// Adapted from agentui (https://github.com/ashish200729/agentui), components/agents/message-scroller.tsx
// Copyright (c) 2026 Saurabh Chauhan. MIT License, see ./LICENSE-agentui.md
// 改造:保留"离底部 followThreshold 以内才跟随、滚轮/触摸/上翻键主动离开、ResizeObserver 跟随内容增长"的状态机;
// 去掉 PreviewRail 导航;新增"回到底部"按钮(离开跟随时出现,点了恢复跟随);reduced motion 下不用平滑滚动;
// 程序滚动的静默期取 --duration-layout 的值,不写字面量时长;log 只播报新增行(additions),流式正文不逐字播报。
import { ArrowDown } from "lucide-react";
import {
  type ComponentPropsWithRef,
  type ReactNode,
  useCallback,
  useEffect,
  useLayoutEffect,
  useRef,
  useState,
} from "react";
import { cn } from "~/lib/utils.ts";
import { useReducedMotion } from "./use-reduced-motion.ts";

export interface MessageScrollerProps extends Omit<ComponentPropsWithRef<"div">, "children"> {
  children: ReactNode;
  /** 用户停在底部附近时,新内容自动滚到底 */
  followOutput?: boolean;
  /** 离底部多少像素内仍算"跟随中" */
  followThreshold?: number;
  onFollowChange?: (following: boolean) => void;
  /** 滚动区的无障碍名称 */
  label?: string;
  /** 还在等待流式内容 */
  busy?: boolean;
  viewportClassName?: string;
  contentClassName?: string;
  /** "回到底部"按钮文案 */
  jumpLabel?: string;
}

function layoutMs(el: Element | null): number {
  if (!el) return 0;
  const v = getComputedStyle(el).getPropertyValue("--duration-layout").trim();
  const n = Number.parseFloat(v);
  return Number.isFinite(n) ? (v.endsWith("ms") ? n : n * 1000) : 0;
}

export function MessageScroller({
  followOutput = true,
  followThreshold = 56,
  onFollowChange,
  label = "对话记录",
  busy,
  viewportClassName,
  contentClassName,
  jumpLabel = "回到底部",
  className,
  children,
  ...props
}: MessageScrollerProps) {
  const reduce = useReducedMotion();
  const viewportRef = useRef<HTMLElement>(null);
  const contentRef = useRef<HTMLDivElement>(null);
  const followingRef = useRef(followOutput);
  const programmatic = useRef(false);
  const timer = useRef<number | undefined>(undefined);
  const [following, setFollowingState] = useState(true);

  const setFollowing = useCallback(
    (next: boolean) => {
      if (followingRef.current === next) return;
      followingRef.current = next;
      setFollowingState(next);
      onFollowChange?.(next);
    },
    [onFollowChange],
  );

  const scrollToEnd = useCallback((behavior: ScrollBehavior) => {
    const vp = viewportRef.current;
    if (!vp) return;
    programmatic.current = true;
    if (typeof vp.scrollTo === "function") vp.scrollTo({ top: vp.scrollHeight, behavior });
    else vp.scrollTop = vp.scrollHeight;
    if (timer.current) window.clearTimeout(timer.current);
    // 平滑滚动的静默期:等 scrollend,兜底取 3 × --duration-layout
    const done = () => {
      programmatic.current = false;
      vp.removeEventListener("scrollend", done);
    };
    vp.addEventListener("scrollend", done, { once: true });
    timer.current = window.setTimeout(done, behavior === "smooth" ? layoutMs(vp) * 3 : 0);
  }, []);

  const lastTop = useRef(0);
  const onScroll = useCallback(() => {
    const vp = viewportRef.current;
    if (!vp) return;
    const up = vp.scrollTop < lastTop.current - 1;
    lastTop.current = vp.scrollTop;
    const nearEnd = vp.scrollHeight - vp.scrollTop - vp.clientHeight <= followThreshold;
    // 回到底部附近:恢复跟随;离开底部:只有"往上翻"才算用户离开(程序平滑滚动与内容增长只会往下)
    if (nearEnd) setFollowing(true);
    else if (up && !programmatic.current) setFollowing(false);
  }, [followThreshold, setFollowing]);

  const leaveLiveEdge = useCallback(() => {
    programmatic.current = false;
  }, []);

  useLayoutEffect(() => {
    followingRef.current = followOutput;
    if (!followOutput) return;
    const f = requestAnimationFrame(() => scrollToEnd("auto"));
    return () => cancelAnimationFrame(f);
  }, [followOutput, scrollToEnd]);

  useEffect(() => {
    const content = contentRef.current;
    if (!content || typeof ResizeObserver === "undefined") return;
    // 首次铺满内容(回放历史)直接跳到底,之后的增长才平滑跟随
    let first = true;
    const ro = new ResizeObserver(() => {
      if (!followOutput || !followingRef.current) return;
      scrollToEnd(reduce || first ? "auto" : "smooth");
      first = false;
    });
    ro.observe(content);
    return () => ro.disconnect();
  }, [followOutput, reduce, scrollToEnd]);

  useEffect(
    () => () => {
      if (timer.current) window.clearTimeout(timer.current);
    },
    [],
  );

  return (
    <div data-slot="message-scroller" className={cn("relative min-h-0", className)} {...props}>
      <section
        ref={viewportRef}
        aria-label={label}
        // biome-ignore lint/a11y/noNoninteractiveTabindex: 滚动区要能用键盘聚焦后方向键滚动
        tabIndex={0}
        onScroll={onScroll}
        onWheel={leaveLiveEdge}
        onTouchStart={leaveLiveEdge}
        onKeyDown={(e) => {
          if (["ArrowUp", "PageUp", "Home"].includes(e.key)) leaveLiveEdge();
        }}
        className={cn(
          "h-full overflow-y-auto overscroll-contain outline-none [overflow-anchor:none] [scrollbar-gutter:stable] focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-inset",
          viewportClassName,
        )}
      >
        <div
          ref={contentRef}
          role="log"
          aria-live="polite"
          aria-relevant="additions"
          aria-busy={busy}
          className={contentClassName}
        >
          {children}
        </div>
      </section>
      {!following ? (
        <button
          type="button"
          onClick={() => {
            setFollowing(true);
            scrollToEnd(reduce ? "auto" : "smooth");
          }}
          className="pg-float absolute bottom-3 left-1/2 z-10 inline-flex h-8 -translate-x-1/2 cursor-pointer items-center gap-1.5 rounded-full px-3 text-meta font-medium text-foreground outline-none animate-in fade-in-0 slide-in-from-bottom-1 duration-base motion-reduce:animate-none focus-visible:ring-2 focus-visible:ring-ring"
        >
          <ArrowDown aria-hidden className="size-3.5" />
          {jumpLabel}
          {busy ? <span className="sr-only">（仍在输出）</span> : null}
        </button>
      ) : null}
    </div>
  );
}