/*{
  "DESCRIPTION": "Digital rain: columns of procedurally-generated dot-matrix glyphs cascade down a black screen, each stream led by a bright white character with a long trail fading from phosphor green to black. The glyphs flicker and mutate as they fall. Every column falls an integer number of screen-heights per cycle so the downpour loops seamlessly.",
  "CREDIT": "glslop agent (Claude)",
  "CATEGORIES": ["matrix", "digital-rain", "text", "glyphs", "retro"],
  "INPUTS": [
    { "NAME": "speed", "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.0, "MAX": 3.0 },
    { "NAME": "cols",  "TYPE": "float", "DEFAULT": 26.0, "MIN": 12.0, "MAX": 48.0 },
    { "NAME": "trail", "TYPE": "float", "DEFAULT": 0.55, "MIN": 0.2, "MAX": 0.9 },
    { "NAME": "hue",   "TYPE": "float", "DEFAULT": 0.34, "MIN": 0.0, "MAX": 1.0 }
  ]
}*/

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

float h11(float x){ return fract(sin(x*127.1) * 43758.5453); }
float h21(vec2 p){ return fract(sin(dot(p, vec2(127.1, 311.7))) * 43758.5453); }
float h31(vec3 p){ return fract(sin(dot(p, vec3(127.1, 311.7, 74.7))) * 43758.5453); }

void main(){
  vec2 res = RENDERSIZE.xy;
  float cw  = res.x / cols;
  vec2  cpx = vec2(cw, cw*1.5);
  vec2  cell = floor(gl_FragCoord.xy / cpx);
  vec2  sub  = fract(gl_FragCoord.xy / cpx);

  float ph = fract(TIME * speed / PERIOD);
  float c  = cell.x;

  // falling stream for this column
  float spd   = 1.0 + floor(h11(c) * 3.0);            // integer screen-heights / loop
  float phase = h11(c + 9.3);
  float rp    = fract(ph*spd + phase);
  float yN    = gl_FragCoord.y / res.y;
  float yHead = 1.0 - rp;                             // head falls top -> bottom
  float dAbove = fract(yN - yHead);                   // 0 at head, grows up the trail
  float trailLen = trail * (0.7 + 0.5*h11(c + 4.0));
  float bright  = pow(smoothstep(trailLen, 0.0, dAbove), 1.3) * (0.7 + 0.6*h11(c+2.1));
  float headness = smoothstep(0.07, 0.0, dAbove);

  // flickering dot-matrix glyph (5x7)
  float gRate  = floor(3.0 + h11(c + 1.7) * 7.0);     // integer mutations / loop
  float gPhase = floor(ph*gRate + h21(cell)*3.0);
  vec2  gi = floor(sub * vec2(5.0, 7.0));
  vec2  gf = fract(sub * vec2(5.0, 7.0));
  float dotmask = step(0.15, gf.x)*step(gf.x, 0.85)*step(0.10, gf.y)*step(gf.y, 0.90);
  float onbit  = step(0.5, h31(vec3(cell, dot(gi, vec2(1.0, 5.0)) + gPhase*31.0)));
  float margin = step(0.05, sub.x)*step(sub.x, 0.95)*step(0.03, sub.y)*step(sub.y, 0.97);
  float glyph  = dotmask * onbit * margin;

  vec3 green = 0.5 + 0.5*cos(TAU*(hue + vec3(0.0, 0.08, 0.02)));  // phosphor green base
  green = vec3(0.16, 1.0, 0.42) * (0.5 + green);                  // bias toward green
  vec3 white = vec3(0.85, 1.0, 0.92);

  vec3 col = mix(green, white, headness) * bright * glyph * 1.7;
  col += white * headness * bright * glyph * 1.2;      // hot white head glyphs
  col += green * glyph * 0.06;                         // faint resident glyphs
  col += white * pow(headness, 1.5) * bright * 0.5;    // head bloom halo
  col += green * bright * 0.05;                        // column ambience

  // subtle scanlines + vignette for CRT feel
  col *= 0.85 + 0.15*sin(gl_FragCoord.y * 3.14159 / 1.5);
  vec2 vv = (2.0*gl_FragCoord.xy - res) / res.y;
  col *= 1.0 - 0.18*dot(vv, vv);

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