float map(vec3 p)
{
    return length(p) - 1.0;
}

float shadow(vec3 ro, vec3 rd)
{
    float res = 1.0;
    float d = 0.02;

    for(int i = 0; i < 32; i++)
    {
        vec3 p = ro + rd * d;

        float ds = map(p);

        res = min(res, 8.0 * ds / d);

        d += clamp(ds, 0.02, 0.2);
    }

    return clamp(res, 0.0, 1.0);
}

vec3 getNormal(vec3 p)
{
    vec2 e = vec2(0.001, 0.0);

    return normalize(vec3(
        map(p + e.xyy) - map(p - e.xyy),
        map(p + e.yxy) - map(p - e.yxy),
        map(p + e.yyx) - map(p - e.yyx)
    ));
}

void mainImage(out vec4 fragColor, in vec2 fragCoord)
{
    vec2 uv = fragCoord.xy / iResolution.xy;

    vec2 p = uv * 2.0 - 1.0;

    p.x *= iResolution.x / iResolution.y;

    vec3 ro = vec3(0.0, 0.0, -4.0);
    vec3 rd = normalize(vec3(p, 1.5));

    float d = 0.0;

    vec3 color = vec3(0.02, 0.04, 0.08);

    for(int i = 0; i < 64; i++)
    {
        vec3 pos = ro + rd * d;

        float ds = map(pos);

        if(ds < 0.001)
        {
            vec3 normal = getNormal(pos);

            vec3 lightDir =
                normalize(vec3(1.0, 1.0, -1.0));

            float diff =
                max(dot(normal, lightDir), 0.0);

            float sh =
                shadow(pos + normal * 0.01, lightDir);

            color += vec3(1.0, 0.3, 0.8)
                     * diff * sh;

            break;
        }

        d += ds;
    }

    fragColor = vec4(color, 1.0);
}