Slider

'use client';

import { useId, useState } from 'react';
import { cn, round, useInView, useLocale, useTexts } from '@/lib/kit';

const TEXTS = { en: { min: 'Minimum', max: 'Maximum' }, de: { min: 'Minimum', max: 'Maximum' } };

/* The thumb and track of a native range input, in both engines */
const RANGE = cn(
  'h-11 w-full cursor-pointer appearance-none bg-transparent outline-none disabled:cursor-not-allowed',
  '[&::-webkit-slider-runnable-track]:h-1.5 [&::-webkit-slider-runnable-track]:rounded-full [&::-webkit-slider-runnable-track]:bg-transparent',
  '[&::-moz-range-track]:h-1.5 [&::-moz-range-track]:rounded-full [&::-moz-range-track]:bg-transparent',
  '[&::-webkit-slider-thumb]:-mt-[7px] [&::-webkit-slider-thumb]:size-5 [&::-webkit-slider-thumb]:appearance-none [&::-webkit-slider-thumb]:rounded-full [&::-webkit-slider-thumb]:border-[1.5px] [&::-webkit-slider-thumb]:border-primary [&::-webkit-slider-thumb]:bg-background [&::-webkit-slider-thumb]:shadow-[var(--shadow-md)] [&::-webkit-slider-thumb]:transition-[box-shadow,scale] [&::-webkit-slider-thumb]:duration-300 [&::-webkit-slider-thumb]:ease-[cubic-bezier(0.34,1.56,0.64,1)]',
  '[&::-moz-range-thumb]:size-5 [&::-moz-range-thumb]:rounded-full [&::-moz-range-thumb]:border-[1.5px] [&::-moz-range-thumb]:border-primary [&::-moz-range-thumb]:bg-background [&::-moz-range-thumb]:shadow-[var(--shadow-md)] [&::-moz-range-thumb]:transition-[box-shadow,scale] [&::-moz-range-thumb]:duration-300',
  // The thumb grows while it is held, with a soft ring around it
  'hover:[&::-webkit-slider-thumb]:shadow-[0_0_0_5px_color-mix(in_oklab,var(--primary)_14%,transparent),var(--shadow-md)] active:[&::-webkit-slider-thumb]:scale-[1.25] active:[&::-webkit-slider-thumb]:shadow-[0_0_0_7px_color-mix(in_oklab,var(--primary)_16%,transparent),var(--shadow-md)] active:[&::-moz-range-thumb]:scale-[1.25]',
  'focus-visible:[&::-webkit-slider-thumb]:shadow-[0_0_0_5px_color-mix(in_oklab,var(--primary)_24%,transparent),var(--shadow-md)] focus-visible:[&::-moz-range-thumb]:shadow-[0_0_0_5px_color-mix(in_oklab,var(--primary)_24%,transparent)]',
  // Keyboard focus: a solid ring on the thumb itself, the part that moves (the soft halo alone was too faint)
  'focus-visible:outline-none',
  'focus-visible:[&::-webkit-slider-thumb]:outline-2 focus-visible:[&::-webkit-slider-thumb]:outline-offset-2 focus-visible:[&::-webkit-slider-thumb]:outline-solid focus-visible:[&::-webkit-slider-thumb]:outline-primary',
  'focus-visible:[&::-moz-range-thumb]:outline-2 focus-visible:[&::-moz-range-thumb]:outline-offset-2 focus-visible:[&::-moz-range-thumb]:outline-solid focus-visible:[&::-moz-range-thumb]:outline-primary',
);

function Track({ from, to, marks, min, max, disabled }: { from: number; to: number; marks?: number[]; min: number; max: number; disabled?: boolean }) {
  const pct = (v: number) => round(((v - min) / (max - min)) * 100);
  // The fill grows to its value once the slider scrolls into view
  const [ref, inView] = useInView<HTMLDivElement>();
  return (
    // Inset by the thumb radius, so 0 % and 100 % sit under the thumb center
    <div ref={ref} aria-hidden className={cn('pointer-events-none absolute inset-x-2.5 top-1/2 h-1.5 -translate-y-1/2 rounded-full bg-[color-mix(in_oklab,var(--foreground)_11%,var(--background))]', disabled && 'opacity-60')}>
      <div className="absolute inset-y-0 overflow-hidden rounded-full" style={{ left: `${pct(from)}%`, width: `${round(pct(to) - pct(from))}%` }}>
        <div className={cn('size-full origin-left rounded-full bg-primary transition-[scale] duration-[900ms] ease-[var(--ease-kit)] motion-reduce:transition-none', !inView && 'scale-x-0')} />
      </div>
      {marks?.map((m) => (
        <span
          key={m}
          className={cn(
            'absolute top-1/2 size-1 -translate-x-1/2 -translate-y-1/2 rounded-full transition-[background-color,scale] duration-300',
            m >= from && m <= to ? 'scale-125 bg-primary-foreground/70' : 'bg-[color-mix(in_oklab,var(--foreground)_30%,transparent)]',
          )}
          style={{ left: `${pct(m)}%` }}
        />
      ))}
    </div>
  );
}

/** The value in a small bubble over the thumb while it is dragged or moved with the keyboard */
function Bubble({ value, min, max, show, children }: { value: number; min: number; max: number; show: boolean; children: React.ReactNode }) {
  const p = (value - min) / (max - min);
  return (
    <span
      aria-hidden
      className={cn(
        'pointer-events-none absolute bottom-[calc(50%+18px)] z-20 -translate-x-1/2 whitespace-nowrap rounded-md bg-foreground px-2 py-1 text-xs font-medium tabular-nums text-background shadow-[var(--shadow-md)] transition-[opacity,scale,translate] duration-200 ease-[var(--ease-kit)] motion-reduce:transition-none',
        'after:absolute after:left-1/2 after:top-full after:-translate-x-1/2 after:border-4 after:border-transparent after:border-t-foreground after:content-[""]',
        show ? 'translate-y-0 scale-100 opacity-100' : 'translate-y-1 scale-90 opacity-0',
      )}
      style={{ left: `calc(10px + (100% - 20px) * ${round(p, 4)})`, transformOrigin: '50% 100%' }}
    >
      {children}
    </span>
  );
}

interface Base {
  label?: React.ReactNode;
  hint?: React.ReactNode;
  min?: number;
  max?: number;
  step?: number;
  /** Format a value for display and screen readers, e.g. (v) => `${v} €` */
  format?: (value: number) => string;
  /** Tick marks with labels under the track */
  marks?: { value: number; label?: string }[];
  /** Show min and max under the track */
  showRange?: boolean;
  disabled?: boolean;
  name?: string;
  labels?: Partial<(typeof TEXTS)['en']>;
  className?: string;
}

export type SliderProps =
  | (Base & { range?: false; value?: number; defaultValue?: number; onValueChange?: (v: number) => void })
  | (Base & { range: true; value?: [number, number]; defaultValue?: [number, number]; onValueChange?: (v: [number, number]) => void });

/**
 * A styled native range input: keyboard, touch and forms for free. One value or a range with two thumbs.
 *   <Slider label="Budget" min={500} max={10000} step={100} format={(v) => `${v} €`} />
 *   <Slider range label="Price" defaultValue={[20, 80]} />
 */
export function Slider(props: SliderProps) {
  const { label, hint, min = 0, max = 100, step = 1, format, marks, showRange, disabled, name, labels, className } = props;
  const t = useTexts(TEXTS, labels);
  const locale = useLocale();
  const id = useId();
  const fmt = format ?? ((v: number) => v.toLocaleString(locale === 'de' ? 'de-DE' : 'en-US'));
  const initial: [number, number] = props.range
    ? props.value ?? props.defaultValue ?? [min, max]
    : [min, (props.value as number | undefined) ?? (props.defaultValue as number | undefined) ?? round((min + max) / 2)];
  const [inner, setInner] = useState<[number, number]>(initial);
  const value: [number, number] = props.range ? props.value ?? inner : [min, (props.value as number | undefined) ?? inner[1]];

  const set = (next: [number, number]) => {
    setInner(next);
    if (props.range) props.onValueChange?.(next);
    else (props.onValueChange as ((v: number) => void) | undefined)?.(next[1]);
  };

  // Which thumb shows its bubble: the one held down, or the one moved with the keyboard
  const [held, setHeld] = useState<'a' | 'b' | null>(null);
  const bubble = (which: 'a' | 'b') => ({
    onPointerDown: () => setHeld(which),
    onPointerUp: () => setHeld(null),
    onPointerCancel: () => setHeld(null),
    onKeyDown: (e: React.KeyboardEvent) => e.key.startsWith('Arrow') || e.key === 'Home' || e.key === 'End' || e.key.startsWith('Page') ? setHeld(which) : undefined,
    onKeyUp: () => setHeld(null),
    onBlur: () => setHeld(null),
  });

  const readout = props.range ? `${fmt(value[0])} ${locale === 'de' ? 'bis' : 'to'} ${fmt(value[1])}` : fmt(value[1]);
  const markValues = marks?.map((m) => m.value);

  return (
    <div className={cn('grid w-full gap-1', className)} role={props.range ? 'group' : undefined} aria-labelledby={props.range && label ? `${id}-label` : undefined}>
      {(
        <div className="flex items-baseline justify-between gap-4">
          {label && (
            <label id={`${id}-label`} htmlFor={props.range ? undefined : `${id}-a`} className="text-sm font-medium text-foreground">
              {label}
            </label>
          )}
          <output htmlFor={props.range ? `${id}-a ${id}-b` : `${id}-a`} className={cn('ms-auto text-sm font-medium tabular-nums text-foreground transition-colors duration-200', held && 'text-[color-mix(in_oklab,var(--primary)_80%,var(--foreground))]')}>
            {readout}
          </output>
        </div>
      )}
      <div className="relative">
        {props.range && <Bubble value={value[0]} min={min} max={max} show={held === 'a'}>{fmt(value[0])}</Bubble>}
        <Bubble value={value[1]} min={min} max={max} show={held === 'b'}>{fmt(value[1])}</Bubble>
        <Track from={props.range ? value[0] : min} to={value[1]} marks={markValues} min={min} max={max} disabled={disabled} />
        {props.range ? (
          <>
            {/* Two inputs on one track. Only the thumbs take pointer events, so both stay reachable. */}
            <input
              id={`${id}-a`}
              type="range"
              min={min}
              max={max}
              step={step}
              value={value[0]}
              disabled={disabled}
              name={name ? `${name}-min` : undefined}
              {...bubble('a')}
              aria-label={`${t.min}${typeof label === 'string' ? `, ${label}` : ''}`}
              aria-valuetext={fmt(value[0])}
              onChange={(e) => set([Math.min(Number(e.target.value), value[1] - step), value[1]])}
              className={cn(RANGE, 'pointer-events-none absolute inset-0 [&::-moz-range-thumb]:pointer-events-auto [&::-webkit-slider-thumb]:pointer-events-auto', value[0] > max - (max - min) * 0.1 && 'z-10')}
            />
            <input
              id={`${id}-b`}
              type="range"
              min={min}
              max={max}
              step={step}
              value={value[1]}
              disabled={disabled}
              name={name ? `${name}-max` : undefined}
              {...bubble('b')}
              aria-label={`${t.max}${typeof label === 'string' ? `, ${label}` : ''}`}
              aria-valuetext={fmt(value[1])}
              onChange={(e) => set([value[0], Math.max(Number(e.target.value), value[0] + step)])}
              className={cn(RANGE, 'pointer-events-none relative [&::-moz-range-thumb]:pointer-events-auto [&::-webkit-slider-thumb]:pointer-events-auto')}
            />
          </>
        ) : (
          <input
            id={`${id}-a`}
            type="range"
            min={min}
            max={max}
            step={step}
            value={value[1]}
            disabled={disabled}
            name={name}
            {...bubble('b')}
            aria-valuetext={fmt(value[1])}
            aria-describedby={hint ? `${id}-hint` : undefined}
            onChange={(e) => set([min, Number(e.target.value)])}
            className={cn(RANGE, 'relative')}
          />
        )}
      </div>
      {marks && marks.some((m) => m.label) && (
        <div aria-hidden className="relative mx-2.5 h-4 text-xs text-subtle-foreground">
          {marks.map((m, i) => (
            <span
              key={m.value}
              className={cn('absolute top-0 whitespace-nowrap', i === 0 ? 'translate-x-[-10px]' : i === marks.length - 1 ? 'translate-x-[calc(-100%+10px)]' : '-translate-x-1/2')}
              style={{ left: `${round(((m.value - min) / (max - min)) * 100)}%` }}
            >
              {m.label}
            </span>
          ))}
        </div>
      )}
      {showRange && !marks && (
        <div aria-hidden className="flex justify-between text-xs tabular-nums text-subtle-foreground">
          <span>{fmt(min)}</span>
          <span>{fmt(max)}</span>
        </div>
      )}
      {hint && (
        <p id={`${id}-hint`} className="mt-1 text-[13px] text-muted-foreground">
          {hint}
        </p>
      )}
    </div>
  );
}