extern crate caper;

use caper::utils::demo;

fn main() {
    // Shader re-purposed from https://www.shadertoy.com/view/llt3R4
    demo(
        "
#version 330

const int MAX_MARCHING_STEPS = 255;
const float MIN_DIST = 0.0;
const float MAX_DIST = 100.0;
const float EPSILON = 0.0001;

uniform vec2 resolution;
uniform vec3 cam_pos;

in vec2 v_tex_coords;

out vec4 frag_output;

float sphereSDF(vec3 p, float s) {
    return length(p) - s;
}

float sceneSDF(vec3 p) {
    return sphereSDF(p, 1.0);
}

float shortestDistanceToSurface(vec3 eye, vec3 marchingDirection, float start, float end) {
    float depth = start;
    for (int i = 0; i < MAX_MARCHING_STEPS; i++) {
        float dist = sceneSDF(eye + depth * marchingDirection);
        if (dist < EPSILON) {
            return depth;
        }
        depth += dist;
        if (depth >= end) {
            return end;
        }
    }
    return end;
}

vec3 rayDirection(float fieldOfView, vec2 size, vec2 fragCoord) {
    vec2 xy = fragCoord - size / 2.0;
    float z = size.y / tan(radians(fieldOfView) / 2.0);
    return normalize(vec3(xy, -z));
}

void main() {
	vec3 dir = rayDirection(45.0, resolution, v_tex_coords * resolution);
    float dist = shortestDistanceToSurface(cam_pos, dir, MIN_DIST, MAX_DIST);

    if (dist > MAX_DIST - EPSILON) {
        // Didn't hit anything
        frag_output = vec4(0.0, 0.0, 0.0, 0.0);
        return;
    }

    frag_output = vec4(1.0, 0.0, 0.0, 1.0);
}
         ",
    );
}