/*{
  "DESCRIPTION": "Ember flow — sparks streaked along a rising curl-noise heat field, traced by line-integral convolution.",
  "CREDIT": "claude-opus-4-8",
  "INPUTS": [
    { "NAME": "scale",    "TYPE": "float", "DEFAULT": 2.4, "MIN": 1.0, "MAX": 6.0 },
    { "NAME": "buoy",     "TYPE": "float", "DEFAULT": 0.8, "MIN": 0.0, "MAX": 2.0 },
    { "NAME": "contrast", "TYPE": "float", "DEFAULT": 1.6, "MIN": 0.6, "MAX": 3.0 },
    { "NAME": "heat",     "TYPE": "float", "DEFAULT": 1.0, "MIN": 0.4, "MAX": 1.8 }
  ]
}*/

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; }

vec2 curl(vec2 p){
  float e = 0.01;
  float a = fbm(p + vec2(0.0, e)), b = fbm(p - vec2(0.0, e));
  float c = fbm(p + vec2(e, 0.0)), d = fbm(p - vec2(e, 0.0));
  return vec2(a - b, d - c) / (2.0 * e);
}

vec2 field(vec2 q, float t){
  vec2 v = curl(q * 0.8 + vec2(0.0, -t * 0.3)) * 0.8;
  v += vec2(0.0, buoy);             // heat rises
  return v;
}

vec3 fire(float h){
  h = clamp(h, 0.0, 1.0);
  vec3 c = mix(vec3(0.0), vec3(0.55, 0.04, 0.0), smoothstep(0.0, 0.3, h));
  c = mix(c, vec3(1.0, 0.35, 0.0), smoothstep(0.3, 0.6, h));
  c = mix(c, vec3(1.0, 0.85, 0.35), smoothstep(0.6, 0.86, h));
  c = mix(c, vec3(1.0, 1.0, 0.92), smoothstep(0.86, 1.0, h));
  return c;
}

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

  // line-integral convolution along the flow, both directions
  float acc = 0.0, wsum = 0.0;
  vec2 pf = q, pb = q;
  for (int i = 0; i < 18; i++){
    float wi = 1.0 - float(i) / 18.0;
    vec2 ff = field(pf, t), fb = field(pb, t);
    pf += ff / (length(ff) + 1e-4) * 0.03;
    pb -= fb / (length(fb) + 1e-4) * 0.03;
    acc += wi * noise(pf * 10.0 + vec2(0.0, t * 2.0));
    acc += wi * noise(pb * 10.0 + vec2(0.0, t * 2.0));
    wsum += 2.0 * wi;
  }
  acc /= wsum;
  float h = pow(clamp((acc - 0.5) * contrast + 0.5, 0.0, 1.0), 1.4) * heat;
  h *= 0.7 + 0.5 * smoothstep(0.8, -0.6, uv.y);    // hotter at the base, cooler as it rises

  vec3 col = fire(h);
  col += vec3(0.1, 0.2, 0.6) * smoothstep(0.93, 1.0, h) * 0.5;   // blue at the hottest cores

  // flying ember sparks riding the heat upward
  vec2 sp = q * 3.0; sp.y += t * 1.6;
  vec2 sid = floor(sp);
  float sh = hash(sid);
  vec2 sc = sid + 0.5 + 0.32 * vec2(sin(sh * 30.0 + t * 2.0), cos(sh * 23.0 + t));
  float sd = length(sp - sc);
  float spark = smoothstep(0.11, 0.0, sd) * step(0.86, sh) * (0.5 + 0.5 * sin(t * 6.0 + sh * 50.0));
  spark *= smoothstep(0.15, 0.5, h);               // only where there's heat to throw them
  col += spark * vec3(1.0, 0.8, 0.45) * 1.6;

  // cool smoke haze drifting off the top
  col = mix(col, col * vec3(0.8, 0.55, 0.5) + vec3(0.02, 0.012, 0.015),
            smoothstep(0.1, 0.95, uv.y) * (1.0 - h) * 0.6);

  col *= 1.0 - 0.32 * dot(uv, uv);
  col = pow(max(col, 0.0), vec3(0.9));
  gl_FragColor = vec4(col, 1.0);
}