#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform float u_time;

float random (in vec2 _st) { 
    return fract(sin(dot(_st.xy,
                         vec2(12.9898,78.233)))* 
        43758.5453123);
}

// Based on Morgan McGuire @morgan3d
// https://www.shadertoy.com/view/4dS3Wd
float noise (in vec2 _st) {
    vec2 i = floor(_st);
    vec2 f = fract(_st);

    // Four corners in 2D of a tile
    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;
}

#define NUM_OCTAVES 5

float fbm ( in vec2 _st) {
    float v = 0.0;
    float a = 0.5;
    vec2 shift = vec2(100.0);
    // Rotate to reduce axial bias
    mat2 rot = mat2(cos(0.5),sin(0.5),
                    -sin(0.5),cos(0.50));
    for (int i = 0; i < NUM_OCTAVES; ++i) {
        v += a * noise(_st);
        _st = rot * _st * 2.0 + shift;
        a *= 0.5;
    }
    return v;
}

void main() {
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    vec3 color = vec3(0.0);

    vec2 pos = vec2(st*5.0+u_time);

    float noise = noise(pos);
    float fbm = fbm(pos);

    color = vec3( mix(noise, fbm, abs(sin(u_time))));

    gl_FragColor = vec4(color,1.0);
}