extends Spatial
class_name SolarSystem

const G = 6.674 * pow(10, -11)

# Because we're dealing with a miniature, we want small masses and distances to have noticeable
# gravitational effects. These multipliers adapt the scale of the game to an earth-like scale.
const mass_multiplier = pow(10, 24)  # thus, the earth would get a mass of 6
const distance_multiplier = 318550   # thus, a radius of 20 results in an earth-like radius

# If the gravity doesn't feel like it should, this scales it. A higher value means that the pull is
# stronger.
const gravity_multiplier = 10.0


# Return the gravity acceleration vector to the closest planet, not taking all other planets into
# account.
# This is useful for keeping something firmly on the planet while it is touching it, or for creating
# a predictable orbit.
func get_closest_gravity_acceleration(position: Vector3) -> Vector3:
	var closest_planet_distance = INF
	var closest_force = 0.0
	
	# Iterate through all planets to find the closest one
	for planet in get_children():
		var pos_to_center = (planet.transform.origin - position)
		var distance = pos_to_center.length()
		
		if distance < closest_planet_distance:
			# This planet is closer than the previous ones -> calculate and save the values
			var force = _gravity(planet.mass * mass_multiplier, distance * distance_multiplier)
			force *= gravity_multiplier
			
			# Multiply by the normalized vector towards the center to give the force a direction
			closest_force = (pos_to_center / distance) * force
			closest_planet_distance = distance
	
	return closest_force


# Return the total gravity acceleration vector experienced at that position.
func get_gravity_acceleration(position: Vector3) -> Vector3:
	var total_force = Vector3.ZERO
	
	# Add each planet's gravity force to the total force
	for planet in get_children():
		var pos_to_center = (planet.transform.origin - position)
		var distance = pos_to_center.length()
		
		var force = _gravity(planet.mass * mass_multiplier, distance * distance_multiplier)
		force *= gravity_multiplier
		
		total_force += (pos_to_center / distance) * force
	
	return total_force


# Return the velocity needed to orbit the given planet
func get_orbit_velocity(position: Vector3, planet):
	var pos_to_center = (planet.global_transform.origin - position)
	var distance = pos_to_center.length()
	
	# raw_velocity is the velocity according to the formula, but we also need to account for the gravity_multiplier.
	var raw_velocity = -sqrt((G * planet.mass * mass_multiplier) / (distance * distance_multiplier))
	var gravity_scale_factor = sqrt(gravity_multiplier / distance_multiplier)
	
	return raw_velocity * gravity_scale_factor


# Formula for gravity
static func _gravity(mass, distance):
	return (G * mass) / (distance * distance)