Section Header

'use client';

import { useEffect, useRef, useState } from 'react';
import { cn, prefersReducedMotion, useInView, useTexts } from '@/lib/kit';
import { Icon } from '@/components/ui/icon';
import { NumberTicker } from '@/components/ui/number-ticker';
import { WordReveal } from '@/components/ui/word-reveal';
import { useScrollProgress } from '@/components/ui/motion';

/*
 * Heads for sections that carry more than a title. The plain left, centered
 * and split heads live in section.tsx (SectionHeader). These six add what a
 * section often needs next to its title:
 *
 *   with-link     a "view all" link on the title's baseline, for rows and carousels
 *   with-filters  filter chips with a gliding marker that drive the content below
 *   chapter       a chapter mark, a rule that draws across while you scroll, title and text in two columns
 *   with-stat     title and text with one big figure that counts up
 *   sticky-side   the head stands in a side column while the content scrolls past, with a progress line
 *   statement     one large sentence whose words light up as you read (once per page)
 *
 * Server-rendered in the final state. Scroll motion reads --kit-progress, so
 * without JavaScript and with reduced motion everything stands finished.
 */

export type SectionHeadingVariant = 'with-link' | 'with-filters' | 'chapter' | 'with-stat' | 'sticky-side' | 'statement';

export interface SectionHeadingProps {
  variant?: SectionHeadingVariant;
  eyebrow?: string;
  /** For statement a plain string, its words light up one after another */
  title: React.ReactNode;
  description?: React.ReactNode;
  /** with-link (and optional on the others): a link next to the title */
  link?: { label: string; href: string };
  /** with-filters */
  filters?: string[];
  defaultFilter?: number;
  onFilterChange?: (index: number) => void;
  /** chapter: the mark, e.g. "02" or "Chapter two" */
  mark?: string;
  /** with-stat: the figure and what it means */
  stat?: { value: number; format?: Intl.NumberFormatOptions; label: string; suffix?: string };
  /** statement: a phrase inside the title in the accent color */
  highlight?: string;
  as?: 'h2' | 'h3';
  /** The section content. sticky-side places it in the right column, the others below the head. */
  children?: React.ReactNode;
  labels?: Partial<(typeof TEXTS)['en']>;
  className?: string;
}

const TEXTS = {
  en: { filters: 'Filter', progress: 'Reading progress of this section' },
  de: { filters: 'Filter', progress: 'Lesefortschritt dieses Abschnitts' },
};

const TITLE = 'text-balance text-h2 font-semibold text-foreground';
const EYEBROW = 'mb-4 text-sm font-medium text-[color-mix(in_oklab,var(--primary)_75%,var(--muted-foreground))]';
const RISE = 'transition-[opacity,transform,filter] duration-700 ease-[var(--ease-kit)] motion-reduce:transition-none';

/** Fades and lifts its children in when the head scrolls into view */
function useEnter() {
  const [ref, inView] = useInView<HTMLDivElement>();
  const hidden = !inView;
  const at = (i: number): { className: string; style: React.CSSProperties } => ({
    className: cn(RISE, hidden && 'translate-y-3 opacity-0 blur-[2px]'),
    style: { transitionDelay: hidden ? '0ms' : `${i * 80}ms` },
  });
  return [ref, at] as const;
}

function ViewAll({ link, className }: { link: NonNullable<SectionHeadingProps['link']>; className?: string }) {
  return (
    <a
      href={link.href}
      className={cn(
        'group/link inline-flex h-11 shrink-0 items-center gap-1.5 whitespace-nowrap rounded-md text-[15px] font-medium text-foreground outline-none focus-visible:outline-solid focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-[var(--ring)]',
        className,
      )}
    >
      <span className="relative">
        {link.label}
        <span aria-hidden className="absolute inset-x-0 -bottom-0.5 h-px origin-left scale-x-0 bg-current transition-transform duration-500 ease-[var(--ease-kit)] group-hover/link:scale-x-100 motion-reduce:transition-none" />
      </span>
      <span className="grid size-7 place-items-center rounded-full border border-border bg-surface text-muted-foreground transition-[transform,background-color,color,border-color] duration-300 ease-[var(--ease-kit)] group-hover/link:translate-x-1 group-hover/link:border-transparent group-hover/link:bg-primary group-hover/link:text-primary-foreground motion-reduce:transition-none">
        <Icon name="arrow-right" size={14} />
      </span>
    </a>
  );
}

function Filters({ filters, value, onChange, label }: { filters: string[]; value: number; onChange: (i: number) => void; label: string }) {
  const refs = useRef<(HTMLButtonElement | null)[]>([]);
  const row = useRef<HTMLDivElement>(null);
  const [box, setBox] = useState<{ x: number; w: number; total: number; glide: boolean } | null>(null);
  const measure = (i: number) => {
    const el = refs.current[i];
    const r = row.current;
    if (el && r) setBox((b) => ({ x: el.offsetLeft, w: el.offsetWidth, total: r.scrollWidth, glide: b !== null }));
  };
  // The marker is measured once the fonts are in, until then the active chip paints itself
  useEffect(() => {
    const go = () => measure(value);
    go();
    document.fonts?.ready.then(go).catch(() => {});
    window.addEventListener('resize', go);
    return () => window.removeEventListener('resize', go);
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, [value]);
  const move = (to: number) => {
    const n = (to + filters.length) % filters.length;
    onChange(n);
    refs.current[n]?.focus();
    refs.current[n]?.scrollIntoView({ block: 'nearest', inline: 'nearest' });
  };
  return (
    <div ref={row} role="radiogroup" aria-label={label} className="kit-no-scrollbar relative -mx-1 flex max-w-full gap-1 overflow-x-auto px-1 py-1">
      {box && (
        <span
          aria-hidden
          className={cn('pointer-events-none absolute top-1 bottom-1 left-0 bg-foreground motion-reduce:transition-none', box.glide && 'transition-[clip-path] duration-500 ease-[var(--ease-kit)]')}
          style={{ width: box.total, clipPath: `inset(0 ${box.total - box.x - box.w}px 0 ${box.x}px round 999px)` }}
        />
      )}
      {filters.map((f, i) => (
        <button
          key={f}
          ref={(el) => {
            refs.current[i] = el;
          }}
          type="button"
          role="radio"
          aria-checked={i === value}
          tabIndex={i === value ? 0 : -1}
          onClick={() => onChange(i)}
          onKeyDown={(e) => {
            if (e.key === 'ArrowRight' || e.key === 'ArrowDown') {
              e.preventDefault();
              move(value + 1);
            } else if (e.key === 'ArrowLeft' || e.key === 'ArrowUp') {
              e.preventDefault();
              move(value - 1);
            }
          }}
          className={cn(
            'relative z-10 h-11 shrink-0 whitespace-nowrap rounded-full border px-4 text-sm font-medium outline-none transition-[color,border-color,background-color] duration-300 focus-visible:outline-solid focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-[var(--ring)] sm:h-10',
            i === value
              ? cn('border-transparent text-background', !box && 'bg-foreground')
              : 'border-border text-muted-foreground hover:border-border-strong hover:text-foreground',
          )}
        >
          {f}
        </button>
      ))}
    </div>
  );
}

/**
 * Reading progress of a section into --kit-progress: 0 when its top passes 30 %
 * of the window, 1 when its end has been read. At the end of the page the end may
 * never reach that line, then the section counts as read once its end is in view.
 */
function useReadProgress<T extends HTMLElement>() {
  const ref = useRef<T>(null);
  useEffect(() => {
    const el = ref.current;
    if (!el) return;
    if (prefersReducedMotion()) {
      el.style.setProperty('--kit-progress', '1');
      return;
    }
    let frame = 0;
    const update = () => {
      frame = 0;
      const r = el.getBoundingClientRect();
      const h = window.innerHeight;
      const atEnd = h + window.scrollY >= document.documentElement.scrollHeight - 2;
      const p = atEnd && r.bottom <= h + 1 ? 1 : Math.min(1, Math.max(0, (h * 0.3 - r.top) / Math.max(1, r.height - h * 0.4)));
      el.style.setProperty('--kit-progress', p.toFixed(4));
    };
    const on = () => {
      if (!frame) frame = requestAnimationFrame(update);
    };
    update();
    window.addEventListener('scroll', on, { passive: true });
    window.addEventListener('resize', on);
    return () => {
      cancelAnimationFrame(frame);
      window.removeEventListener('scroll', on);
      window.removeEventListener('resize', on);
    };
  }, []);
  return ref;
}

/**
 * A section head with a link, filters, a chapter mark, a figure, a sticky side
 * column or a statement that lights up while you read.
 *   <SectionHeading variant="with-link" title="Latest stories" link={{ label: 'All stories', href: '/blog' }}>…</SectionHeading>
 */
export function SectionHeading({ variant = 'with-link', eyebrow, title, description, link, filters = [], defaultFilter = 0, onFilterChange, mark, stat, highlight, as: Tag = 'h2', children, labels, className }: SectionHeadingProps) {
  const t = useTexts(TEXTS, labels);
  const [ref, at] = useEnter();
  const [filter, setFilter] = useState(defaultFilter);
  const progress = useScrollProgress<HTMLDivElement>({ start: 0.05, end: 0.4 });
  const side = useReadProgress<HTMLDivElement>();
  const pickFilter = (i: number) => {
    setFilter(i);
    onFilterChange?.(i);
  };

  if (variant === 'statement') {
    const text = typeof title === 'string' ? title : '';
    return (
      <div className={cn('max-w-[58rem]', className)}>
        {eyebrow && <p className={EYEBROW}>{eyebrow}</p>}
        {text ? (
          <WordReveal as={Tag} text={text} trigger="scroll" highlight={highlight} scrollRange={[0.1, 0.55]} className="text-balance text-[clamp(1.875rem,1.2rem+2.6vw,3.25rem)] font-semibold leading-[1.12] tracking-[-0.025em] text-foreground" />
        ) : (
          <Tag className="text-balance text-[clamp(1.875rem,1.2rem+2.6vw,3.25rem)] font-semibold leading-[1.12] tracking-[-0.025em] text-foreground">{title}</Tag>
        )}
        {(description || link) && (
          <div className="mt-8 flex flex-col gap-4 sm:flex-row sm:items-center sm:gap-8">
            {description && <p className="max-w-[32rem] text-pretty text-[15px] leading-relaxed text-muted-foreground">{description}</p>}
            {link && <ViewAll link={link} />}
          </div>
        )}
        {children && <div className="mt-12 sm:mt-16">{children}</div>}
      </div>
    );
  }

  if (variant === 'sticky-side') {
    return (
      <div ref={side} className={cn('grid grid-cols-1 gap-10 lg:grid-cols-[minmax(0,5fr)_minmax(0,7fr)] lg:gap-16', className)}>
        <div className="lg:relative">
          <div ref={ref} className="lg:sticky lg:top-24">
            {eyebrow && <p {...at(0)} className={cn(EYEBROW, at(0).className)}>{eyebrow}</p>}
            <Tag {...at(1)} className={cn(TITLE, at(1).className)}>
              {title}
            </Tag>
            {description && (
              <p {...at(2)} className={cn('mt-5 max-w-[28rem] text-pretty text-lead text-muted-foreground', at(2).className)}>
                {description}
              </p>
            )}
            {link && (
              <div {...at(3)} className={cn('mt-6', at(3).className)}>
                <ViewAll link={link} />
              </div>
            )}
            <div aria-hidden className="mt-10 hidden h-px w-full max-w-[16rem] overflow-hidden bg-border lg:block">
              <div className="h-full w-full origin-left bg-primary" style={{ transform: 'scaleX(var(--kit-progress, 1))' }} />
            </div>
          </div>
        </div>
        <div className="min-w-0">{children}</div>
      </div>
    );
  }

  if (variant === 'chapter') {
    return (
      <div ref={progress} className={cn('', className)}>
        <div ref={ref} className="flex items-center gap-4">
          {mark && (
            <span {...at(0)} className={cn('shrink-0 text-[13px] font-semibold tabular-nums leading-none tracking-[0.04em] text-[color-mix(in_oklab,var(--primary)_80%,var(--foreground))]', at(0).className)}>
              {mark}
            </span>
          )}
          <span aria-hidden className="h-px min-w-0 flex-1 origin-left bg-border-strong motion-reduce:transform-none" style={{ transform: 'scaleX(var(--kit-progress, 1))' }} />
          {eyebrow && <span className="shrink-0 text-[13px] leading-none text-muted-foreground">{eyebrow}</span>}
        </div>
        <div className="mt-8 grid grid-cols-1 gap-5 sm:mt-10 lg:grid-cols-2 lg:items-end lg:gap-16">
          <Tag {...at(1)} className={cn(TITLE, at(1).className)}>
            {title}
          </Tag>
          {(description || link) && (
            <div {...at(2)} className={cn('lg:pb-1.5', at(2).className)}>
              {description && <p className="max-w-[32rem] text-pretty text-lead text-muted-foreground">{description}</p>}
              {link && <ViewAll link={link} className="mt-4" />}
            </div>
          )}
        </div>
        {children && <div className="mt-12 sm:mt-14">{children}</div>}
      </div>
    );
  }

  if (variant === 'with-stat') {
    return (
      <div className={className}>
        <div ref={ref} className="grid grid-cols-1 gap-8 border-b border-border pb-10 sm:pb-12 lg:grid-cols-[minmax(0,1fr)_auto] lg:items-end lg:gap-16">
          <div className="min-w-0">
            {eyebrow && <p {...at(0)} className={cn(EYEBROW, at(0).className)}>{eyebrow}</p>}
            <Tag {...at(1)} className={cn(TITLE, 'max-w-[40rem]', at(1).className)}>
              {title}
            </Tag>
            {description && (
              <p {...at(2)} className={cn('mt-5 max-w-[34rem] text-pretty text-lead text-muted-foreground', at(2).className)}>
                {description}
              </p>
            )}
          </div>
          {stat && (
            <div {...at(3)} className={cn('flex items-end gap-4 lg:flex-col lg:items-end lg:gap-2 lg:text-right', at(3).className)}>
              <p className="text-[clamp(3rem,2.2rem+3vw,4.75rem)] font-semibold leading-[0.9] tracking-[-0.04em] text-foreground tabular-nums">
                <NumberTicker value={stat.value} format={stat.format} duration={1600} />
                {stat.suffix && <span className="kit-accent-text">{stat.suffix}</span>}
              </p>
              <p className="max-w-[14rem] pb-1 text-[13px] leading-snug text-muted-foreground lg:pb-0">{stat.label}</p>
            </div>
          )}
        </div>
        {children && <div className="mt-10 sm:mt-12">{children}</div>}
      </div>
    );
  }

  if (variant === 'with-filters') {
    return (
      <div className={className}>
        <div ref={ref} className="flex flex-col gap-6 lg:flex-row lg:items-end lg:justify-between lg:gap-12">
          <div className="min-w-0">
            {eyebrow && <p {...at(0)} className={cn(EYEBROW, at(0).className)}>{eyebrow}</p>}
            <Tag {...at(1)} className={cn(TITLE, at(1).className)}>
              {title}
            </Tag>
            {description && (
              <p {...at(2)} className={cn('mt-4 max-w-[34rem] text-pretty text-lead text-muted-foreground', at(2).className)}>
                {description}
              </p>
            )}
          </div>
          {filters.length > 0 && (
            <div {...at(3)} className={cn('min-w-0 shrink-0', at(3).className)}>
              <Filters filters={filters} value={filter} onChange={pickFilter} label={t.filters} />
            </div>
          )}
        </div>
        {children && <div className="mt-8 sm:mt-10">{children}</div>}
      </div>
    );
  }

  /* with-link */
  return (
    <div ref={progress} className={className}>
      <div ref={ref} className="flex flex-col gap-4 sm:flex-row sm:items-end sm:justify-between sm:gap-10">
        <div className="min-w-0">
          {eyebrow && <p {...at(0)} className={cn(EYEBROW, at(0).className)}>{eyebrow}</p>}
          <Tag {...at(1)} className={cn(TITLE, at(1).className)}>
            {title}
          </Tag>
          {description && (
            <p {...at(2)} className={cn('mt-4 max-w-[34rem] text-pretty text-lead text-muted-foreground', at(2).className)}>
              {description}
            </p>
          )}
        </div>
        {link && (
          <div {...at(3)} className={cn('-mb-2 sm:-mb-1', at(3).className)}>
            <ViewAll link={link} />
          </div>
        )}
      </div>
      <div aria-hidden className="mt-8 h-px bg-border sm:mt-10">
        <div className="h-px origin-left bg-[linear-gradient(90deg,var(--primary),color-mix(in_oklab,var(--primary)_10%,transparent))]" style={{ transform: 'scaleX(var(--kit-progress, 1))' }} />
      </div>
      {children && <div className="mt-8 sm:mt-10">{children}</div>}
    </div>
  );
}