Rain

// <RainBackground>: soft rain falling on a slant at two depths, with ripples spreading in the
// puddles along the bottom. Animated.

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

export const rainTheme = {
  skyTop: '#9fb3c4',
  skyBottom: '#dbe4ec',
  drop: '#ffffff',
  ground: '#b7c6d3',
  ripple: '#ffffff',
};
export type RainTheme = typeof rainTheme;

const LAYERS = [
  { count: 46, length: [10, 16], fall: [1.2, 1.6], width: 1, opacity: 0.45 },
  { count: 26, length: [20, 30], fall: [0.7, 0.95], width: 1.6, opacity: 0.75 },
];

const rand = seeded(61);
const between = ([a, b]: number[]) => a + rand() * (b - a);
const DROPS = LAYERS.flatMap(layer =>
  Array.from({ length: layer.count }, () => ({
    // Start further right than the box: the slant carries drops left as they fall.
    x: r2(rand() * 125),
    y: r2(rand() * 100),
    length: r2(between(layer.length)),
    fall: r2(between(layer.fall)),
    delay: r2(rand() * 3),
    width: layer.width,
    opacity: layer.opacity,
  })),
);

const RIPPLES = Array.from({ length: 14 }, () => ({
  x: r2(3 + rand() * 94),
  y: r2(6 + rand() * 30),
  size: r2(14 + rand() * 18),
  time: r2(1.6 + rand() * 1.4),
  delay: r2(rand() * 3),
}));

/** Soft rain falling on a slant, with ripples in the puddles below. */
export function RainBackground(props: BackgroundProps<RainTheme>) {
  return (
    <BackgroundFrame
      {...props}
      defaultTheme={rainTheme}
      layer={
        <>
          <div className={styles.sky} />
          {DROPS.map((d, i) => (
            <div
              key={i}
              className={`${base.particle} ${styles.fall}`}
              style={{ left: `${d.x}%`, '--y': `${d.y}%`, animationDuration: `${d.fall}s`, animationDelay: `${-d.delay}s` } as CSSProperties}
            >
              <span className={styles.streak} style={{ height: d.length, width: d.width, opacity: d.opacity }} />
            </div>
          ))}
          <div className={styles.ground}>
            {RIPPLES.map((r, i) => (
              <span
                key={i}
                className={styles.ripple}
                style={{ left: `${r.x}%`, bottom: `${r.y}%`, width: r.size, height: r.size * 0.32, animationDuration: `${r.time}s`, animationDelay: `${-r.delay}s` }}
              />
            ))}
          </div>
        </>
      }
    />
  );
}

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

/* Drops fall the layer's full height plus a little, drifting left by a fifth of that: a slant
   of about 11°, which the streak matches. */
.fall {
  animation: fall linear infinite;
  will-change: transform;
}

.streak {
  display: block;
  border-radius: 1px;
  background: linear-gradient(transparent, var(--drop));
  transform: rotate(11.3deg);
}

.ground {
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  height: 56px;
  background: linear-gradient(transparent, var(--ground) 40%);
}

.ripple {
  position: absolute;
  border: 1px solid var(--ripple);
  border-radius: 50%;
  transform: translateX(-50%) scale(0.1);
  opacity: 0;
  animation: ripple ease-out infinite;
}

@keyframes fall {
  from { transform: translate(0, -40px); }
  to   { transform: translate(calc(-0.2 * (100cqh + 80px)), calc(100cqh + 40px)); }
}
@keyframes ripple {
  0%   { transform: translateX(-50%) scale(0.1); opacity: 0; }
  10%  { opacity: 0.9; }
  100% { transform: translateX(-50%) scale(1); opacity: 0; }
}

// 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); }
}