/*{
  "DESCRIPTION": "Borealis: curtains of aurora ripple and fold over a still night — green low, violet high — above a starfield and a dark mountain horizon with a faint mirror in the water below. Folds and shimmer run on a circular phase so it loops seamlessly over 16s.",
  "CREDIT": "glslop agent (Claude)",
  "CATEGORIES": ["aurora", "sky", "atmospheric", "neon", "landscape"],
  "INPUTS": [
    { "NAME": "speed",  "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.0, "MAX": 3.0 },
    { "NAME": "glow",   "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.3, "MAX": 3.0 },
    { "NAME": "hue",    "TYPE": "float", "DEFAULT": 0.0,  "MIN": 0.0, "MAX": 1.0 },
    { "NAME": "curtains","TYPE": "long", "DEFAULT": 3,    "MIN": 1,   "MAX": 6 },
    { "NAME": "zoom",   "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.6, "MAX": 1.6 }
  ]
}*/

#define PI  3.14159265359
#define TAU 6.28318530718
#define MAXC 6

const float PERIOD = 16.0;

float hash(vec2 p){ return fract(sin(dot(p, vec2(127.1, 311.7))) * 43758.5453); }
float noise(vec2 p){
  vec2 i = floor(p), f = fract(p); f = f * f * (3.0 - 2.0 * f);
  float a = hash(i), b = hash(i + vec2(1, 0)), c = hash(i + vec2(0, 1)), d = hash(i + vec2(1, 1));
  return mix(mix(a, b, f.x), mix(c, d, f.x), f.y);
}
float fbm(vec2 p){ float s = 0.0, a = 0.5; for (int i = 0; i < 4; i++){ s += a * noise(p); p *= 2.0; a *= 0.5; } return s; }

void main(){
  vec2 uv = (gl_FragCoord.xy - 0.5 * RENDERSIZE.xy) / RENDERSIZE.y;
  uv /= zoom;

  float ph = fract(TIME * speed / PERIOD);
  float th = TAU * ph;
  vec2 orb = 0.5 * vec2(cos(th), sin(th));     // circular phase -> seamless noise scroll

  // --- night sky gradient + stars ---
  vec3 col = mix(vec3(0.02, 0.03, 0.07), vec3(0.0, 0.01, 0.03), uv.y * 0.5 + 0.5);
  vec2 sp = uv * 12.0;
  vec2 sg = floor(sp);
  float sh = hash(sg);
  float star = smoothstep(0.10, 0.0, length(fract(sp) - vec2(hash(sg + 1.0), hash(sg + 2.0))));
  col += vec3(0.9, 0.95, 1.0) * star * step(0.88, sh) * (0.6 + 0.4 * sin(th * 3.0 + sh * 30.0));

  // --- aurora curtains ---
  vec3 aur = vec3(0.0);
  for (int i = 0; i < MAXC; i++){
    if (i >= curtains) break;
    float fi = float(i);
    // the curtain's horizontal centre folds with height (seamless ripple)
    float fold = 0.35 * sin(uv.y * 1.6 + th + fi * 2.1)
               + 0.18 * sin(uv.y * 3.7 - th * 2.0 + fi);
    float cx = -0.1 + 0.28 * (fi - float(curtains - 1) * 0.5) + fold;
    // vertical light streaks within the curtain (sharper = more filamentary)
    float streak = fbm(vec2((uv.x - cx) * 7.0 + fi * 9.0, uv.y * 1.4) + orb);
    streak = pow(streak, 1.7) + 0.5 * pow(streak, 4.0);
    // horizontal envelope (how wide the curtain is) and a soft top
    float env = exp(-pow((uv.x - cx) / 0.24, 2.0));
    float vfade = smoothstep(1.0, -0.45, uv.y) * smoothstep(-0.55, -0.15, uv.y);  // sits above horizon
    float a = streak * env * vfade;
    // colour: vivid green low -> cyan -> violet/magenta high
    float hgt = clamp(uv.y * 0.85 + 0.45, 0.0, 1.0);
    vec3 c = mix(vec3(0.1, 1.0, 0.35), vec3(0.55, 0.1, 1.0), hgt);
    c = mix(c, vec3(1.0, 0.25, 0.7), smoothstep(0.55, 1.0, hgt) * 0.7);
    aur += c * a;
  }
  col += aur * 2.6 * glow;

  // --- dark mountain horizon + faint water mirror ---
  float horizon = -0.28 + 0.04 * fbm(vec2(uv.x * 2.0 + 3.0, 0.0)) - 0.02;
  float land = smoothstep(0.01, -0.01, uv.y - horizon);
  // water reflection of the aurora (below horizon, dimmed + rippled)
  if (uv.y < horizon){
    vec2 ruv = vec2(uv.x, 2.0 * horizon - uv.y);
    ruv.x += 0.02 * sin(uv.y * 30.0 + th);
    float rh = clamp(ruv.y * 0.9 + 0.4, 0.0, 1.0);
    vec3 rc = mix(vec3(0.15, 1.0, 0.45), vec3(0.5, 0.15, 0.95), rh);
    float renv = 0.0;
    for (int i = 0; i < MAXC; i++){ if (i >= curtains) break; float fi = float(i);
      float fold = 0.35 * sin(ruv.y * 1.6 + th + fi * 2.1) + 0.18 * sin(ruv.y * 3.7 - th * 2.0 + fi);
      float cx = -0.1 + 0.28 * (fi - float(curtains - 1) * 0.5) + fold;
      renv += exp(-pow((ruv.x - cx) / 0.22, 2.0)) * pow(fbm(vec2((ruv.x - cx) * 6.0 + fi * 9.0, ruv.y * 1.6) + orb), 2.2);
    }
    col += rc * renv * 0.18 * glow * smoothstep(horizon - 0.5, horizon, uv.y);
  }
  col = mix(col, vec3(0.005, 0.01, 0.02), land);   // mountains in shadow

  col = col / (1.0 + col);
  col = pow(col, vec3(0.85));
  col *= 1.0 - 0.18 * dot(uv * zoom, uv * zoom);
  gl_FragColor = vec4(col, 1.0);
}