const float IOR_AIR = 1.0;
const float IOR_WATER = 1.333;

float poolHeight = 1.0;
uniform vec3 light;
uniform vec3 sphereCenter;
uniform float sphereRadius;
uniform sampler2D tiles;
uniform sampler2D causticTex;
uniform sampler2D water;

vec2 intersectCube(vec3 origin, vec3 ray, vec3 cubeMin, vec3 cubeMax) {
    vec3 tMin = (cubeMin - origin) / ray;
    vec3 tMax = (cubeMax - origin) / ray;

    vec3 t1 = min(tMin, tMax);
    vec3 t2 = max(tMin, tMax);

    float tNear = max(max(t1.x, t1.y), t1.z);
    float tFar = min(min(t2.x, t2.y), t2.z);

    return vec2(tNear, tFar);
}

float intersectSphere(vec3 origin, vec3 ray, vec3 sphereCenter, float sphereRadius) {
    vec3 toSphere = origin - sphereCenter;

    float a = dot(ray, ray);
    float b = 2.0 * dot(toSphere, ray);
    float c = dot(toSphere, toSphere) - sphereRadius * sphereRadius;

    float discriminant = b * b - 4.0 * a * c;
    if (discriminant > 0.0) {
        float t = (-b - sqrt(discriminant)) / (2.0 * a);
        if (t > 0.0) {
            return t;
        }
    }

    return 1.0e6;
}

uniform sampler2D causticTex;
vec3 getSphereColor(vec3 point) {
    vec3 color = vec3(0.5);

    /* ambient occlusion with walls */
    color *= 1.0 - 0.9 / pow((1.0 + sphereRadius - abs(point.x)) / sphereRadius, 3.0);
    color *= 1.0 - 0.9 / pow((1.0 + sphereRadius - abs(point.z)) / sphereRadius, 3.0);
    color *= 1.0 - 0.9 / pow((point.y + 1.0 + sphereRadius) / sphereRadius, 3.0);

    /* caustics */
    vec3 sphereNormal = (point - sphereCenter) / sphereRadius;
    vec3 refractedLight = refract(-light, vec3(0.0, 1.0, 0.0), IOR_AIR / IOR_WATER);
    float diffuse = max(0.0, dot(-refractedLight, sphereNormal)) * 0.5;
    vec4 info = texture2D(water, point.xz * 0.5 + 0.5);
    if (point.y < info.r) {
        vec4 caustic = texture2D(causticTex, 0.75 * (point.xz - point.y * refractedLight.xz / refractedLight.y) * 0.5 + 0.5);
        diffuse *= caustic.r * 4.0;
    }
    color += diffuse;

    return color;
}

uniform sampler2D tiles;
vec3 getWallColor(vec3 point) {
    float scale = 0.5;

    vec3 wallColor;
    vec3 normal;
    if (abs(point.x) > 0.999) {
        wallColor = texture2D(tiles, point.yz * 0.5 + vec2(1.0, 0.5)).rgb;
        normal = vec3(-point.x, 0.0, 0.0);
    } else if (abs(point.z) > 0.999) {
        wallColor = texture2D(tiles, point.yx * 0.5 + vec2(1.0, 0.5)).rgb;
        normal = vec3(0.0, 0.0, -point.z);
    } else {
        wallColor = texture2D(tiles, point.xz * 0.5 + 0.5).rgb;
        normal = vec3(0.0, 1.0, 0.0);
    }

    scale /= length(point); /* pool ambient occlusion */
    scale *= 1.0 - 0.9 / pow(length(point - sphereCenter) / sphereRadius, 4.0); /* sphere ambient occlusion */

    /* caustics */
    vec3 refractedLight = -refract(-light, vec3(0.0, 1.0, 0.0), IOR_AIR / IOR_WATER);
    float diffuse = max(0.0, dot(refractedLight, normal));
    vec4 info = texture2D(water, point.xz * 0.5 + 0.5);
    if (point.y < info.r) {
        vec4 caustic = texture2D(causticTex, 0.75 * (point.xz - point.y * refractedLight.xz / refractedLight.y) * 0.5 + 0.5);
        scale += diffuse * caustic.r * 2.0 * caustic.g;
    } else {
        /* shadow for the rim of the pool */
        vec2 t = intersectCube(point, refractedLight, vec3(-1.0, -poolHeight, -1.0), vec3(1.0, 2.0, 1.0));
        diffuse *= 1.0 / (1.0 + exp(-200.0 / (1.0 + 10.0 * (t.y - t.x)) * (point.y + refractedLight.y * t.y - 2.0 / 12.0)));

        scale += diffuse * 0.5;
    }

    return wallColor * scale;
}

uniform samplerCube sky;
vec3 getSurfaceRayColor(vec3 origin, vec3 ray, vec3 waterColor) {
    vec3 color;
    float q = intersectSphere(origin, ray, sphereCenter, sphereRadius);

    if (q < 1.0e6) {
        color = getSphereColor(origin + ray * q);
    } else if (ray.y < 0.0) {
        vec2 t = intersectCube(origin, ray, vec3(-1.0, -poolHeight, -1.0), vec3(1.0, 2.0, 1.0));
        color = getWallColor(origin + ray * t.y);
    } else {
        vec2 t = intersectCube(origin, ray, vec3(-1.0, -poolHeight, -1.0), vec3(1.0, 2.0, 1.0));
        vec3 hit = origin + ray * t.y;
        if (hit.y < 2.0 / 12.0) {
        color = getWallColor(hit);
        } else {
        color = textureCube(sky, ray).rgb;
        color += vec3(pow(max(0.0, dot(light, ray)), 5000.0)) * vec3(10.0, 8.0, 6.0);
        }
    }

    if (ray.y < 0.0) {
        color *= waterColor;
    }

    return color;
}