx, item.ny)
			return pos, normal
		end
	end
end

function Physics:project(from, direction, distance)
	local key_points = {}
	table.insert(key_points, from)

	local distance_traveled = 0
	local position = from
	-- While the ray hasen't traveled the required amount
	while math.abs(distance_traveled - distance) > 0.01 do
		-- Check if it collides with anything
		local destination = position + direction * (distance-distance_traveled)
		local intersect, normal = self:castRay(position, destination)
		local new_position = intersect or destination
		-- Update how much the ray has moved in total
		distance_traveled = distance_traveled + (position - new_position).length
		position = new_position

		-- If it collided change its travel direction
		if normal then
			-- sx and sy and just number for flipped the direction when something
			-- bounces.
			-- When `normal.x` is zero, sx = 1, otherwise sx = -1. Same with sy
			direction.x = direction.x * (1 - 2*math.abs(normal.x))
			direction.y = direction.y * (1 - 2*math.abs(normal.y))
		end

		-- Save this point where it stopped/collided
		table.insert(key_points, new_position)
	end

	return key_points
end

function Physics:queryRect(pos, size, filter)
	return self.bump:queryRect(pos.x, pos.y, size.x, size.y, filter)
end

return Physics