Silk Coil

"use client";

import { useEffect, useRef } from "react";
import { mountShaderSurface } from "@/lib/shader-surface";

/**
 * A live WebGL variation on `silk-drape`'s drape family that swaps hung
 * fabric for wound fabric: the same fold math rides a helical phase instead
 * of a purely vertical one, so the bands wind around a column as height
 * increases rather than hanging straight down — a twisted silk rope filling
 * the frame rather than a curtain. A fake-cylindrical rim shading (brighter
 * at the centre, darker toward the left/right edges) sells the roundness the
 * helix alone doesn't.
 *
 * The pointer's contract is grip, not parting: its x leans the whole column
 * sideways and speeds the twist's travel, the way pulling a held rope skews
 * its axis and spins it faster; its y drops a bright cinch ring at that
 * height, as if a hand were gripping the coil there.
 *
 * One of the reusable background fields, a sibling of `SilkField`,
 * `AuroraVeil` and `QuiltField` in the same fabric family. Drop it into any
 * `position: relative`/`isolate` parent — it fills the box. Built on
 * `lib/shader-surface.ts`, so degradation is already handled: no WebGL, a
 * blocked or lost context, a hidden tab, or `prefers-reduced-motion` all
 * leave the CSS `.iris-silkcoilfield__floor` underneath visible.
 *
 * Same reasoning as `SilkField` for the palette: it IS the site's amber
 * accent ramp, read live via `colors`, not a fixed departure from it.
 *
 * Reading guard: identical mechanism to `SilkField` — off by default
 * (`null`), on when `guardSelector` resolves to an element.
 */
const FRAG = `
uniform vec2  u_res;
uniform float u_time;
uniform float u_scale;
uniform vec3  u_pointer;  /* x, y (0 bottom .. 1 top), presence 0..1 */

uniform vec3 u_deep;    /* --color-accent-950, the top of the field */
uniform vec3 u_umber;   /* --color-accent-800, upper-mid */
uniform vec3 u_gold;    /* --color-accent-600, lower-mid */
uniform vec3 u_pale;    /* --color-accent-300, the base */

uniform vec4  u_readA;
uniform float u_guard;

const float FOLDS   = 5.0;
const float THREADS = 18.0;
const float TWIST   = 2.6;   /* how sharply the bands wind per unit height */

void main() {
  vec2 res = u_res / u_scale;
  vec2 uv  = gl_FragCoord.xy / u_scale / res;          /* 0..1, y up */
  float aspect = res.x / res.y;
  vec2 P = (uv - 0.5) * vec2(aspect, 1.0);

  /* grip: the pointer's x leans the column's axis, its presence speeds the
     twist's travel — the two things gripping and pulling a wound rope
     actually does, as distinct from silk-drape's parting pull. */
  float lean  = (u_pointer.x - 0.5) * u_pointer.z * 0.35;
  float speed = 1.0 + u_pointer.z * 0.9;

  vec2 wp = vec2(P.x - lean, P.y);

  float warp  = fbm(vec2(wp.x * 1.3 + wp.y * 0.6, wp.y * 0.7 + u_time * 0.02)) * 0.26;
  float twist = wp.y * TWIST + u_time * 0.05 * speed;
  float phase = wp.x * FOLDS + twist + warp;

  float coarse = sin(phase * 3.14159265);
  float fine   = sin((wp.x * THREADS + twist * 1.6 + warp * 1.6) * 3.14159265);
  float drape  = clamp(coarse * 0.7 + fine * 0.3, -1.0, 1.0);
  drape = sign(drape) * pow(abs(drape), 0.82);

  /* fake cylindrical shading: brighter at the column's own centre, darker
     toward its left/right edges — what reads as "round" rather than "flat"
     under the fold pattern riding on top of it */
  float rim = 1.0 - pow(clamp(abs(uv.x - 0.5) * 2.0, 0.0, 1.0), 1.6);

  vec3 base = u_pale;
  base = mix(base, u_gold,  smoothstep(0.10, 0.52, uv.y));
  base = mix(base, u_umber, smoothstep(0.42, 0.80, uv.y));
  base = mix(base, u_deep,  smoothstep(0.70, 1.05, uv.y));

  vec3 col = base * (0.5 + 0.55 * (0.5 + 0.5 * drape)) * mix(0.55, 1.08, rim);

  /* the cinch: a bright ring at the pointer's own height, as if a grip had
     closed around the coil there */
  float cinch = u_pointer.z * exp(-pow((uv.y - u_pointer.y) * 3.4, 2.0));
  col += u_pale * cinch * mix(0.15, 0.5, rim) * (0.4 + 0.6 * (0.5 + 0.5 * drape));

  col = max(col, 0.0);

  /* ---- the reading guard (see SilkField for the full rationale) ---- */
  vec2 rd = abs(uv - u_readA.xy) / max(u_readA.zw, vec2(0.02));
  float m = mix(max(rd.x, rd.y), length(rd), 0.4);
  float band = 1.0 - smoothstep(0.72, 2.1, m);
  col = mix(col, holdUnder(col, 0.09), band * u_guard);

  col += (bayer8(gl_FragCoord.xy) - 0.5) * (2.2 / 255.0);

  gl_FragColor = vec4(col, 1.0);
}
`;

export interface SilkCoilFieldProps {
  className?: string;
  /**
   * CSS selector for the block the reading guard should keep readable,
   * resolved against `document`. `null` (the default) turns the guard off.
   */
  guardSelector?: string | null;
}

export function SilkCoilField({ className, guardSelector = null }: SilkCoilFieldProps) {
  const canvasRef = useRef<HTMLCanvasElement>(null);

  useEffect(() => {
    const canvas = canvasRef.current;
    if (!canvas) return;

    const guardOn = guardSelector != null;
    let guardEl: Element | null | undefined;
    const readGuardEl = () => {
      if (guardEl === undefined) {
        guardEl = guardOn ? document.querySelector(guardSelector as string) : null;
      }
      return guardEl;
    };

    return mountShaderSurface(canvas, {
      fragment: FRAG,
      colors: {
        u_deep: "--color-accent-950",
        u_umber: "--color-accent-800",
        u_gold: "--color-accent-600",
        u_pale: "--color-accent-300",
      },
      uniforms: ["u_readA", "u_guard"],
      onInit: (gl, u) => {
        if (u.u_readA) gl.uniform4f(u.u_readA, 0.5, 0.5, 0.44, 0.32);
        if (u.u_guard) gl.uniform1f(u.u_guard, guardOn ? 1 : 0);
      },
      onFrame: (gl, u, s) => {
        if (!guardOn || !u.u_readA) return;
        let cx = 0.5, cy = 0.5, hw = 0.44, hh = 0.32;
        const el = readGuardEl();
        const { rect } = s;
        if (el && rect.width > 0 && rect.height > 0) {
          const r = el.getBoundingClientRect();
          const padX = rect.width * 0.09;
          const padY = rect.height * 0.11;
          cx = (r.left + r.width / 2 - rect.left) / rect.width;
          cy = 1 - (r.top + r.height / 2 - rect.top) / rect.height;
          hw = (r.width / 2 + padX) / rect.width;
          hh = (r.height / 2 + padY) / rect.height;
        }
        gl.uniform4f(u.u_readA, cx, cy, hw, hh);
      },
      onPainted: () => canvas.setAttribute("data-shader", "on"),
      /* No onIdle — same contract as SilkField: once painted, the last
         frame stays on screen while the surface is parked off-view. */
      onLost: () => canvas.removeAttribute("data-shader"),
      maxPixels: 2_200_000,
      dprCap: 1.5,
    });
  }, [guardSelector]);

  return (
    <div className={`absolute inset-0 overflow-hidden${className ? ` ${className}` : ""}`} aria-hidden="true">
      <div className="absolute inset-0 [background:radial-gradient(_70%_100%_at_50%_50%,transparent_40%,oklch(0_0_0_/_0.45)_100%_),linear-gradient(_180deg,var(--color-accent-950)_0%,var(--color-accent-800)_42%,var(--color-accent-600)_72%,var(--color-accent-300)_100%_)]" />
      <canvas ref={canvasRef} className="absolute inset-0 w-full h-full opacity-0 transition-opacity duration-[--duration-slow] ease-[--ease-standard] data-[shader=on]:opacity-100" />
    </div>
  );
}