Sideways scroll

'use client';

import { useCallback, useEffect, useRef, useState } from 'react';
import { cn, round, useTexts } from '@/lib/kit';
import { Eyebrow } from '@/components/ui/section';
import { Button } from '@/components/ui/button';
import { Icon, type IconName } from '@/components/ui/icon';
import { Reveal } from '@/components/ui/reveal';
import { useScrollFade } from '@/components/ui/motion';
import { seg, useLive, useSceneProgress } from '@/components/ui/scroll-scene';

/*
 * A section that moves sideways while the page scrolls down. The section sticks for as long as
 * its row needs to pass, one pixel of scrolling moves the row by one pixel, a line and a counter
 * show how far you are. From 768 px on. On phones every variant has its own layout (a list, a
 * timeline downwards or a swipe row). Without JavaScript and with reduced motion the row is a
 * normal row you scroll sideways with touch, trackpad or keyboard.
 *
 *   cards     service or feature cards with an icon and a number
 *   timeline  years on a line that fills while you pass them
 *   gallery   pictures of different widths with a large word gliding slower behind them
 *   chapters  wide panels, one chapter each, text beside a picture
 *   products  a shop row with prices and a closing tile to the full collection
 */

export type SidewaysScrollVariant = 'cards' | 'timeline' | 'gallery' | 'chapters' | 'products';

export interface SidewaysItem {
  title: string;
  text?: string;
  /** Small line above the title: category, place, a date */
  eyebrow?: string;
  icon?: IconName;
  media?: React.ReactNode;
  /** timeline: the year or date in large figures */
  year?: string;
  /** products: the price as written, e.g. "€34" */
  price?: string;
  /** products: a short label on the picture, e.g. "New" */
  tag?: string;
  href?: string;
}

export interface SidewaysScrollProps extends Omit<React.HTMLAttributes<HTMLElement>, 'title'> {
  variant?: SidewaysScrollVariant;
  eyebrow?: string;
  title?: React.ReactNode;
  description?: string;
  items: SidewaysItem[];
  action?: { label: string; href: string };
  /** gallery: a large word behind the pictures */
  backdrop?: string;
  labels?: Partial<(typeof TEXTS)['en']>;
}

const TEXTS = {
  en: { region: 'Scrollable row', of: 'of', more: 'Learn more', all: 'See the whole collection' },
  de: { region: 'Seitlich scrollbare Reihe', of: 'von', more: 'Mehr erfahren', all: 'Ganze Kollektion ansehen' },
};

const pad = (n: number) => String(n).padStart(2, '0');
/** Where an item counts as reached: its left edge passes this share of the window width */
const LINE = 0.42;

interface Mark {
  a: number;
  b: number;
}

/**
 * Measures the row once it is live (and on resize): how far it moves and at which progress each
 * item reaches the reading line. No measuring while scrolling.
 */
function useRow(live: boolean) {
  const row = useRef<HTMLDivElement>(null);
  const [dist, setDist] = useState(0);
  const [marks, setMarks] = useState<Mark[]>([]);
  useEffect(() => {
    const el = row.current;
    if (!el || !live) {
      setDist(0);
      setMarks([]);
      return;
    }
    const measure = () => {
      const items = Array.from(el.querySelectorAll<HTMLElement>(':scope > [data-item]'));
      const last = items[items.length - 1];
      if (!last) return;
      const padEnd = parseFloat(getComputedStyle(el).paddingRight) || 0;
      const d = Math.max(0, Math.round(last.offsetLeft + last.offsetWidth + padEnd - el.clientWidth));
      const line = el.clientWidth * LINE;
      setDist(d);
      // Items near the end never reach the line, so the marks are squeezed until the last one is reached at 92 %
      const raw = items.map((it) => (it.offsetLeft - line) / Math.max(1, d));
      const s = Math.min(1, 0.92 / Math.max(0.0001, raw[raw.length - 1]));
      setMarks(items.map((it, i) => {
        // already past the line at the start: always reached
        if (raw[i] <= 0) return { a: -1, b: -0.999 };
        const a = Math.min(1, raw[i] * s);
        return { a: round(a, 4), b: round(Math.min(1, Math.max(a + 0.0001, a + (it.offsetWidth / Math.max(1, d)) * s)), 4) };
      }));
    };
    measure();
    const ro = new ResizeObserver(measure);
    ro.observe(el);
    return () => ro.disconnect();
  }, [live]);
  return { row, dist, marks };
}

/** 0 → 1 while --p runs through an item's mark. Finished without JavaScript. */
const through = (m?: Mark) => (m ? seg(m.a, m.b) : '1');

function Header({ eyebrow, title, description, action, aside }: Pick<SidewaysScrollProps, 'eyebrow' | 'title' | 'description' | 'action'> & { aside?: React.ReactNode }) {
  return (
    <div className="kit-container">
      <div className="flex flex-col gap-6 md:flex-row md:items-end md:justify-between md:gap-10">
        <Reveal className="max-w-[40rem]">
          {eyebrow && <Eyebrow>{eyebrow}</Eyebrow>}
          {title && <h2 className="text-balance text-h2 font-semibold text-foreground">{title}</h2>}
          {description && <p className="mt-4 max-w-[34rem] text-pretty text-lead text-muted-foreground">{description}</p>}
        </Reveal>
        {(aside || action) && (
          <div className="flex shrink-0 items-center gap-5">
            {aside}
            {action && (
              <Button href={action.href} variant="secondary" icon="arrow-right">
                {action.label}
              </Button>
            )}
          </div>
        )}
      </div>
    </div>
  );
}

/* ---------- Items ---------- */

function Card({ it, i, mark }: { it: SidewaysItem; i: number; mark?: Mark }) {
  return (
    <article className="group relative flex h-full flex-col justify-between gap-10 overflow-hidden rounded-2xl border border-border bg-card p-6 shadow-[var(--highlight),var(--shadow-sm)] transition-[border-color,transform] duration-500 ease-[var(--ease-kit)] hover:-translate-y-1 hover:border-border-strong motion-reduce:transition-none sm:p-7">
      <div className="flex items-start justify-between">
        <span className="flex size-11 items-center justify-center rounded-xl border border-border bg-surface text-primary transition-colors duration-500 group-hover:bg-[color-mix(in_oklab,var(--primary)_12%,var(--surface))]">
          <Icon name={it.icon ?? 'sparkle'} size={20} />
        </span>
        {/* The number lights up when the card reaches the reading line */}
        <span className="relative text-[13px] font-medium tabular-nums text-subtle-foreground">
          {pad(i + 1)}
          <span aria-hidden data-p className="absolute inset-0 text-primary" style={{ opacity: through(mark) }}>
            {pad(i + 1)}
          </span>
        </span>
      </div>
      <div>
        {it.eyebrow && <p className="text-[12.5px] font-medium text-muted-foreground">{it.eyebrow}</p>}
        <h3 className="mt-2 text-balance text-[1.5rem] font-semibold leading-tight tracking-[-0.015em] text-foreground">{it.title}</h3>
        {/* Room for three lines in the row, so the titles of all cards stand on one line */}
        {it.text && <p className="mt-3 text-pretty text-[15px] leading-relaxed text-muted-foreground md:min-h-[4.875em]">{it.text}</p>}
        {it.href && (
          <a href={it.href} className="mt-5 inline-flex min-h-11 items-center gap-1.5 text-[14px] font-medium text-foreground after:absolute after:inset-0 focus-visible:outline-none [&:focus-visible]:after:rounded-2xl [&:focus-visible]:after:outline-2 [&:focus-visible]:after:outline-solid [&:focus-visible]:after:-outline-offset-2 [&:focus-visible]:after:outline-primary">
            {it.title}
            <Icon name="arrow-right" size={15} className="transition-transform duration-300 group-hover:translate-x-0.5" />
          </a>
        )}
      </div>
    </article>
  );
}

function Year({ it, i, n, mark, live }: { it: SidewaysItem; i: number; n: number; mark?: Mark; live: boolean }) {
  return (
    <div className="relative pl-8 md:pl-0 md:pt-10">
      {/* The line: downwards on phones, across from 768 px. Each item draws its own piece. */}
      <span aria-hidden className={cn('absolute left-[7px] top-2 w-px bg-border md:left-0 md:right-0 md:top-[7px] md:h-px md:w-auto', i === n - 1 ? 'bottom-0 md:right-auto md:w-12' : '-bottom-10 md:-right-0')} />
      <span
        aria-hidden
        data-p
        className={cn('absolute left-[7px] top-2 w-px origin-top bg-primary md:left-0 md:right-0 md:top-[7px] md:h-px md:w-auto md:origin-left', i === n - 1 ? 'bottom-0 md:right-auto md:w-12' : '-bottom-10 md:-right-0')}
        style={{ transform: live ? `scaleX(${through(mark)})` : undefined }}
      />
      <span aria-hidden className="absolute left-0 top-0 flex size-[15px] items-center justify-center rounded-full border border-border-strong bg-background">
        <span data-p className="size-[7px] rounded-full bg-primary" style={{ opacity: live ? `calc(0.15 + ${through(mark)} * 0.85)` : undefined }} />
      </span>
      <p data-p className="text-[clamp(2.75rem,2rem+3vw,5.5rem)] font-semibold leading-none tracking-[-0.04em] tabular-nums text-foreground" style={{ opacity: live ? `calc(0.35 + ${through(mark)} * 0.65)` : undefined }}>
        {it.year}
      </p>
      <h3 className="mt-4 text-balance text-[17px] font-semibold text-foreground">{it.title}</h3>
      {/* Three lines of room in the row, so the pictures below stand on one line */}
      {it.text && <p className="mt-2 max-w-[22rem] text-pretty text-[14.5px] leading-relaxed text-muted-foreground md:min-h-[4.875em]">{it.text}</p>}
      {it.media && <div className="mt-5 h-36 max-w-[22rem] overflow-hidden rounded-lg bg-surface md:h-[min(10rem,20svh)] [&>*]:size-full">{it.media}</div>}
    </div>
  );
}

const GALLERY_W = ['md:w-[26rem]', 'md:w-[19rem]', 'md:w-[32rem]', 'md:w-[22rem]', 'md:w-[28rem]', 'md:w-[20rem]'];

function Picture({ it, i }: { it: SidewaysItem; i: number }) {
  return (
    <figure className="group">
      <div className="relative h-[min(26rem,62vw)] overflow-hidden rounded-xl bg-surface md:h-[min(24rem,40svh)] [&>*]:size-full [&>*]:transition-transform [&>*]:duration-700 [&>*]:ease-[var(--ease-kit)] group-hover:[&>*]:scale-[1.04] motion-reduce:[&>*]:transition-none">{it.media}</div>
      <figcaption className="mt-4 flex items-baseline justify-between gap-4">
        <span className="min-w-0">
          <span className="block truncate text-[15px] font-semibold text-foreground">{it.title}</span>
          {it.text && <span className="block truncate text-[13.5px] text-muted-foreground">{it.text}</span>}
        </span>
        <span className="shrink-0 text-[12.5px] tabular-nums text-subtle-foreground">{it.eyebrow ?? pad(i + 1)}</span>
      </figcaption>
    </figure>
  );
}

function Chapter({ it, i, t }: { it: SidewaysItem; i: number; t: (typeof TEXTS)['en'] }) {
  return (
    <article className="grid h-full grid-cols-1 overflow-hidden rounded-2xl border border-border bg-card shadow-[var(--highlight),var(--shadow-sm)] lg:grid-cols-[minmax(0,1fr)_minmax(0,1.1fr)]">
      <div className="flex flex-col justify-between gap-8 p-6 sm:p-8 lg:p-10">
        <p className="flex items-center gap-3 text-[13px] font-medium tabular-nums text-muted-foreground">
          <span className="text-[color-mix(in_oklab,var(--primary)_80%,var(--foreground))]">{pad(i + 1)}</span>
          <span aria-hidden className="h-px w-8 bg-border-strong" />
          {it.eyebrow && <span className="text-foreground">{it.eyebrow}</span>}
        </p>
        <div>
          <h3 className="text-balance text-[clamp(1.625rem,1.2rem+1.3vw,2.375rem)] font-semibold leading-[1.1] tracking-[-0.025em] text-foreground">{it.title}</h3>
          {it.text && <p className="mt-4 max-w-[28rem] text-pretty text-[15.5px] leading-relaxed text-muted-foreground">{it.text}</p>}
          {it.href && (
            <a href={it.href} className="-mx-2 mt-6 inline-flex min-h-11 items-center gap-1.5 rounded-md px-2 text-[14px] font-medium text-foreground hover:underline hover:underline-offset-4 focus-visible:outline-2 focus-visible:outline-solid focus-visible:-outline-offset-2 focus-visible:outline-primary">
              {t.more}
              <span className="sr-only">, {it.title}</span>
              <Icon name="arrow-right" size={15} />
            </a>
          )}
        </div>
      </div>
      {it.media && <div className="relative min-h-[14rem] overflow-hidden border-t border-border bg-surface lg:border-l lg:border-t-0 [&>*]:absolute [&>*]:inset-0 [&>*]:size-full">{it.media}</div>}
    </article>
  );
}

function Product({ it }: { it: SidewaysItem }) {
  return (
    <article className="group relative">
      <div className="relative aspect-[4/5] overflow-hidden rounded-xl bg-surface [&>*]:size-full [&>*]:transition-transform [&>*]:duration-700 [&>*]:ease-[var(--ease-kit)] group-hover:[&>*]:scale-[1.04] motion-reduce:[&>*]:transition-none">
        {it.media}
        {it.tag && <span className="absolute left-3 top-3 !size-auto rounded-full bg-background px-2.5 py-1 text-[11.5px] font-medium text-foreground shadow-[var(--shadow-sm)]">{it.tag}</span>}
      </div>
      <div className="mt-3 flex items-start justify-between gap-3">
        <div className="min-w-0">
          <h3 className="truncate text-[14.5px] font-medium text-foreground">
            <a href={it.href ?? '#'} className="after:absolute after:inset-0 focus-visible:outline-none [&:focus-visible]:after:rounded-xl [&:focus-visible]:after:outline-2 [&:focus-visible]:after:outline-solid [&:focus-visible]:after:outline-offset-4 [&:focus-visible]:after:outline-primary">
              {it.title}
            </a>
          </h3>
          {it.eyebrow && <p className="truncate text-[13px] text-muted-foreground">{it.eyebrow}</p>}
        </div>
        {it.price && <p className="shrink-0 text-[14.5px] font-semibold tabular-nums text-foreground">{it.price}</p>}
      </div>
    </article>
  );
}

/* ---------- Section ---------- */

const ITEM_W: Record<SidewaysScrollVariant, string> = {
  cards: 'w-full md:h-[min(22rem,48svh)] md:w-[21rem] lg:w-[23rem]',
  timeline: 'w-full md:w-[21rem] md:pr-10 lg:w-[24rem]',
  gallery: 'w-[78%] shrink-0 snap-start sm:w-[22rem]',
  chapters: 'w-full md:h-[min(30rem,60svh)] md:w-[min(82vw,44rem)] lg:w-[min(72vw,58rem)]',
  products: 'w-[62%] shrink-0 snap-start sm:w-[15rem] md:w-[16rem]',
};

/** On phones: a list downwards (cards, timeline, chapters) or a swipe row (gallery, products) */
const PHONE: Record<SidewaysScrollVariant, 'list' | 'row'> = { cards: 'list', timeline: 'list', gallery: 'row', chapters: 'list', products: 'row' };

/** A section that scrolls sideways while the page scrolls down. */
export function SidewaysScroll({ variant = 'cards', eyebrow, title, description, items, action, backdrop, labels, className, ...rest }: SidewaysScrollProps) {
  const t = useTexts(TEXTS, labels);
  const live = useLive('(min-width: 768px)');
  const { row, dist, marks } = useRow(live);
  const running = live && dist > 0;
  const n = items.length;
  const [active, setActive] = useState(0);
  // The counter walks evenly from the first to the last item
  const back = useRef<HTMLParagraphElement>(null);
  // The row and the word behind it move by a transform written straight onto them. A custom
  // property on the row would restyle every picture in it on every frame.
  const onProgress = useCallback(
    (p: number) => {
      setActive(Math.round(p * (n - 1)));
      if (row.current) row.current.style.transform = `translate3d(${round(-p * dist, 1)}px, 0, 0)`;
      if (back.current) back.current.style.transform = `translate3d(${round(-p * dist * 0.35, 1)}px, 0, 0)`;
    },
    [n, dist, row],
  );
  const track = useSceneProgress<HTMLDivElement>(running, { onProgress, self: false });
  useEffect(() => {
    if (running) return;
    row.current?.style.removeProperty('transform');
    back.current?.style.removeProperty('transform');
  }, [running, row]);
  const [fadeRef, fade] = useScrollFade<HTMLDivElement>();
  const phone = PHONE[variant];

  // Tab into an item that sits outside the window: scroll the page to the progress that shows it
  const onFocus = (e: React.FocusEvent<HTMLDivElement>) => {
    if (!running || !track.current) return;
    const item = (e.target as HTMLElement).closest<HTMLElement>('[data-item]');
    if (!item) return;
    const i = Number(item.dataset.item);
    const m = marks[i];
    if (!m) return;
    const top = track.current.getBoundingClientRect().top + window.scrollY;
    const travel = track.current.offsetHeight - window.innerHeight;
    window.scrollTo({ top: Math.round(top + Math.max(0, Math.min(1, m.a + 0.02)) * travel), behavior: 'instant' as ScrollBehavior });
  };

  const counter = (
    <div className={cn('hidden items-center gap-4 md:flex', !running && 'md:hidden')} aria-hidden={!running}>
      <span className="w-16 text-[12.5px] font-medium tabular-nums text-muted-foreground">
        {pad(active + 1)} / {pad(n)}
      </span>
      <span className="relative h-0.5 w-32 overflow-hidden rounded-full bg-border lg:w-44">
        <span data-p className="absolute inset-0 origin-left rounded-full bg-primary" style={{ transform: 'scaleX(var(--p, 1))' }} />
      </span>
      <span className="sr-only">
        {active + 1} {t.of} {n}
      </span>
    </div>
  );

  const rowClass = cn(
    'relative flex gap-5 md:gap-6',
    variant === 'timeline' && 'gap-10 md:gap-0',
    variant === 'gallery' && 'gap-4 md:gap-8',
    // padding lines the first item up with the container, the last one ends on the same margin
    'px-6 lg:px-[max(2rem,calc((100%-var(--container-kit))/2))]',
    phone === 'list' ? 'max-md:flex-col' : 'kit-no-scrollbar snap-x snap-mandatory overflow-x-auto scroll-px-6 max-md:pb-2 md:snap-none',
    running ? 'w-full overflow-visible will-change-transform' : 'md:kit-no-scrollbar md:overflow-x-auto',
    !running && fade,
  );

  const content = (
    <div
      ref={(el) => {
        row.current = el;
        fadeRef.current = el;
      }}
      role={running ? undefined : 'region'}
      aria-label={running ? undefined : t.region}
      tabIndex={running || phone === 'list' ? undefined : 0}
      onFocus={onFocus}
      className={cn(rowClass, 'focus-visible:outline-2 focus-visible:outline-solid focus-visible:-outline-offset-2 focus-visible:outline-primary')}
    >
      {items.map((it, i) => (
        <div key={i} data-item={i} className={cn('relative shrink-0 md:snap-start', running && 'will-change-transform', ITEM_W[variant], variant === 'gallery' && GALLERY_W[i % GALLERY_W.length])}>
          {variant === 'cards' && <Card it={it} i={i} mark={running ? marks[i] : undefined} />}
          {variant === 'timeline' && <Year it={it} i={i} n={n} mark={marks[i]} live={running} />}
          {variant === 'gallery' && <Picture it={it} i={i} />}
          {variant === 'chapters' && <Chapter it={it} i={i} t={t} />}
          {variant === 'products' && <Product it={it} />}
        </div>
      ))}
      {variant === 'products' && action && (
        <div data-item={n} className={cn('relative shrink-0 snap-start', ITEM_W.products)}>
          <a href={action.href} className="group flex aspect-[4/5] flex-col items-center justify-center gap-4 rounded-xl border border-dashed border-border-strong text-center transition-colors duration-500 hover:border-primary hover:bg-[color-mix(in_oklab,var(--primary)_6%,transparent)] focus-visible:outline-2 focus-visible:outline-solid focus-visible:-outline-offset-2 focus-visible:outline-primary">
            <span className="flex size-12 items-center justify-center rounded-full bg-primary text-primary-foreground transition-transform duration-500 group-hover:translate-x-1">
              <Icon name="arrow-right" size={20} />
            </span>
            <span className="max-w-[10rem] text-balance text-[14.5px] font-medium text-foreground">{t.all}</span>
          </a>
        </div>
      )}
    </div>
  );

  return (
    <section {...rest} className={cn('relative overflow-x-clip', !running && 'py-20 sm:py-24 lg:py-32', className)}>
      <div ref={track} className="relative" style={running ? { height: `calc(100svh + ${dist}px)` } : undefined}>
        <div className={cn(running && 'sticky top-0 flex h-svh flex-col justify-center gap-10 overflow-clip py-10 lg:gap-14')}>
          <Header eyebrow={eyebrow} title={title} description={description} action={variant === 'products' ? undefined : action} aside={counter} />
          <div className={cn('relative z-10', !running && 'mt-12 sm:mt-14')}>{content}</div>
          {/* gallery: a large word behind the lower edge of the pictures, gliding at a third of their speed */}
          {variant === 'gallery' && backdrop && (
            <p
              ref={back}
              aria-hidden
              className="pointer-events-none relative -mt-[0.8em] hidden select-none whitespace-nowrap pl-[max(2rem,calc((100%-var(--container-kit))/2))] text-[clamp(5rem,10vw,10rem)] font-semibold leading-none tracking-[-0.05em] text-[color-mix(in_oklab,var(--foreground)_9%,transparent)] md:block"
            >
              {backdrop}
            </p>
          )}
        </div>
      </div>
    </section>
  );
}