import { useCallback, useEffect, useLayoutEffect, useRef, useState } from 'react'
import { computeLogWindow, isNearBottom } from '../lib/logWindow'

/** 每行固定高度（与 CSS 中的 .log-line 一致），虚拟滚动据此换算位置。 */
export const LOG_ROW_HEIGHT = 20
const LOG_VERTICAL_PADDING = 6
const OVERSCAN_ROWS = 24
const FOLLOW_THRESHOLD_PX = 24

interface LogViewerProps {
  /** 已经切好的日志行；内容会被原地追加，靠 version 触发重渲染。 */
  lines: readonly string[]
  /** 每次内容变化递增，用于在"跟随中"时滚到底部。 */
  version: number
  emptyLabel?: string
}

/**
 * 虚拟滚动的日志面板：只渲染视口附近的行，长日志下 DOM 节点数量恒定。
 *
 * 行高固定、不自动换行（长行横向滚动），这样滚动位置可以直接由行号换算，
 * 不需要逐行测量高度。用户往上翻时停止跟随新日志，并给出"回到最新"按钮。
 */
export function LogViewer({
  lines,
  version,
  emptyLabel = '等待日志输出...',
}: LogViewerProps) {
  const viewportRef = useRef<HTMLDivElement>(null)
  const frameRef = useRef(0)
  const followingRef = useRef(true)
  const [following, setFollowing] = useState(true)
  const [metrics, setMetrics] = useState({ scrollTop: 0, height: 0 })

  useEffect(() => {
    const node = viewportRef.current
    if (!node) {
      return
    }
    const sync = () => {
      setMetrics((previous) =>
        previous.height === node.clientHeight
          ? previous
          : { ...previous, height: node.clientHeight },
      )
    }
    sync()
    if (typeof ResizeObserver === 'undefined') {
      return
    }
    const observer = new ResizeObserver(sync)
    observer.observe(node)
    return () => observer.disconnect()
  }, [])

  useEffect(() => {
    return () => {
      if (frameRef.current) {
        cancelAnimationFrame(frameRef.current)
      }
    }
  }, [])

  const handleScroll = useCallback(() => {
    if (frameRef.current) {
      return
    }
    frameRef.current = requestAnimationFrame(() => {
      frameRef.current = 0
      const node = viewportRef.current
      if (!node) {
        return
      }
      const atBottom = isNearBottom(
        node.scrollTop,
        node.clientHeight,
        node.scrollHeight,
        FOLLOW_THRESHOLD_PX,
      )
      followingRef.current = atBottom
      setFollowing(atBottom)
      setMetrics({ scrollTop: node.scrollTop, height: node.clientHeight })
    })
  }, [])

  // 有新内容时贴住底部；同步更新滚动位置，避免窗口还停在旧位置闪一下。
  useLayoutEffect(() => {
    if (!followingRef.current) {
      return
    }
    const node = viewportRef.current
    if (!node) {
      return
    }
    node.scrollTop = node.scrollHeight
    setMetrics({ scrollTop: node.scrollTop, height: node.clientHeight })
  }, [version])

  const jumpToLatest = useCallback(() => {
    const node = viewportRef.current
    if (!node) {
      return
    }
    node.scrollTop = node.scrollHeight
    followingRef.current = true
    setFollowing(true)
    setMetrics({ scrollTop: node.scrollTop, height: node.clientHeight })
  }, [])

  const { start, end } = computeLogWindow(
    lines.length,
    metrics.scrollTop,
    metrics.height,
    LOG_ROW_HEIGHT,
    OVERSCAN_ROWS,
  )
  const canvasHeight =
    lines.length * LOG_ROW_HEIGHT + LOG_VERTICAL_PADDING * 2

  return (
    <div className="log-viewer-shell">
      <div
        className="log-viewer"
        ref={viewportRef}
        onScroll={handleScroll}
        role="log"
        aria-label="运行日志"
        tabIndex={0}
      >
        {lines.length === 0 ? (
          <p className="log-empty">{emptyLabel}</p>
        ) : (
          <div className="log-canvas" style={{ height: canvasHeight }}>
            <div
              className="log-window"
              style={{ transform: `translateY(${start * LOG_ROW_HEIGHT}px)` }}
            >
              {lines.slice(start, end).map((line, index) => (
                <div className="log-line" key={start + index}>
                  {line}
                </div>
              ))}
            </div>
          </div>
        )}
      </div>
      {lines.length > 0 && !following ? (
        <button
          type="button"
          className="log-jump-latest"
          onClick={jumpToLatest}
        >
          回到最新 ↓
        </button>
      ) : null}
    </div>
  )
}