/*{
  "DESCRIPTION": "Aurora over a mountain range, mirrored in a still rippling lake under a field of stars.",
  "CREDIT": "claude-opus-4-8",
  "INPUTS": [
    { "NAME": "speed",     "TYPE": "float", "DEFAULT": 0.5, "MIN": 0.0, "MAX": 2.0 },
    { "NAME": "bands",     "TYPE": "float", "DEFAULT": 5.0, "MIN": 2.0, "MAX": 9.0 },
    { "NAME": "intensity", "TYPE": "float", "DEFAULT": 0.8, "MIN": 0.2, "MAX": 1.6 },
    { "NAME": "hue",       "TYPE": "float", "DEFAULT": 0.4, "MIN": 0.0, "MAX": 1.0 }
  ]
}*/

const float HY = 0.46;   // horizon height (uv.y)

float hash(vec2 p){ p = fract(p * vec2(123.34, 456.21)); p += dot(p, p + 45.32); return fract(p.x * p.y); }
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, 0.0)), c = hash(i + vec2(0.0, 1.0)), d = hash(i + vec2(1.0, 1.0));
  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 = p * 2.0 + 3.0; a *= 0.5; } return s; }
vec3 palette(float t){ return 0.5 + 0.5 * cos(6.28318 * (t + vec3(0.0, 0.33, 0.67) + hue)); }

// mountain top height above the horizon for column x (two stacked ridges)
float ridge(float x){
  float far  = 0.06 + 0.05 * fbm(vec2(x * 2.2 + 11.0, 0.0));
  float near = 0.02 + 0.11 * fbm(vec2(x * 1.4 - 5.0, 7.0));
  return max(far, near);
}

vec3 auroraCol(float x, float y, float t){
  vec3 c = vec3(0.0);
  for (int i = 0; i < 9; i++){
    if (float(i) >= bands) break;
    float fi = float(i);
    float yC = 0.64 + 0.07 * fi;
    float wob = 0.10 * noise(vec2(x * 1.5 + fi, t * 0.5 + fi * 3.0)) + 0.04 * sin(x * 3.0 + t + fi);
    float d = abs(y - (yC + wob));
    float curt = exp(-d * 15.0);
    curt *= 0.55 + 0.45 * noise(vec2(x * 22.0, t + fi * 5.0));   // vertical streaks
    c += curt * palette(0.12 * fi + 0.3 + 0.1 * x) * intensity;
  }
  return c * smoothstep(HY - 0.02, HY + 0.12, y);
}

// the scene above the horizon (sky + stars + aurora + mountains) at coordinate (x, y)
vec3 sceneAbove(vec2 q, float t){
  float x = q.x, y = q.y;
  vec3 col = mix(vec3(0.03, 0.04, 0.10), vec3(0.0, 0.0, 0.015), clamp((y - 0.4) / 0.6, 0.0, 1.0));
  // stars (upper sky)
  vec2 g = floor(vec2(x, y) * 130.0);
  float h = hash(g);
  col += smoothstep(0.986, 1.0, h) * (0.6 + 0.4 * sin(t * 3.0 + h * 40.0)) * smoothstep(0.5, 0.75, y);
  // aurora
  col += auroraCol(x, y, t);
  // mountains (solid below the ridge outline)
  float top = HY + ridge(x);
  if (y < top){
    float k = top - y;                                   // depth into the mountain body
    vec3 mnt = mix(vec3(0.05, 0.06, 0.11), vec3(0.01, 0.01, 0.03), smoothstep(0.0, 0.16, k));
    mnt += auroraCol(x, top, t) * 0.35 * exp(-k * 26.0); // aurora rim-light on the peaks
    col = mnt;
  }
  return col;
}

void main(){
  vec2 uv = gl_FragCoord.xy / RENDERSIZE;
  float asp = RENDERSIZE.x / RENDERSIZE.y;
  float x = (uv.x - 0.5) * asp;
  float t = TIME * speed;

  vec3 col;
  if (uv.y >= HY){
    col = sceneAbove(vec2(x, uv.y), t);
  } else {
    // lake: mirror the scene across the horizon with ripple distortion
    float depth = HY - uv.y;                              // 0 at shore -> grows downward
    float ripple = 0.010 * sin(uv.y * 55.0 - t * 2.0) * (0.3 + depth * 1.5);
    ripple += 0.006 * (fbm(vec2(x * 7.0, uv.y * 18.0 - t)) - 0.5);
    vec2 rc = vec2(x + ripple, 2.0 * HY - uv.y + ripple * 0.4);
    vec3 refl = sceneAbove(rc, t);
    refl *= vec3(0.62, 0.72, 0.92);                       // water tint
    refl = mix(refl, vec3(0.0, 0.02, 0.05), smoothstep(0.0, HY, depth) * 0.55);  // fade with depth
    float spark = pow(max(0.0, fbm(vec2(x * 26.0, uv.y * 26.0 - t * 3.0)) - 0.55), 2.0);
    refl += spark * 0.5 * vec3(0.7, 0.9, 1.0);            // specular glints
    col = refl;
  }

  col *= 1.0 - 0.25 * dot(uv - 0.5, uv - 0.5);            // vignette
  col = pow(max(col, 0.0), vec3(0.9));
  gl_FragColor = vec4(col, 1.0);
}