partsys.h

#pragma once

#include <vector>

#include "vec.h"
#include "SDL_gpu.h"

struct PartSys {

	GPU_Image* texture;

	struct Particle {
		Particle() {}
		int sprite;
		vec pos;
		vec vel;
		float ttl;
		float scale;
		float rotation;
		float rotation_vel;
		float alpha;
		inline bool Update(float dt, const PartSys& system) {
			ttl -= dt;
			if (ttl < 0) return true;
			vel += system.acc * dt;
			pos += vel * dt;
			scale += system.scale_vel * dt;
			if (scale < 0.f) {
				if (system.scale_vel < 0.f) {
					return true;
				}
				else {
					scale = 0.0001f;
				}
			}
			rotation += rotation_vel * dt;
			alpha += system.alpha_vel * dt;
			if (alpha < 0.f) {
				if (system.alpha_vel < 0.f) {
					return true;
				} else {
					alpha = 0.f;
				}
			}
			else if (system.bounce_alpha > 0) {
				if (alpha > 2 * system.bounce_alpha) {
					return true;
				}
			} else if (alpha > 1.f) {
				alpha = 1.f;
			}
			return false;
		}
	};

	vec pos = vec();
	//float spawn_radius = 0.f; // TODO: Implement

	vec max_vel = vec();
	vec min_vel = vec();

	vec acc = vec();

	float min_ttl = 1.f;
	float max_ttl = 1.f;

	float min_interval = 0.2f;
	float max_interval = 0.2f;

	float min_scale = 1.f;
	float max_scale = 1.f;

	float scale_vel = 0.f;

	float min_rotation = 0.f;
	float max_rotation = 0.f;

	float min_rotation_vel = 0.f;
	float max_rotation_vel = 0.f;

	float alpha = 1.f;
	float alpha_vel = 0.f;

	float bounce_alpha = -1.f; //max alpha, at which it starts going back to 0

	float time = 0.f;

	PartSys(GPU_Image* t) {
		SetTexture(t);
		particles.reserve(80); //~3kb memory usage
	}
	void SetTexture(GPU_Image* t) { texture = t; }

	void AddSprite(const GPU_Rect& rect) {
		sprites.emplace_back(rect);
	}

	void Spawn(float dt) { SpawnWithExternalTimer(time, dt); }
	void SpawnWithExternalTimer(float& timer, float dt);
	void UpdateParticles(float dt); //Doesn't create new particles, use Spawn(), SpawnWithExternalTimer(), AddParticle() or AddParticles()
	void Draw() const;

	Particle& AddParticle();
	inline void AddParticles(int n) {
		while (n-- > 0) {
			AddParticle();
		}
	}

	void DrawImGUI(const char* title = "Particles");
	void Clear() { particles.clear(); }

	void FlipX() {
		float aux = max_vel.x;
		max_vel.x = -min_vel.x;
		min_vel.x = -aux;
		acc.x = -acc.x;
		aux = max_rotation_vel;
		max_rotation_vel = -min_rotation_vel;
		min_rotation_vel = -aux;
	}

	std::vector<Particle> particles;
private:
	mutable std::vector<GPU_Rect> sprites;
};