// Author @patriciogv - 2015
// http://patriciogonzalezvivo.com
#ifdef GL_ES
precision mediump float;
#endif

#define PI 3.14159265358979323846

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);
}

vec2 truchetPattern(in vec2 _st, in float _index){
    _index = fract(((_index-0.5)*2.0));
    if (_index > 0.75) {
        _st = vec2(1.0) - _st;
    } else if (_index > 0.5) {
        _st = vec2(1.0-_st.x,_st.y);
    } else if (_index > 0.25) {
        _st = 1.0-vec2(1.0-_st.x,_st.y);
    }
    return _st;
}

void main() {
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    st *= 10.0;
    // st = (st-vec2(5.0))*(abs(sin(u_time*0.2))*5.);
    // st.x += u_time*3.0;

    vec2 ivec = floor(st);  // integer
    vec2 fvec = fract(st);  // fraction

    vec2 tile = truchetPattern(fvec, random( ivec ));

    float color = 0.0;

    // Maze
    color = smoothstep(tile.x-0.3,tile.x,tile.y)-
            smoothstep(tile.x,tile.x+0.3,tile.y);

    // Circles
    // color = (step(length(tile),0.6) -
    //          step(length(tile),0.4) ) +
    //         (step(length(tile-vec2(1.)),0.6) -
    //          step(length(tile-vec2(1.)),0.4) );

    // Truchet (2 triangles)
    // color = step(tile.x,tile.y);

    gl_FragColor = vec4(vec3(color),1.0);
}