Number ticker

'use client';

import { useEffect, useRef, useState } from 'react';
import { cn, prefersReducedMotion, useLocale } from '@/lib/kit';

export type NumberTickerVariant = 'count' | 'roll';
type Tag = 'span' | 'p' | 'div' | 'strong' | 'dd';

export interface NumberTickerProps {
  /** The value to arrive at */
  value: number;
  /** Where counting starts. Default 0. */
  from?: number;
  /** count: the number counts up (default). roll: every digit rolls like an odometer. */
  variant?: NumberTickerVariant;
  /** Intl.NumberFormat options, e.g. { style: 'currency', currency: 'EUR' } or { style: 'percent' } */
  format?: Intl.NumberFormatOptions;
  /** A locale for Intl, e.g. 'de-CH'. Default: the kit locale (de: de-DE, en: en-US). */
  locale?: string;
  /** Milliseconds the count takes. Default 1800. */
  duration?: number;
  /** Milliseconds before it starts. Default 0. */
  delay?: number;
  /**
   * load: right away. view (or in-view): when it scrolls in (default).
   * scroll: the number follows the scroll position, it counts up on the
   * way into the window and back down when you scroll up.
   */
  trigger?: 'load' | 'view' | 'in-view' | 'scroll';
  /** With trigger scroll: the part of the way through the window that counts, 0 to 1. Default [0.05, 0.5]. */
  scrollRange?: [number, number];
  /** Element to render. Default span. */
  as?: Tag;
  className?: string;
}

/*
 * The final number is always in the box, invisible, so the box has its
 * final width from the first paint and the counting number sits on top,
 * aligned to the end like an odometer. Digits are tabular.
 * count: hidden until hydration (only with scripting and motion), so the
 * final value does not flash before counting from zero.
 * roll: pure CSS, every digit column rolls from 0 to its digit.
 */
const CSS = `
.kitfx-nt{display:inline-grid;font-variant-numeric:tabular-nums;white-space:nowrap}
.kitfx-nt>*{grid-area:1/1}
.kitfx-nt-ghost{visibility:hidden}
.kitfx-nt-live{justify-self:end}
.kitfx-nt-col{display:inline-block;position:relative;clip-path:inset(0 -.1em)}
.kitfx-nt-col>.kitfx-nt-strip{position:absolute;left:0;right:0;top:0;display:flex;flex-direction:column;transform:translateY(calc(var(--n) * -1lh));animation:kitfx-nt-roll var(--kitfx-nt-dur) cubic-bezier(.3,.6,.15,1) calc(var(--kitfx-nt-delay) + var(--k) * 70ms) both}
.kitfx-nt-strip>span{height:1lh;text-align:center}
.kitfx-nt-col>.kitfx-nt-digit{visibility:hidden}
.kitfx-nt[data-mode=wait] .kitfx-nt-strip{animation:none;transform:translateY(0)}
.kitfx-nt[data-mode=scroll] .kitfx-nt-strip{animation:none;transform:translateY(calc(var(--n) * var(--kitfx-p,1) * -1lh));transition:transform .25s linear}
@keyframes kitfx-nt-roll{from{transform:translateY(0)}}
@keyframes kitfx-nt-hold{from{visibility:hidden}}
@media (scripting:enabled) and (prefers-reduced-motion:no-preference){.kitfx-nt[data-pending] .kitfx-nt-live{animation:kitfx-nt-hold 0s 2.5s both}}
@media (prefers-reduced-motion:reduce){.kitfx-nt-col>.kitfx-nt-strip{animation:none;transition:none}}`;

const LOCALES = { en: 'en-US', de: 'de-DE' } as const;

/** Decimals of a number as written, so 3.8 counts as 0.0 … 3.8 and not 3.81234 */
const decimalsOf = (n: number) => {
  const s = String(n);
  const i = s.indexOf('.');
  return i < 0 ? 0 : Math.min(s.length - i - 1, 4);
};

const easeOut = (t: number) => 1 - Math.pow(1 - t, 4);

/**
 * Scroll link. Writes --kitfx-p (0 to 1) on the element: 0 when it has
 * travelled `start` of its way through the window, 1 at `end` (0 is its top
 * at the bottom edge, 1 its bottom at the top edge). The value is scaled to
 * the part of that way the page can still scroll, so a line at the end of a
 * short page still finishes. Never with reduced motion, then the CSS
 * fallback 1 keeps the final state.
 */
function useScrollLink(ref: { current: HTMLElement | null }, on: boolean, range: readonly [number, number], onChange?: (p: number) => void) {
  const [start, end] = range;
  const listener = useRef(onChange);
  listener.current = onChange;
  useEffect(() => {
    const el = ref.current;
    if (!el || !on || prefersReducedMotion()) return;
    let frame = 0;
    const span = Math.max(0.0001, end - start);
    const at = (top: number, h: number, vh: number) => Math.min(1, Math.max(0, ((vh - top) / (h + vh) - start) / span));
    const update = () => {
      frame = 0;
      const r = el.getBoundingClientRect();
      const vh = window.innerHeight;
      const rest = Math.max(0, document.documentElement.scrollHeight - vh - window.scrollY);
      const most = at(r.top - rest, r.height, vh);
      const p = most < 0.02 ? 1 : Math.min(1, at(r.top, r.height, vh) / most);
      el.style.setProperty('--kitfx-p', p.toFixed(4));
      listener.current?.(p);
    };
    const onScroll = () => {
      if (!frame) frame = requestAnimationFrame(update);
    };
    update();
    window.addEventListener('scroll', onScroll, { passive: true });
    window.addEventListener('resize', onScroll);
    return () => {
      cancelAnimationFrame(frame);
      window.removeEventListener('scroll', onScroll);
      window.removeEventListener('resize', onScroll);
      el.style.removeProperty('--kitfx-p');
    };
  }, [ref, on, start, end]);
}


/**
 * Counts up to a value when it scrolls into view, formatted for the locale
 * with Intl (48.210 € in German, $48,210 in English). Screen readers get the
 * final value once. Without JavaScript and with reduced motion the final
 * value stands there.
 *   <NumberTicker value={48210} format={{ style: 'currency', currency: 'EUR', maximumFractionDigits: 0 }} className="text-h1" />
 */
export function NumberTicker({ value, from = 0, variant = 'count', format, locale, duration = 1800, delay = 0, trigger = 'view', scrollRange = [0.05, 0.5], as: Tag = 'span', className }: NumberTickerProps) {
  const kitLocale = useLocale();
  const loc = locale ?? LOCALES[kitLocale];
  const digits = decimalsOf(value);
  const opts: Intl.NumberFormatOptions = {
    ...(format?.style === 'percent' ? {} : { minimumFractionDigits: digits, maximumFractionDigits: digits }),
    ...format,
  };
  const fmt = new Intl.NumberFormat(loc, opts);
  const final = fmt.format(value);

  const ref = useRef<HTMLElement>(null);
  const [current, setCurrent] = useState(value);
  const [pending, setPending] = useState(variant === 'count');
  const scroll = trigger === 'scroll';
  const [mode, setMode] = useState<'play' | 'wait' | 'scroll'>(scroll ? 'scroll' : 'play');
  // Scroll: the count eases out over the way, so the last digits settle slowly
  useScrollLink(ref, scroll, scrollRange, (p) => {
    if (variant === 'count') setCurrent(p >= 1 ? value : from + (value - from) * p * (2 - p));
  });

  useEffect(() => {
    setPending(false);
    const el = ref.current;
    if (!el || scroll || prefersReducedMotion()) return;
    const below = el.getBoundingClientRect().top >= window.innerHeight * 0.9;

    if (variant === 'roll') {
      // CSS rolls from the first paint. Further down, hold it until it scrolls in.
      if (trigger === 'load' || !below) return;
      setMode('wait');
      const io = new IntersectionObserver(
        ([e]) => {
          if (!e?.isIntersecting) return;
          setMode('play');
          io.disconnect();
        },
        { rootMargin: '0px 0px -10% 0px' },
      );
      io.observe(el);
      return () => io.disconnect();
    }

    let raf = 0;
    let timer = 0;
    setCurrent(from);
    const run = () => {
      const start = performance.now();
      const tick = (now: number) => {
        const t = Math.min(1, (now - start) / duration);
        setCurrent(t >= 1 ? value : from + (value - from) * easeOut(t));
        if (t < 1) raf = requestAnimationFrame(tick);
      };
      raf = requestAnimationFrame(tick);
    };
    const go = () => {
      timer = window.setTimeout(run, delay);
    };
    if (trigger === 'load' || !below) {
      go();
      return () => {
        window.clearTimeout(timer);
        cancelAnimationFrame(raf);
      };
    }
    const io = new IntersectionObserver(
      ([e]) => {
        if (!e?.isIntersecting) return;
        io.disconnect();
        go();
      },
      { rootMargin: '0px 0px -10% 0px' },
    );
    io.observe(el);
    return () => {
      io.disconnect();
      window.clearTimeout(timer);
      cancelAnimationFrame(raf);
    };
  }, [value, from, duration, delay, trigger, variant, scroll]);

  const vars = { '--kitfx-nt-dur': `${duration}ms`, '--kitfx-nt-delay': `${delay}ms` } as React.CSSProperties;

  let live: React.ReactNode;
  if (variant === 'roll') {
    const chars = Array.from(final);
    let k = 0;
    live = chars.map((c, i) => {
      if (!/\d/.test(c)) return <span key={i}>{c}</span>;
      // Every column turns at least once, the ones to the right start a little later
      return (
        <span key={i} className="kitfx-nt-col" style={{ '--n': 10 + Number(c), '--k': k++ } as React.CSSProperties}>
          <span className="kitfx-nt-digit">{c}</span>
          <span className="kitfx-nt-strip">
            {Array.from({ length: 20 }, (_, j) => (
              <span key={j}>{j % 10}</span>
            ))}
          </span>
        </span>
      );
    });
  } else {
    live = current === value ? final : fmt.format(current);
  }

  return (
    <Tag ref={ref as never} className={cn('kitfx-nt', className)} data-pending={pending ? '' : undefined} data-mode={mode} style={vars}>
      <style dangerouslySetInnerHTML={{ __html: CSS }} />
      <span className="sr-only">{final}</span>
      <span aria-hidden className="kitfx-nt-ghost">
        {final}
      </span>
      <span aria-hidden className="kitfx-nt-live">
        {live}
      </span>
    </Tag>
  );
}