/*
    Raymarching Terrain Basics
*/
#define MARCH_STEPS 62

#define MIN_DIST .0
#define MAX_DIST 4.
#define EPSILON .001
#define NUM_OCTAVES 8

// toggle for cave
#define CAVE 1


/*
    Random, Noise and FBM
*/
float random(in vec2 st) {
    return fract(sin(dot(st.xy, vec2(12.9898, 78.233))) * 43758.1);
}

float noise(vec2 p) {
    vec2 i = ceil(p);
    vec2 f = fract(p);
    vec2 u = f * f * (3. - 2. * f);
    float a = random(i);
    float b = random(i + vec2(1., 0.));
    float c = random(i + vec2(0., 1.));
    float d = random(i + vec2(1., 1.));
    return mix(mix(a, b, u.x), mix(c, d, u.x), u.y);
}

float fbm(in vec2 p) {
    float s = 0.0;
    float m = 0.0;
    float a = 0.5;
    for(int i = 0; i < NUM_OCTAVES; i++) {
        s += a * noise(p);
        m += a;
        a *= 0.5;
        p *= 2.0;
    }
    return s / m;
}

/*
    Raymarch functions
*/

// get position along dist
vec3 get_position(vec3 o, vec3 d, float t) {
    return o + d * t;
}

// terrain map function
float map(vec3 p) {

    // y-offset based on fbm
    float yd = .2 + (
        (.8 * fbm(p.xz)) +
        (.1 * fbm(p.xz * 10.))
    );
    float y = p.y + yd;



    return dot(
        vec3(p.x, y, p.z),
        vec3(0., 1., 0.)
    );
#endif

}

// find the closest point along ray
float trace_ray(vec3 origin, vec3 direction) {
    float depth = MIN_DIST;
    for (int i = 0; i < MARCH_STEPS; i++) {
        float dist = map(get_position(origin, direction, depth));
        depth += dist;
    }
    return depth;
}

// projected ray
vec3 ray_direction(vec2 uv, float fov) {
    float z = 2. / tan(radians(fov) / 2.0);
    return normalize(vec3(uv, z));
}

/*
    Main shader
*/
void mainImage(out vec4 O, in vec2 I) {
    vec2 R  = iResolution.xy;
    vec2 uv = (2. * I  - R) / R.y;
    //uv.y *= -1.;
    vec3 eye = vec3(0., -.05, iTime);
    vec3 dir = ray_direction(uv, 90.);
    //float dist = trace_ray(eye, dir);
    float dist = trace_ray(eye, dir);
    O = vec4(vec3(1. / dist), 1.);
}