"use client";

/**
 * The scrolling thread: date separators, grouped bubbles, and infinite scroll
 * upwards into older history.
 *
 * Two scroll behaviours matter and are easy to get wrong:
 *
 *  1. Loading older messages PREPENDS content, which would otherwise shove the
 *     reader down the page. We record scrollHeight before the prepend and add
 *     the difference back afterwards, so the message they were reading stays
 *     under their eyes.
 *
 *  2. A new message should only auto-scroll if they were already at the
 *     bottom. Yanking someone away from history they are reading is worse than
 *     making them tap to catch up.
 */

import { useCallback, useEffect, useLayoutEffect, useRef } from "react";
import { groupMessages } from "@/lib/chat-grouping";
import { MessageGroup } from "@/components/chat/message-group";
import type { ChatMessage, ChatPeer, Receipts } from "@/lib/chat";

/** Distance from the bottom still considered "at the bottom". */
const AT_BOTTOM_PX = 120;
/** Distance from the top that triggers loading older history. */
const LOAD_MORE_PX = 200;

function dayLabel(iso: string): string {
  const d = new Date(iso);
  const today = new Date();
  const yesterday = new Date(today);
  yesterday.setDate(today.getDate() - 1);
  const sameDay = (a: Date, b: Date) => a.toDateString() === b.toDateString();
  if (sameDay(d, today)) return "Today";
  if (sameDay(d, yesterday)) return "Yesterday";
  return d.toLocaleDateString(undefined, {
    day: "numeric",
    month: "short",
    year: d.getFullYear() === today.getFullYear() ? undefined : "numeric",
  });
}

export function MessageList({
  messages,
  selfId,
  senderFor,
  receipts,
  isGroup,
  hasMore,
  loadingMore,
  onLoadMore,
  onReply,
  onMenu,
  starredIds,
  onRetry,
  nameFor,
}: {
  /** Ascending by seq — oldest first. */
  messages: ChatMessage[];
  selfId: string;
  /** Resolves a message's sender for their avatar and name. */
  senderFor: (senderId: string | null | undefined) => ChatPeer | undefined;
  receipts: Receipts | null;
  isGroup: boolean;
  hasMore: boolean;
  loadingMore: boolean;
  onLoadMore: () => void;
  onReply: (message: ChatMessage) => void;
  onMenu: (message: ChatMessage, point: { x: number; y: number }) => void;
  starredIds: Set<string>;
  onRetry: (message: ChatMessage) => void;
  nameFor: (senderId: string | null | undefined) => string;
}) {
  const scrollRef = useRef<HTMLDivElement>(null);
  const atBottomRef = useRef(true);
  // Height captured just before older messages are prepended.
  const prependAnchorRef = useRef<number | null>(null);
  const prevOldestRef = useRef<string | null>(null);
  const prevNewestRef = useRef<string | null>(null);

  const oldest = messages[0]?.id ?? null;
  const newest = messages[messages.length - 1]?.id ?? null;

  const onScroll = useCallback(() => {
    const el = scrollRef.current;
    if (!el) return;
    atBottomRef.current =
      el.scrollHeight - el.scrollTop - el.clientHeight < AT_BOTTOM_PX;

    if (el.scrollTop < LOAD_MORE_PX && hasMore && !loadingMore) {
      // Capture the anchor before the parent prepends, so the layout effect
      // below can restore the reading position.
      prependAnchorRef.current = el.scrollHeight;
      onLoadMore();
    }
  }, [hasMore, loadingMore, onLoadMore]);

  // Runs before paint, so the correction is never visible as a jump.
  useLayoutEffect(() => {
    const el = scrollRef.current;
    if (!el) return;

    const prependedOlder =
      prevOldestRef.current !== null && oldest !== prevOldestRef.current;
    const appendedNewer =
      prevNewestRef.current !== null && newest !== prevNewestRef.current;
    const firstRender = prevNewestRef.current === null && newest !== null;

    if (prependedOlder && prependAnchorRef.current !== null) {
      el.scrollTop += el.scrollHeight - prependAnchorRef.current;
      prependAnchorRef.current = null;
    } else if (firstRender) {
      el.scrollTop = el.scrollHeight; // open at the newest message
    } else if (appendedNewer && atBottomRef.current) {
      el.scrollTop = el.scrollHeight;
    }

    prevOldestRef.current = oldest;
    prevNewestRef.current = newest;
  }, [oldest, newest, messages.length]);

  // Keep pulling while the thread is still shorter than its viewport —
  // otherwise a short first page leaves nothing to scroll and history stalls.
  useEffect(() => {
    const el = scrollRef.current;
    if (!el || loadingMore || !hasMore) return;
    if (el.scrollHeight <= el.clientHeight) onLoadMore();
  }, [messages.length, loadingMore, hasMore, onLoadMore]);

  const rows = groupMessages(messages);

  return (
    <div
      ref={scrollRef}
      onScroll={onScroll}
      // overflow-x-hidden is load-bearing: `overflow-y: auto` computes the
      // other axis to `auto` rather than `visible`, so the swipe-to-reply
      // transform would otherwise give the thread a horizontal scrollbar.
      className="flex-1 overflow-x-hidden overflow-y-auto overscroll-contain px-4 py-4"
    >
      {loadingMore && (
        <p className="pb-3 text-center text-xs text-muted-foreground">
          Loading earlier messages…
        </p>
      )}

      <div className="flex flex-col gap-3">
        {rows.map((row) =>
          row.kind === "date" ? (
            <div key={row.key} className="flex justify-center py-1">
              <span className="rounded-full bg-muted px-3 py-1 text-[11px] font-medium text-muted-foreground">
                {dayLabel(row.at)}
              </span>
            </div>
          ) : row.messages[0]?.kind === "system" ? (
            <div key={row.key} className="flex justify-center">
              <span className="max-w-[80%] text-center text-[11px] text-muted-foreground">
                {row.messages[0].body}
              </span>
            </div>
          ) : (
            <MessageGroup
              key={row.key}
              messages={row.messages}
              mine={row.senderId === selfId}
              sender={senderFor(row.senderId)}
              receipts={receipts}
              showAvatar={isGroup}
              selfId={selfId}
              onReply={onReply}
              onMenu={onMenu}
              starredIds={starredIds}
              onRetry={onRetry}
              nameFor={nameFor}
            />
          ),
        )}
      </div>
    </div>
  );
}