Motorway

"use client";

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

/**
 * A live WebGL variation on `RoadField`'s night-road family that trades the
 * family's usual four-or-so streaks for a genuine wide highway: six full
 * lanes of light trail, symmetric across the bundle, converging on the same
 * kind of real perspective projection toward a vanishing point inside the
 * frame. Five dashed dividers ride between the six lanes rather than
 * `RoadField`'s single one, and the colour itself widens with the road — cool
 * white-gold in the two centre lanes, warming to full amber at the outer
 * edges, the way a broad sodium-lit carriageway actually reads wider than it
 * is deep.
 *
 * The lane count is a fixed loop of six rather than a prop: this is one
 * specific, wider piece in the family (`night-road` is the narrow one,
 * `rainline` the wet one, this the six-lane one), not a parametric "road
 * generator" — same reasoning `RoadField` and `ContraflowField` already
 * apply to their own fixed lane counts.
 *
 * Pointer contract carried over from `RoadField` unchanged — steering and
 * throttle, not a new gesture:
 *  - steering — the pointer's x pans the vanishing point.
 *  - throttle — the pointer's y speeds every lane's flicker and dash travel
 *    the nearer it sits to the bottom edge.
 * The same warm headlight bloom rides the cursor.
 *
 * One of the reusable background fields, a sibling of `RoadField`,
 * `ContraflowField` and `RainlineField` in the same trail family. Drop it
 * into any `position: relative`/`isolate` parent — it fills the box. Built
 * on `lib/shader-surface.ts`, so every degradation path is already handled:
 * no WebGL, a blocked or lost context, a hidden tab, or
 * `prefers-reduced-motion` all leave the CSS `.iris-motorwayfield__floor`
 * underneath visible (a still composed frame in the same palette, never a
 * blank box).
 *
 * Fixed palette, not the token ramp — same reasoning as `RoadField`: the
 * near-white flare core and the cool/warm widening live nowhere in the
 * accent ramp, so the CSS floor carries the colour rather than a live token
 * read.
 *
 * Reading guard: when `guardSelector` resolves to an element, the field
 * measures that block every frame and clamps its own luminance under a
 * ceiling inside that region (hue and saturation untouched), so copy laid
 * over the field holds WCAG AA with no overlay layer. `null` (the default)
 * turns the guard off — for decorative use where nothing sits on top.
 */

/* Palette, sRGB 0–1. Uniforms, not tokens — see the note above. */
const PALETTE: Record<string, [number, number, number]> = {
  u_black: [0.013, 0.013, 0.015], // the near-black asphalt between the lanes
  u_haze: [0.22, 0.12, 0.06], // the warm haze pooling under the bundle near the camera
  u_white: [0.92, 0.88, 0.78], // the cool white-gold of the two centre lanes
  u_gold: [0.98, 0.72, 0.32], // the mid-bundle colour
  u_amber: [0.99, 0.46, 0.14], // the hotter outer-lane colour
  u_flare: [1.0, 0.97, 0.9], // the near-white core riding every lane
  u_line: [0.83, 0.8, 0.72], // the dashed lane dividers
};

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_black;
uniform vec3 u_haze;
uniform vec3 u_white;
uniform vec3 u_gold;
uniform vec3 u_amber;
uniform vec3 u_flare;
uniform vec3 u_line;

/* Reading region for the contrast guard: xy = centre (uv, y up),
   zw = half-extent. Measured from the live copy block every frame.
   u_guard is 0 when the guard is off. */
uniform vec4  u_readA;
uniform float u_guard;

const float HORIZON = 0.72;  /* uv.y of the vanishing point */
const float VP_X    = 0.0;   /* the point's x offset from centre, P-space */
const float ROAD_W  = 0.86;  /* road half-width at the camera — wider than RoadField's, six lanes */
const float MIN_BW  = 0.004; /* width floor so the bundle never divides by zero */

float trail(float dx, float sigma, float wz, float seed, float spd, out float core) {
  float s2 = sigma * sigma + 1e-7;
  float body = exp(-dx * dx / s2);
  core = exp(-dx * dx / (s2 * 0.085));
  float flicker = 0.77 + 0.23 * sin(wz * 0.5 - u_time * spd + seed * 6.4);
  return body * flicker;
}

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);

  /* steering: the pointer's x pans the vanishing point. throttle: the
     pointer's y doubles as an accelerator — nearer the bottom edge (closer
     to the camera) reads as pressing it down. Same contract as RoadField. */
  float steer = (u_pointer.x - 0.5) * u_pointer.z * 0.5;
  float speed = 1.0 + u_pointer.z * (1.0 - u_pointer.y) * 2.4;
  float vpX = VP_X + steer;

  float d = uv.y / HORIZON;
  float invZ = pow(clamp(1.0 - d, 0.0, 1.0), 1.35);
  float aboveHorizon = 1.0 - smoothstep(0.9, 1.02, d);
  float wz = (1.0 - invZ) / max(invZ, 0.02);

  float roadCenterX = vpX * (1.0 - invZ);
  float bw = max(ROAD_W * invZ, MIN_BW);

  vec3 col = u_black;

  /* a warm haze pooling under the whole bundle near the camera — nothing
     here is pure black, the discipline the rest of the family keeps */
  float haze = smoothstep(0.55, 0.0, d) * exp(-abs(P.x - roadCenterX) * 0.7);
  col += u_haze * haze * 0.12;

  /* six lanes, symmetric across the bundle's half-width — offsets of
     -2.5..2.5 in steps of 1, scaled to the bundle so they converge to the
     same point together. Colour widens outward: white-gold at the two
     centre lanes, full amber by the outer pair. */
  float coreSum = 0.0;
  for (int i = 0; i < 6; i++) {
    float k = (float(i) - 2.5) / 3.0;            /* -0.833 .. 0.833 */
    float laneX = roadCenterX + k * bw;
    float seed = float(i) * 1.9 + 1.1;
    float spd = (0.82 + 0.05 * float(i)) * speed;
    float core;
    float e = trail(P.x - laneX, bw * 0.075, wz, seed, spd, core);
    float t = abs(k) / 0.833;                     /* 0 centre .. 1 edge */
    vec3 laneColor = mix(u_white, mix(u_gold, u_amber, smoothstep(0.4, 1.0, t)), smoothstep(0.0, 0.55, t));
    col += laneColor * e * 1.55 * aboveHorizon;
    coreSum += core;
  }
  col += u_flare * coreSum * aboveHorizon * 0.6;

  /* five dashed dividers riding between the six lanes, spaced and animated
     in world depth so they compress toward the horizon exactly the way the
     lanes converge */
  for (int j = 0; j < 5; j++) {
    float k = (float(j) - 2.0) / 3.0;             /* -0.667 .. 0.667 */
    float lineX = roadCenterX + k * bw;
    float dashSigma = bw * 0.018 + 0.001;
    float dxD = P.x - lineX;
    float lineBody = exp(-dxD * dxD / (dashSigma * dashSigma + 1e-7));
    float dp = fract(wz * 0.55 - u_time * 0.9 * speed + float(j) * 0.2);
    float dashOn = clamp(smoothstep(0.0, 0.05, dp) - smoothstep(0.55, 0.6, dp), 0.0, 1.0);
    col += u_line * lineBody * dashOn * aboveHorizon * 0.9;
  }

  /* the touch: a warm headlight bloom with a near-white core, the same
     "your touch joined the light" move RoadField makes */
  vec2 ptr = (u_pointer.xy - 0.5) * vec2(aspect, 1.0);
  vec2 toPtr = P - ptr;
  float dPtr = dot(toPtr, toPtr);
  col += u_amber * u_pointer.z * exp(-dPtr * 9.0) * 0.5;
  col += u_flare * u_pointer.z * exp(-dPtr * 40.0) * 0.4;

  col = max(col, 0.0);

  /* ---- the reading guard (see TileField 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 MotorwayFieldProps {
  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 MotorwayField({ className, guardSelector = null }: MotorwayFieldProps) {
  const canvasRef = useRef<HTMLCanvasElement>(null);

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

    const guardOn = guardSelector != null;

    /* The block the reading guard protects. Looked up once, lazily. */
    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,
      uniforms: [...Object.keys(PALETTE), "u_readA", "u_guard"],
      onInit: (gl, u) => {
        for (const name of Object.keys(PALETTE)) {
          if (u[name]) gl.uniform3fv(u[name], PALETTE[name]);
        }
        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 RoadField / RainlineField: once
         painted, the last frame stays on screen while the surface is parked
         off-view, so scrolling away and back does not crossfade to the
         static floor. A lost context still clears it below. */
      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(_46%_50%_at_50%_72%,oklch(0.94_0.03_85)_0%,transparent_12%_),radial-gradient(_58%_68%_at_50%_72%,oklch(0.85_0.09_75)_0%,transparent_32%_),radial-gradient(_80%_92%_at_50%_72%,oklch(0.62_0.16_55)_0%,transparent_58%_),radial-gradient(_105%_105%_at_50%_72%,oklch(0.18_0.05_40)_0%,transparent_78%_),oklch(0.032_0.008_60)] after:content-[''] after:absolute after:inset-0 after:[background:repeating-linear-gradient(_90deg,transparent_0_3.2%,oklch(0.1_0.02_60_/_0.55)_3.2%_4.6%_)] after:[-webkit-mask-image:radial-gradient(_72%_90%_at_50%_72%,transparent_0_10%,#000_40%,transparent_80%_)] after:[mask-image:radial-gradient(_72%_90%_at_50%_72%,transparent_0_10%,#000_40%,transparent_80%_)]" />
      <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>
  );
}