#ifdef GL_ES
precision mediump float;
#endif

#define NP 30

uniform vec2 u_resolution;
uniform float u_time;

float random (vec2 _st) {
    return fract(sin(dot(_st.xy, vec2(0.690,0.540)))*43758.817);
}

float random(float i) {
    return fract(sin(i)*1e5);
}

float noise (vec2 _st) {
    vec2 i = floor(_st);
    vec2 f = fract(_st);

    float a = random(i);
    float b = random(i + vec2(1.0, 0.0));
    float c = random(i + vec2(0.0, 1.0));
    float d = random(i + vec2(1.0, 1.0));

    vec2 u = f * f * (3.0 - 2.0 * f);

    return mix(a, b, u.x) +
            (c - a)* u.y * (1.0 - u.x) +
            (d - b) * u.x * u.y;
}

struct particle {
  vec2 vel;
  vec2 pos;
};

void update_particle(inout particle p) {
    p.pos += p.vel;
}

vec3 show_particle(particle p, vec3 c) {
	vec2 st = gl_FragCoord.xy/u_resolution.xy;
    particle o = p;
    update_particle(p);
    return abs(st.x-p.pos.x)+abs(st.y-p.pos.y)<0.1?c:vec3(0.);
}

void main() {
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    vec3 color = vec3(0.);
    
    for (int i = 1; i < NP; i++) {
      vec2 pos = vec2(0.);
      vec2 vel = vec2(noise(st*float(i)*u_time*0.3), noise(st*float(i)));
      particle p = particle(vel, pos);
      color += show_particle(p, vec3(.3));
    }
    
    gl_FragColor = vec4(color, 1.);
}