import {
  type RefObject,
  useCallback,
  useEffect,
  useRef,
  useState,
} from "react";
import { API_URL } from "../config/api.ts";

export interface UserResult {
  id: string;
  username: string;
  avatarMime: string | null;
}

interface MentionState {
  open: boolean;
  query: string;
  start: number;
  results: UserResult[];
  selectedIndex: number;
}

const CLOSED: MentionState = {
  open: false,
  query: "",
  start: 0,
  results: [],
  selectedIndex: 0,
};

function getMentionQuery(
  value: string,
  pos: number,
): { query: string; start: number } | null {
  const textBefore = value.slice(0, pos);
  // Match @word at end of text before cursor, not preceded by [ ( or word char
  const match = textBefore.match(/(?<![[(A-Za-z0-9_])@([A-Za-z0-9_]*)$/);
  if (!match || match[1].length === 0) return null;
  const start = pos - match[0].length;
  return { query: match[1], start };
}

export function useMentionAutocomplete(
  value: string,
  onChange: (v: string) => void,
  textareaRef: RefObject<HTMLTextAreaElement | null>,
) {
  const [state, setState] = useState<MentionState>(CLOSED);
  const debounceRef = useRef<ReturnType<typeof setTimeout> | null>(null);
  const cursorRef = useRef<number>(0);

  const handleMentionChange = useCallback(
    (e: React.ChangeEvent<HTMLTextAreaElement>) => {
      const newValue = e.target.value;
      const pos = e.target.selectionStart ?? newValue.length;
      cursorRef.current = pos;
      onChange(newValue);

      const mention = getMentionQuery(newValue, pos);
      if (!mention) {
        setState(CLOSED);
        if (debounceRef.current) clearTimeout(debounceRef.current);
        return;
      }

      setState((s) => ({
        ...s,
        open: false,
        query: mention.query,
        start: mention.start,
        selectedIndex: 0,
      }));

      if (debounceRef.current) clearTimeout(debounceRef.current);
      debounceRef.current = setTimeout(async () => {
        try {
          const res = await fetch(
            `${API_URL}/api/users/search?q=${
              encodeURIComponent(mention.query)
            }`,
          );
          const body = await res.json();
          if (body.success && body.data.length > 0) {
            setState((s) =>
              s.query === mention.query
                ? { ...s, open: true, results: body.data, selectedIndex: 0 }
                : s
            );
          } else {
            setState(CLOSED);
          }
        } catch {
          setState(CLOSED);
        }
      }, 150);
    },
    [onChange],
  );

  const doSelect = useCallback(
    (username: string, start: number, cursorPos: number) => {
      const before = value.slice(0, start);
      const after = value.slice(cursorPos);
      onChange(`${before}@${username} ${after}`);
      setState(CLOSED);
      if (debounceRef.current) clearTimeout(debounceRef.current);
      setTimeout(() => {
        const el = textareaRef.current;
        if (el) {
          const newPos = start + username.length + 2; // @ + username + space
          el.focus();
          el.setSelectionRange(newPos, newPos);
        }
      }, 0);
    },
    [value, onChange, textareaRef],
  );

  const handleMentionKeyDown = useCallback(
    (e: React.KeyboardEvent<HTMLTextAreaElement>) => {
      if (!state.open || state.results.length === 0) return;
      if (e.key === "ArrowDown") {
        e.preventDefault();
        setState((s) => ({
          ...s,
          selectedIndex: Math.min(s.selectedIndex + 1, s.results.length - 1),
        }));
      } else if (e.key === "ArrowUp") {
        e.preventDefault();
        setState((s) => ({
          ...s,
          selectedIndex: Math.max(s.selectedIndex - 1, 0),
        }));
      } else if (
        (e.key === "Enter" || e.key === "Tab") && !e.ctrlKey && !e.metaKey
      ) {
        e.preventDefault();
        const user = state.results[state.selectedIndex];
        if (user) doSelect(user.username, state.start, cursorRef.current);
      } else if (e.key === "Escape") {
        e.preventDefault();
        setState(CLOSED);
      }
    },
    [state, doSelect],
  );

  const handleMentionSelect = useCallback(
    (username: string) => {
      doSelect(username, state.start, cursorRef.current);
    },
    [doSelect, state.start],
  );

  useEffect(() => {
    return () => {
      if (debounceRef.current) clearTimeout(debounceRef.current);
    };
  }, []);

  return {
    mentionOpen: state.open,
    mentionResults: state.results,
    mentionSelectedIndex: state.selectedIndex,
    handleMentionChange,
    handleMentionKeyDown,
    handleMentionSelect,
  };
}