// Author:
// Title:

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform float u_time;

float random (vec2 st) {
    return fract(sin(0.00175*3.14 + dot(st.xy,
                                                vec2(12.9898,78.233)))*
            43758.5453);
}

void main() {
    vec2 uv = gl_FragCoord.xy/u_resolution.xy;
    // to make it maintain the same scale for all resolutons (probably)
    // uv.x *= u_resolution.x/u_resolution.y;

    // count of squares
    float count = 3.0;
    // so every square behaves like original UV
    vec2 cell = fract(uv * count);

    // Angle vector, just for getting different rotation
    // angle for every square.
    vec2 angle_v = ceil(uv*count); // it will be (0,1), (2,1), etc

    float angle = 0.0;

    // trying to get 1D value from 2D vector
    angle = (angle_v.x + angle_v.y);

    // angle = random(angle_v);
    // if (angle <= 0.25) {
    //     angle = 0.0;
    // } else if (angle <= 0.5) {
    //     angle = 3.14 * 0.5;
    // } else if (angle <= 0.75) {
    //     angle = 3.14;
    // } else {
    //     angle = 3.14 * 1.5;
    // }

    // animate angle
    angle *= sin(3.14*u_time*0.25);

    // rotate cell
    // 1. translate to new origin
    cell = (cell - 0.5);
    // 2. rotate vector
    cell = vec2(
        cell.x*cos(angle) - cell.y*sin(angle)
        , cell.x*sin(angle) + cell.y*cos(angle)
    );
    // 3. translate it back
    cell += 0.5;

    vec3 color = vec3(0.);
    color = vec3(cell.x,cell.y,0.0);

    gl_FragColor = vec4(color,1.0);
}