#ifdef GL_ES
precision highp float;
#endif

// These are defined by Kodelife, or whatever environment you are using.
uniform float time;
uniform vec2 resolution;

varying vec3 v_normal;
varying vec2 v_texcoord;

// Define some constants
const int steps = 128; // This is the maximum amount a ray can march.
const float smallNumber = 0.001;
const float maxDist = 10.; // This is the maximum distance a ray can travel.

float scene(vec3 position){
    // So this is different from the sphere equation above in that I am
    // splitting the position into its three different positions
    // and adding a 10th of a cos wave to the x position so it oscillates left
    // to right and a (positive) sin wave to the z position
    // so it will go back and forth.
    float sphere = length(
        vec3(
            position.x + cos(time)/10.,
            position.y,
            position.z+ sin(time) +1.)
        )-0.5;

    // This is different from the ground equation because the UV is only
    // between -1 and 1 we want more than 1/2pi of a wave per length of the
    // screen so we multiply the position by a factor of 10 inside the trig
    // functions. Since sin and cos oscillate between -1 and 1, that would be
    // the entire height of the screen so we divide by a factor of 10.
    float ground = position.y + sin(position.x * 10.) / 10.
                              + cos(position.z * 10.) / 10. + 1.;

    // We want to return whichever one is closest to the ray, so we return the
    // minimum distance.
    return min(sphere,ground);
}

vec4 trace (vec3 origin, vec3 direction){

    float dist = 0.;
    float totalDistance = 0.;
    vec3 positionOnRay = origin;

    for(int i = 0 ; i < steps; i++){

        dist = scene(positionOnRay);

        // Advance along the ray trajectory the amount that we know the ray
        // can travel without going through an object.
        positionOnRay += dist * direction;

        // Total distance is keeping track of how much the ray has traveled
        // thus far.
        totalDistance += dist;

        // If we hit an object or are close enough to an object,
        if (dist < smallNumber){
            // return the distance the ray had to travel normalized so be white
            // at the front and black in the back.
            return 1. - (vec4(totalDistance) / maxDist);

        }

        if (totalDistance > maxDist){

            return vec4(0.); // Background color.
        }
    }

    return vec4(0.);// Background color.
}

// main is a reserved function that is going to be called first
void main(void)
{
    // We are redefining the UV coordinates (aka texcoords) to be 0,0 in the
    // middle of the screen this is because its easier to work with the camera at
    // (0,0) instead of (0.5,0.5) for the SDFs
    vec2 uv = -1. + 2. * v_texcoord;

    // Unfortunately our screens are not square so we must account for that.
    uv.x *= (resolution.x / resolution.y);

    vec3 rayOrigin = vec3(uv, 0.);
    vec3 camOrigin = vec3(0., 0., -1.);
    vec3 direction = camOrigin + rayOrigin;

    // This reserved variable is what we must set the final color to
    gl_FragColor = trace(rayOrigin, direction);
}