/*{
  "DESCRIPTION": "An Earth-like world from orbit: an analytic sphere clothed in fbm continents and oceans, with a day/night terminator, a scatter of amber city lights across the dark hemisphere, a drifting cloud layer, and a soft blue atmosphere halo glowing at the limb. The planet turns once on a tilted axis for a seamless loop, hanging in a field of stars.",
  "CREDIT": "glslop agent (Claude)",
  "CATEGORIES": ["planet", "space", "procedural", "atmosphere", "3d"],
  "INPUTS": [
    { "NAME": "speed", "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.0, "MAX": 3.0 },
    { "NAME": "sea",   "TYPE": "float", "DEFAULT": 0.52, "MIN": 0.3, "MAX": 0.7 },
    { "NAME": "clouds","TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.0, "MAX": 2.0 },
    { "NAME": "zoom",  "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.6, "MAX": 1.8 }
  ]
}*/

#define PI  3.14159265359
#define TAU 6.28318530718
const float PERIOD = 16.0;

float hash3(vec3 p){ return fract(sin(dot(p, vec3(127.1, 311.7, 74.7))) * 43758.5453); }
float noise3(vec3 p){
  vec3 i = floor(p), f = fract(p);
  f = f*f*(3.0 - 2.0*f);
  float n000 = hash3(i+vec3(0,0,0)), n100 = hash3(i+vec3(1,0,0));
  float n010 = hash3(i+vec3(0,1,0)), n110 = hash3(i+vec3(1,1,0));
  float n001 = hash3(i+vec3(0,0,1)), n101 = hash3(i+vec3(1,0,1));
  float n011 = hash3(i+vec3(0,1,1)), n111 = hash3(i+vec3(1,1,1));
  return mix(mix(mix(n000,n100,f.x), mix(n010,n110,f.x), f.y),
             mix(mix(n001,n101,f.x), mix(n011,n111,f.x), f.y), f.z);
}
float fbm3(vec3 p){
  float s = 0.0, a = 0.5;
  for(int i = 0; i < 6; i++){ s += a*noise3(p); p = p*2.03 + 11.1; a *= 0.5; }
  return s;
}
mat3 rotY(float a){ float c=cos(a), s=sin(a); return mat3(c,0,-s, 0,1,0, s,0,c); }
mat3 rotX(float a){ float c=cos(a), s=sin(a); return mat3(1,0,0, 0,c,-s, 0,s,c); }

void main(){
  vec2 uv = (2.0*gl_FragCoord.xy - RENDERSIZE.xy) / RENDERSIZE.y;
  uv /= zoom;

  float ph = fract(TIME * speed / PERIOD);
  float th = TAU * ph;

  vec3 ro = vec3(0.0, 0.0, 3.0);
  vec3 rd = normalize(vec3(uv, -1.7));
  vec3 sun = normalize(vec3(0.75, 0.35, 0.45));

  float R = 1.0;
  float b = dot(ro, rd);
  float c = dot(ro, ro) - R*R;
  float disc = b*b - c;

  vec3 col = vec3(0.0);

  // starfield background
  vec3 sdir = rd;
  float st = hash3(floor(sdir*220.0));
  col += vec3(0.8,0.85,1.0) * smoothstep(0.9965, 1.0, st) * 0.9;
  col += vec3(0.02,0.02,0.05);

  mat3 spin = rotX(0.42) * rotY(-th);                   // tilted axis, one turn

  if(disc > 0.0){
    float t = -b - sqrt(disc);
    vec3 pos = ro + rd*t;
    vec3 n = normalize(pos);
    vec3 sp = spin * n;                                  // rotated surface coords

    // terrain
    float h = fbm3(sp*2.4);
    float land = smoothstep(sea-0.02, sea+0.04, h);
    float elev = (h - sea) / max(0.001, 1.0 - sea);

    vec3 ocean = mix(vec3(0.01,0.05,0.18), vec3(0.04,0.22,0.42), smoothstep(0.0,1.0,h/sea));
    vec3 lowland = vec3(0.10, 0.32, 0.10);
    vec3 highland= vec3(0.36, 0.30, 0.16);
    vec3 snow    = vec3(0.9, 0.92, 0.95);
    vec3 ground = mix(lowland, highland, smoothstep(0.1, 0.5, elev));
    ground = mix(ground, snow, smoothstep(0.55, 0.8, elev) + smoothstep(0.75, 0.95, abs(sp.y)));
    vec3 surf = mix(ocean, ground, land);

    // clouds (second layer, drifts a touch faster -> integer multiple to loop)
    vec3 cp = rotY(-th*2.0) * (rotX(0.42)*n);
    float cl = smoothstep(0.55, 0.85, fbm3(cp*3.1 + 5.0)) * clouds;

    float diff = dot(n, sun);
    float day = smoothstep(-0.1, 0.25, diff);

    // specular sun glint on oceans
    vec3 h2 = normalize(sun - rd);
    float spec = pow(max(dot(n, h2), 0.0), 40.0) * (1.0 - land);

    vec3 lit = surf * (0.05 + 1.05*max(diff, 0.0));
    lit += vec3(1.0, 0.95, 0.8) * spec * day * 1.2;
    lit = mix(lit, vec3(0.95,0.97,1.0)*(0.1+max(diff,0.0)), cl);   // clouds catch light

    // night side: amber city lights on land
    float night = smoothstep(0.15, -0.15, diff);
    float cityMask = step(0.86, hash3(floor(sp*60.0))) * land;
    lit += vec3(1.0, 0.7, 0.3) * cityMask * night * 1.4;
    lit += vec3(0.6,0.3,0.1) * land * night * 0.03;

    // atmosphere rim (fresnel) - blue glow toward the limb, brightest on day side
    float rim = pow(1.0 - max(dot(n, -rd), 0.0), 3.0);
    lit += vec3(0.25, 0.5, 1.0) * rim * (0.3 + 0.9*day) * 1.3;

    col = lit;
  } else {
    // atmospheric halo just outside the disc
    float perp = sqrt(max(0.0, dot(ro,ro) - b*b));       // ray closest distance to centre
    float halo = smoothstep(1.35, R, perp) * step(R, perp);
    // light the halo on the sunlit limb
    vec3 limbDir = normalize(ro + rd*(-b));
    float lit = smoothstep(-0.2, 0.6, dot(limbDir, sun));
    col += vec3(0.2, 0.45, 1.0) * halo * (0.2 + 0.9*lit) * 1.1;
  }

  col = col / (1.0 + col);
  col = pow(col, vec3(0.85));
  gl_FragColor = vec4(col, 1.0);
}