import { useCallback, useEffect, useRef, useState } from 'react'

const KEY = 'superbill.split'

/**
 * Draggable two-pane split, PDF on the left and values on the right.
 *
 * The ratio is remembered: how much page a reviewer needs beside the values
 * depends on their screen and their document, and re-dragging it every session
 * would be a small tax paid hundreds of times.
 *
 * The drag listeners live on window, not the handle, so the pointer can outrun
 * the 6px handle mid-drag without dropping it.
 *
 * Below `md`, side-by-side stops being usable at all — squeezing both panes
 * into a 375px screen leaves the PDF at ~165px wide, unreadable. Below that
 * breakpoint this becomes two full-width tabs instead of a shrinking split;
 * both panes stay mounted (toggling `hidden`/`flex`, not unmounting) so
 * switching tabs never resets the PDF viewer's scroll or zoom.
 */
export function SplitPane({
  left,
  right,
  min = 25,
  max = 75,
  ratio,
}: {
  left: React.ReactNode
  right: React.ReactNode
  min?: number
  max?: number
  /**
   * Force a split for as long as it is set, without overwriting the reviewer's
   * remembered one — fullscreen widens the page and docks the values to the
   * right, and leaving it must restore the ratio they chose, not this one.
   */
  ratio?: number
}) {
  const [saved, setSaved] = useState(() => {
    const stored = Number(localStorage.getItem(KEY))
    // The scan is the thing being read; the values are short. The page gets
    // the larger share by default.
    return stored >= min && stored <= max ? stored : 60
  })
  const pct = ratio ?? saved
  const setPct = setSaved
  const [dragging, setDragging] = useState(false)
  const [mobileTab, setMobileTab] = useState<'left' | 'right'>('left')
  const box = useRef<HTMLDivElement>(null)

  useEffect(() => {
    localStorage.setItem(KEY, String(saved))
  }, [saved])

  const onMove = useCallback(
    (e: MouseEvent) => {
      const rect = box.current?.getBoundingClientRect()
      if (!rect) return
      const next = ((e.clientX - rect.left) / rect.width) * 100
      setPct(Math.min(max, Math.max(min, next)))
    },
    [min, max],
  )

  useEffect(() => {
    if (!dragging) return
    const stop = () => setDragging(false)
    window.addEventListener('mousemove', onMove)
    window.addEventListener('mouseup', stop)
    // The PDF pane has its own drag-to-pan handler; without this it would
    // interpret the divider drag as a pan and text would select across both
    // panes on the way.
    document.body.style.userSelect = 'none'
    document.body.style.cursor = 'col-resize'
    return () => {
      window.removeEventListener('mousemove', onMove)
      window.removeEventListener('mouseup', stop)
      document.body.style.userSelect = ''
      document.body.style.cursor = ''
    }
  }, [dragging, onMove])

  return (
    <div className="flex min-h-0 flex-1 flex-col">
      {/* Tab bar: mobile only. The drag handle is the equivalent affordance on
          desktop, so this disappears at md instead of coexisting with it. */}
      <div className="flex shrink-0 border-b border-line-soft bg-rail md:hidden">
        <button
          onClick={() => setMobileTab('left')}
          aria-current={mobileTab === 'left'}
          className="flex-1 border-b-2 py-2.5 font-sora text-[13px] font-semibold transition-colors"
          style={{
            borderColor: mobileTab === 'left' ? 'var(--accent-1)' : 'transparent',
            color: mobileTab === 'left' ? 'var(--ink)' : 'var(--muted)',
          }}
        >
          Source page
        </button>
        <button
          onClick={() => setMobileTab('right')}
          aria-current={mobileTab === 'right'}
          className="flex-1 border-b-2 py-2.5 font-sora text-[13px] font-semibold transition-colors"
          style={{
            borderColor: mobileTab === 'right' ? 'var(--accent-1)' : 'transparent',
            color: mobileTab === 'right' ? 'var(--ink)' : 'var(--muted)',
          }}
        >
          Fields to review
        </button>
      </div>

      <div ref={box} className="flex min-h-0 flex-1 flex-col md:flex-row">
        <div
          style={{ '--split-w': `${pct}%` } as React.CSSProperties}
          className={`min-h-0 min-w-0 flex-1 border-b border-line md:w-[var(--split-w)] md:flex-none md:border-b-0 md:border-r md:border-line ${
            mobileTab === 'left' ? 'flex' : 'hidden'
          } md:flex`}
        >
          {left}
        </div>

        <div
          onMouseDown={() => setDragging(true)}
          onDoubleClick={() => setPct(52)}
          title="Drag to resize — double-click to reset"
          className={`hidden w-1.5 shrink-0 cursor-col-resize transition-colors md:block ${
            dragging ? 'bg-accent-1' : 'bg-line hover:bg-muted'
          }`}
        />

        {/* Pointer events off during the drag, so neither pane steals the move. */}
        <div
          style={{
            '--split-w': `${100 - pct}%`,
            pointerEvents: dragging ? 'none' : undefined,
          } as React.CSSProperties}
          className={`min-h-0 min-w-0 flex-1 md:w-[var(--split-w)] md:flex-none ${
            mobileTab === 'right' ? 'flex' : 'hidden'
          } md:flex`}
        >
          {right}
        </div>
      </div>
    </div>
  )
}