// "Fireworks" by Martijn Steinrucken aka BigWings - 2015
// License Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

#define PI 3.141592653589793238
#define TWOPI 6.283185307179586
#define S(x,y,z) smoothstep(x,y,z)
#define B(x,y,z,w) S(x-z, x+z, w)*S(y+z, y-z, w)
#define saturate(x) clamp(x,0.,1.)

#define NUM_EXPLOSIONS 8.
#define NUM_PARTICLES 70.


// Noise functions by Dave Hoskins 
#define MOD3 vec3(.1031,.11369,.13787)
vec3 hash31(float p) {
   vec3 p3 = fract(vec3(p) * MOD3);
   p3 += dot(p3, p3.yzx + 19.19);
   return fract(vec3((p3.x + p3.y)*p3.z, (p3.x+p3.z)*p3.y, (p3.y+p3.z)*p3.x));
}
float hash12(vec2 p){
	vec3 p3  = fract(vec3(p.xyx) * MOD3);
    p3 += dot(p3, p3.yzx + 19.19);
    return fract((p3.x + p3.y) * p3.z);
}

float circ(vec2 uv, vec2 pos, float size) {
	uv -= pos;
    
    size *= size;
    return S(size*1.1, size, dot(uv, uv));
}

float light(vec2 uv, vec2 pos, float size) {
	uv -= pos;
    
    size *= size;
    return size/dot(uv, uv);
}

vec3 explosion(vec2 uv, vec2 p, float seed, float t) {
	
    vec3 col = vec3(0.);
    
    vec3 en = hash31(seed);
    vec3 baseCol = en;
    for(float i=0.; i<NUM_PARTICLES; i++) {
    	vec3 n = hash31(i)-.5;
       
		vec2 startP = p-vec2(0., t*t*.1);        
        vec2 endP = startP+normalize(n.xy)*n.z;
        
        
        float pt = 1.-pow(t-1., 2.);
        vec2 pos = mix(p, endP, pt);    
        float size = mix(.01, .005, S(0., .1, pt));
        size *= S(1., .1, pt);
        
        float sparkle = (sin((pt+n.z)*100.)*.5+.5);
        sparkle = pow(sparkle, pow(en.x, 3.)*50.)*mix(0.01, .01, en.y*n.y);
      
    	//size += sparkle*B(.6, 1., .1, t);
        size += sparkle*B(en.x, en.y, en.z, t);
        
        col += baseCol*light(uv, pos, size);
    }
    
    return col;
}

vec3 Rainbow(vec3 c) {
	
    float t=iTime;
    
    float avg = (c.r+c.g+c.b)/3.;
    c = avg + (c-avg)*sin(vec3(0., .333, .666)+t);
    
    c += sin(vec3(.4, .3, .3)*t + vec3(1.1244,3.43215,6.435))*vec3(.4, .1, .5);
    
    return c;
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
	vec2 uv = fragCoord.xy / iResolution.xy;
	uv.x -= .5;
    uv.x *= iResolution.x/iResolution.y;
    
    float n = hash12(uv+10.);
    float t = iTime*.5;
    
    vec3 c = vec3(0.);
    
    for(float i=0.; i<NUM_EXPLOSIONS; i++) {
    	float et = t+i*1234.45235;
        float id = floor(et);
        et -= id;
        
        vec2 p = hash31(id).xy;
        p.x -= .5;
        p.x *= 1.6;
        c += explosion(uv, p, id, et);
    }
    c = Rainbow(c);
    
    fragColor = vec4(c, 1.);
}