Snowfall

// <SnowfallBackground>: snow falling at three depths onto soft drifts. Animated.
// Far flakes are small and slow, near flakes are six-armed and spin as they sway.

import type { CSSProperties } from 'react';
import { BackgroundFrame, backgroundStyles as base, r2, seeded, type BackgroundProps } from './background';
import styles from './snowfall-background.module.css';

export const snowfallTheme = {
  skyTop: '#bcd3ee',
  skyBottom: '#eef4fb',
  flake: '#ffffff',
  drift: '#ffffff',
  driftShadow: '#d9e6f4',
};
export type SnowfallTheme = typeof snowfallTheme;

const LAYERS = [
  { count: 40, size: [2, 3.5], fall: [18, 26], sway: [6, 14], opacity: 0.6 },
  { count: 24, size: [4, 6], fall: [12, 17], sway: [12, 24], opacity: 0.85 },
  { count: 12, size: [12, 18], fall: [8, 12], sway: [18, 34], opacity: 1 },
];

const rand = seeded(41);
const between = ([a, b]: number[]) => a + rand() * (b - a);
const FLAKES = LAYERS.flatMap((layer, depth) =>
  Array.from({ length: layer.count }, () => ({
    depth,
    x: r2(rand() * 100),
    y: r2(rand() * 100),
    size: r2(between(layer.size)),
    fall: r2(between(layer.fall)),
    delay: r2(rand() * 30),
    sway: r2(between(layer.sway)),
    swayTime: r2(2 + rand() * 3),
    spin: r2(6 + rand() * 8),
    opacity: layer.opacity,
  })),
);

// Six arms, each with a small V near the tip.
const ARM = 'M0 0 V-9 M0 -6 L-2.5 -8.5 M0 -6 L2.5 -8.5';
function Flake({ size }: { size: number }) {
  return (
    <svg className={styles.flake} width={size} height={size} viewBox="-10 -10 20 20" fill="none" strokeWidth="1.6" strokeLinecap="round">
      {[0, 60, 120, 180, 240, 300].map(a => <path key={a} d={ARM} transform={`rotate(${a})`} />)}
    </svg>
  );
}

/** Snow falling at three depths onto soft drifts. */
export function SnowfallBackground(props: BackgroundProps<SnowfallTheme>) {
  return (
    <BackgroundFrame
      {...props}
      defaultTheme={snowfallTheme}
      layer={
        <>
          <div className={styles.sky} />
          {FLAKES.map((f, i) => (
            <div
              key={i}
              className={`${base.particle} ${styles.fall}`}
              style={{ left: `${f.x}%`, '--y': `${f.y}%`, animationDuration: `${f.fall}s`, animationDelay: `${-f.delay}s`, opacity: f.opacity } as CSSProperties}
            >
              <div className={styles.sway} style={{ '--sway': `${f.sway}px`, animationDuration: `${f.swayTime}s` } as CSSProperties}>
                {f.depth === 2 ? (
                  <span className={styles.spin} style={{ animationDuration: `${f.spin}s` }}>
                    <Flake size={f.size} />
                  </span>
                ) : (
                  <span className={styles.dot} style={{ width: f.size, height: f.size }} />
                )}
              </div>
            </div>
          ))}
          <svg className={styles.drifts} viewBox="0 0 400 72" preserveAspectRatio="none">
            <path fill="var(--drift-shadow)" d="M0 40 C60 18 110 30 160 34 S260 14 320 28 S380 30 400 24 V72 H0 Z" />
            <path fill="var(--drift)" d="M0 52 C50 36 100 46 150 48 S240 32 300 42 S370 46 400 40 V72 H0 Z" />
          </svg>
        </>
      }
    />
  );
}

.sky {
  position: absolute;
  inset: 0;
  background: linear-gradient(var(--sky-top), var(--sky-bottom));
}

.fall {
  animation: fall linear infinite;
  will-change: transform;
}

.sway { animation: sway ease-in-out infinite alternate; }
.spin { animation: spin linear infinite; display: block; }

.dot {
  display: block;
  border-radius: 50%;
  background: var(--flake);
  box-shadow: 0 0 2px rgb(90 120 160 / 0.25);
}

.flake {
  display: block;
  stroke: var(--flake);
  filter: drop-shadow(0 0 1px rgb(90 120 160 / 0.35));
}

.drifts {
  position: absolute;
  left: 0;
  bottom: 0;
  width: 100%;
  height: 72px;
}

@keyframes fall {
  from { transform: translateY(-24px); }
  to   { transform: translateY(calc(100cqh + 24px)); }
}
@keyframes sway {
  from { transform: translateX(calc(var(--sway) * -1)); }
  to   { transform: translateX(var(--sway)); }
}
@keyframes spin { to { transform: rotate(360deg); } }

// Shared frame for every background: theming, layering and pausing.
// Backgrounds are pure server components: no client JavaScript.

import type { CSSProperties, ReactNode } from 'react';
import styles from './background.module.css';

export { styles as backgroundStyles };

/** Deterministic PRNG so particles render identically on server and client. */
export function seeded(seed: number): () => number {
  return () => {
    seed = (seed + 0x6d2b79f5) | 0;
    let t = Math.imul(seed ^ (seed >>> 15), 1 | seed);
    t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  };
}

/** Round for stable, compact inline styles. */
export const r2 = (n: number) => Math.round(n * 100) / 100;

/** Props every background accepts. `T` is the background's theme (colour tokens). */
export type BackgroundProps<T> = {
  /** Override any of the background's colours. */
  theme?: Partial<T>;
  /** Freeze animated backgrounds. Static backgrounds ignore it. */
  paused?: boolean;
  className?: string;
  style?: CSSProperties;
  /** Your content. The background paints behind it. */
  children?: ReactNode;
};

// baseColor → --base-color, blob1 → --blob-1
const toVar = (key: string) => `--${key.replace(/([A-Z])/g, '-$1').replace(/(\d+)/g, '-$1').toLowerCase()}`;

type BackgroundFrameProps<T extends Record<string, string>> = BackgroundProps<T> & {
  defaultTheme: T;
  /** The texture itself, rendered in an absolutely positioned layer behind the children. */
  layer: ReactNode;
};

export function BackgroundFrame<T extends Record<string, string>>({
  defaultTheme,
  layer,
  theme,
  paused,
  className,
  style,
  children,
}: BackgroundFrameProps<T>) {
  const vars: Record<string, string> = {};
  for (const [key, value] of Object.entries({ ...defaultTheme, ...theme })) {
    if (value) vars[toVar(key)] = value;
  }
  const classes = [styles.root, paused && styles.paused, className].filter(Boolean).join(' ');

  return (
    <div className={classes} style={{ ...vars, ...style } as CSSProperties}>
      <div className={styles.layer} aria-hidden="true">
        {layer}
      </div>
      {children}
    </div>
  );
}

/* The root establishes a stacking context so the layer sits behind children
   without escaping behind the page. */
.root {
  position: relative;
  isolation: isolate;
}

.layer {
  position: absolute;
  inset: 0;
  z-index: -1;
  overflow: hidden;
  pointer-events: none;
  /* Lets particles move by the layer's own size: 100cqh is its full height, 100cqw its width. */
  container-type: size;
}

/* A particle starts just above the layer at a random horizontal position. */
.particle {
  position: absolute;
  top: 0;
}

.paused,
.paused * { animation-play-state: paused !important; }

@media (prefers-reduced-motion: reduce) {
  .layer,
  .layer * { animation: none !important; }
  /* Without motion, particles rest at a scattered position instead of the top edge. */
  .particle { top: var(--y, 0); }
}