Night Road

"use client";

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

/**
 * A live WebGL field of long-exposure light trails converging on a road —
 * four warm red-to-amber streaks and one branching steel-grey outlier,
 * racing up a real perspective toward a vanishing point inside the frame, a
 * dashed lane divider foreshortening correctly as it recedes. Above the
 * vanishing point is open black sky, exactly like the reference photograph
 * this was built from.
 *
 * The perspective is a genuine projection, not a texture: every lane centre
 * and every line's width is derived from one `invZ(depth)` term that goes 1
 * at the camera (the bottom edge) to 0 exactly at the vanishing point, so
 * the bundle narrows to a single point rather than fading out early. World
 * depth (`wz`, the inverse of that same term) drives the dash spacing and
 * the flicker along each trail, which is why the dashes compress correctly
 * as they approach the horizon instead of staying evenly spaced in screen
 * space.
 *
 * Two pointer interactions, both literal driving metaphors rather than a
 * generic hover glow:
 *  - steering — the pointer's x pans the vanishing point, so the whole road
 *    swings under you the way it does turning a wheel.
 *  - throttle — the pointer's y doubles as an accelerator: the nearer it
 *    sits to the bottom edge (closer to the camera), the faster the dashes
 *    and the trail flicker run.
 * A third, smaller touch: the cursor itself drops a warm headlight bloom
 * with a near-white core, the same "your touch joined the light" move
 * `ArcLightsField` and `RakeField` make with their own pointer lights.
 *
 * One of the reusable background fields (`TileField`, `SilkField`,
 * `RakeField`, `FilamentField`, `ArcLightsField`). 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-roadfield__floor` underneath visible (a still
 * composed frame in the same palette, never a blank box).
 *
 * Like `RakeField` / `FilamentField` (and unlike `SilkField`), the palette is
 * a fixed set passed as uniforms rather than read from the token ramp — the
 * near-white flare core and the steel outlier 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: [0.014, 0.011, 0.011], // the near-black ground between the trails
  u_ember: [0.28, 0.045, 0.03], // the dim inner-lane red and the road haze
  u_crimson: [0.78, 0.1, 0.06], // the mid-warm trail colour
  u_amber: [0.98, 0.42, 0.11], // the hotter outer-lane trail and pointer bloom
  u_flare: [1.0, 0.9, 0.76], // the near-white core riding every trail's centre
  u_steel: [0.58, 0.63, 0.7], // the branching outlier lane
  u_line: [0.82, 0.78, 0.7], // the dashed lane divider
};

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_ember;
uniform vec3 u_crimson;
uniform vec3 u_amber;
uniform vec3 u_flare;
uniform vec3 u_steel;
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 = 0.70;  /* uv.y of the vanishing point */
const float VP_X    = 0.085; /* the point's x offset from centre, P-space */
const float ROAD_W  = 0.62;  /* road half-width at the camera, P-space */
const float MIN_BW  = 0.004; /* width floor so the bundle never divides by zero */

/* One light trail. 'dx' is the pixel's screen-space offset from the lane
   centre, 'sigma' its half-width. 'wz' (world depth) drives a slow flicker
   along its length, so the trail reads as still travelling rather than a
   painted-on streak — 'spd' lets the throttle speed that travel up. */
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 * 0.08));
  float flicker = 0.76 + 0.24 * sin(wz * 0.5 - u_time * spd + seed * 7.0);
  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, the way turning a
     wheel swings the road under you. u_pointer is already eased upstream
     (shader-surface's shared pointer), so this needs no smoothing of its
     own. throttle: the pointer's y doubles as an accelerator — nearer the
     bottom edge (closer to the camera) reads as pressing it down. */
  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;

  /* depth: 0 at the camera, 1 exactly at the vanishing point. A power curve
     so the convergence accelerates the way a real lens does, not a ramp. */
  float d = uv.y / HORIZON;
  float invZ = pow(clamp(1.0 - d, 0.0, 1.0), 1.35);      /* 1 near .. 0 far */
  float aboveHorizon = 1.0 - smoothstep(0.9, 1.02, d);   /* fades to black sky */

  /* world depth: the inverse of invZ, capped rather than left to explode —
     grows quickly near the horizon, which is what makes the dashes and the
     flicker compress correctly as they recede. */
  float wz = (1.0 - invZ) / max(invZ, 0.02);

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

  /* five lane centres in absolute screen space, as offsets of the bundle's
     own half-width — so they converge to the same point together. The
     outlier gets an extra term that grows with distance instead: it drifts
     off the bundle rather than joining it, the branching grey streak that
     peels away in the reference photograph. */
  float lane1 = roadCenterX - bw * 0.86;
  float lane2 = roadCenterX - bw * 0.30;
  float lane3 = roadCenterX + bw * 0.34;
  float lane4 = roadCenterX + bw * 0.80;
  float laneDash = roadCenterX - bw * 0.06;
  float laneOutlier = roadCenterX + bw * 0.95 + 0.16 * (1.0 - invZ) * (1.0 - invZ);

  float c1, c2, c3, c4, cO;
  float e1 = trail(P.x - lane1, bw * 0.100, wz, 1.3, 0.9 * speed, c1);
  float e2 = trail(P.x - lane2, bw * 0.085, wz, 2.7, 0.8 * speed, c2);
  float e3 = trail(P.x - lane3, bw * 0.115, wz, 4.1, 1.0 * speed, c3);
  float e4 = trail(P.x - lane4, bw * 0.090, wz, 5.5, 0.85 * speed, c4);
  float eO = trail(P.x - laneOutlier, bw * 0.05 + 0.0015, wz, 6.9, 0.7 * speed, cO) * 0.8;

  vec3 col = u_black;

  /* a faint warm haze under the bundle near the camera — nothing here is
     pure black, the same discipline RakeField and FilamentField use */
  float haze = smoothstep(0.55, 0.0, d) * exp(-abs(P.x - roadCenterX) * 1.2);
  col += u_ember * haze * 0.10;

  col += mix(u_ember, u_crimson, 0.5) * e1 * 1.5 * aboveHorizon;
  col += u_crimson * e2 * 1.6 * aboveHorizon;
  col += mix(u_crimson, u_amber, 0.5) * e3 * 1.6 * aboveHorizon;
  col += u_amber * e4 * 1.5 * aboveHorizon;
  col += u_steel * eO * 1.1 * aboveHorizon;

  float hotCore = (c1 + c2 + c3 + c4) * aboveHorizon;
  col += u_flare * hotCore * 0.7;
  col += u_flare * cO * aboveHorizon * 0.3;

  /* the dashed lane divider — spaced and animated in world depth, so it
     compresses toward the horizon exactly the way the trails converge. */
  float dashSigma = bw * 0.045 + 0.0012;
  float dxD = P.x - laneDash;
  float lineBody = exp(-dxD * dxD / (dashSigma * dashSigma + 1e-7));
  float dp = fract(wz * 0.55 - u_time * 0.9 * speed);
  float dashOn = clamp(smoothstep(0.0, 0.05, dp) - smoothstep(0.55, 0.60, dp), 0.0, 1.0);
  col += u_line * lineBody * dashOn * aboveHorizon * 1.3;

  /* the touch: a warm headlight bloom with a near-white core, the same
     "your touch joined the light" move ArcLightsField and RakeField make */
  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 RoadFieldProps {
  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 RoadField({ className, guardSelector = null }: RoadFieldProps) {
  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 SilkField / RakeField: 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(_38%_55%_at_58%_70%,oklch(0.9_0.05_70)_0%,transparent_10%_),radial-gradient(_46%_70%_at_58%_70%,oklch(0.68_0.19_40)_0%,transparent_30%_),radial-gradient(_70%_95%_at_58%_70%,oklch(0.42_0.19_30)_0%,transparent_55%_),radial-gradient(_100%_100%_at_58%_70%,oklch(0.16_0.06_30)_0%,transparent_75%_),oklch(0.03_0.01_30)] after:content-[''] after:absolute after:inset-0 after:[background:repeating-linear-gradient(90deg,transparent_0_5%,oklch(0.08_0.02_30_/_0.6)_5%_8%)] after:[mask-image:radial-gradient(60%_85%_at_58%_70%,transparent_0_12%,#000_40%,transparent_78%)] after:[-webkit-mask-image:radial-gradient(60%_85%_at_58%_70%,transparent_0_12%,#000_40%,transparent_78%)]" />
      <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>
  );
}