/*{
  "DESCRIPTION": "A druse of quartz: a cluster of hexagonal crystal points fanning out from a common base, raymarched from exact hex-prism distance fields. Translucent amethyst bodies with bright Fresnel edges, thin-film iridescence sliding across the facets, and a violet inner glow. The cluster turns once for a seamless loop.",
  "CREDIT": "glslop agent (Claude)",
  "CATEGORIES": ["crystal", "raymarch", "mineral", "iridescence", "3d"],
  "INPUTS": [
    { "NAME": "speed",  "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.0, "MAX": 3.0 },
    { "NAME": "hue",    "TYPE": "float", "DEFAULT": 0.74, "MIN": 0.0, "MAX": 1.0 },
    { "NAME": "irid",   "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.0, "MAX": 2.0 },
    { "NAME": "glowAmt","TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.0, "MAX": 2.0 },
    { "NAME": "count",  "TYPE": "long",  "DEFAULT": 7,    "MIN": 3,   "MAX": 9 }
  ]
}*/

#define PI  3.14159265359
#define TAU 6.28318530718
#define GA  2.39996323
const float PERIOD = 16.0;

vec3 pal(float t){ return 0.5 + 0.5*cos(TAU*(t + vec3(0.0, 0.33, 0.66))); }

mat3 rotY(float a){ float c=cos(a), s=sin(a); return mat3(c,0,-s, 0,1,0, s,0,c); }

// rotation mapping local +z onto world direction d (columns = basis)
mat3 align(vec3 d){
  vec3 z = normalize(d);
  vec3 up = abs(z.y) < 0.98 ? vec3(0.0,1.0,0.0) : vec3(1.0,0.0,0.0);
  vec3 x = normalize(cross(up, z));
  vec3 y = cross(z, x);
  return mat3(x, y, z);
}

// IQ hex prism: hexagon in xy, extruded along z; h = (radius, halfDepth)
float sdHexPrism(vec3 p, vec2 h){
  const vec3 k = vec3(-0.8660254, 0.5, 0.57735);
  p = abs(p);
  p.xy -= 2.0*min(dot(k.xy, p.xy), 0.0)*k.xy;
  vec2 d = vec2(length(p.xy - vec2(clamp(p.x, -k.z*h.x, k.z*h.x), h.x))*sign(p.y - h.x),
                p.z - h.y);
  return min(max(d.x, d.y), 0.0) + length(max(d, 0.0));
}

// one crystal point: hex column carved to a hexagonal pyramid tip at +z
float crystal(vec3 q, float len, float rad){
  float apex = len + rad*1.9;
  float body = sdHexPrism(q - vec3(0.0, 0.0, apex*0.5), vec2(rad, apex*0.5));
  float ax = apex - q.z;                               // depth below the apex
  float cone = length(q.xy) - max(0.0, ax)*0.60;       // allowed radius shrinks to the point
  return max(body, cone);
}

float map(vec3 p){
  float d = 1e9;
  for(int i = 0; i < 9; i++){
    if(i >= int(count)) break;
    float fi = float(i);
    float tilt = 0.10 + 0.62*fract(fi*0.37 + 0.13);     // lean from vertical (wide splay)
    float az   = fi*GA;
    vec3 dir = vec3(sin(tilt)*cos(az), cos(tilt), sin(tilt)*sin(az));
    float len = 0.85 + 0.55*fract(fi*0.61);
    float rad = 0.085 + 0.04*fract(fi*0.53);
    vec3 base = dir*0.12;                               // shared-ish base
    vec3 q = (p - base) * align(dir);                  // = transpose(align)*(p-base)
    d = min(d, crystal(q, len, rad));
  }
  return d;
}

vec3 normal(vec3 p){
  vec2 e = vec2(0.0012, 0.0);
  return normalize(vec3(
    map(p+e.xyy) - map(p-e.xyy),
    map(p+e.yxy) - map(p-e.yxy),
    map(p+e.yyx) - map(p-e.yyx)));
}

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

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

  mat3 spin = rotY(th);                                  // one turn -> seamless

  vec3 ro = spin * vec3(0.0, 0.45, 2.35);
  vec3 ta = vec3(0.0, 0.62, 0.0);
  vec3 fw = normalize(ta - ro);
  vec3 rt = normalize(cross(vec3(0,1,0), fw));
  vec3 up = cross(fw, rt);
  vec3 rd = normalize(uv.x*rt + uv.y*up + 2.0*fw);

  float t = 0.0;
  float glow = 0.0;
  bool hit = false;
  for(int i = 0; i < 90; i++){
    vec3 p = ro + rd*t;
    float d = map(p);
    glow += glowAmt * 0.012 / (1.0 + 28.0*d*d);          // volumetric edge glow
    if(d < 0.0006*t){ hit = true; break; }
    t += d*0.8;
    if(t > 8.0) break;
  }

  vec3 col = vec3(0.0);
  vec3 bg = mix(vec3(0.02,0.015,0.045), vec3(0.005,0.005,0.02), uv.y*0.5+0.5);
  col = bg;

  if(hit){
    vec3 p = ro + rd*t;
    vec3 n = normal(p);
    float fres = pow(1.0 - max(dot(n, -rd), 0.0), 2.5);

    vec3 body = vec3(0.42, 0.20, 0.72);                  // amethyst body
    vec3 irc  = 0.5 + 0.5*cos(TAU*(0.6 + 0.6*fres + 0.4*n.y) + vec3(0.0, 1.8, 3.4));
    col  = body * (0.5 + 0.7*(0.5+0.5*n.y));             // lit body
    col += vec3(0.45, 0.22, 0.85) * (1.0 - fres) * 0.55; // violet inner translucency
    col += irc * (0.20 + 0.7*fres) * irid;               // thin-film on the facets
    col += vec3(1.0, 0.96, 1.0) * pow(fres, 2.0) * 0.8;  // bright facet edges
    float sheen = max(dot(n, normalize(vec3(0.5, 0.85, 0.35))), 0.0);
    col += vec3(0.95, 0.85, 1.0) * pow(sheen, 16.0) * 0.9; // sharp specular glints
  }

  col += vec3(0.5, 0.26, 0.95) * glow * 1.1;             // violet inner/edge glow

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