import {
  forwardRef,
  useCallback,
  useEffect,
  useImperativeHandle,
  useMemo,
  useRef,
  useState,
  type DragEvent,
} from "react";
import {
  MDXEditor,
  type MDXEditorMethods,
  headingsPlugin,
  imagePlugin,
  linkDialogPlugin,
  linkPlugin,
  listsPlugin,
  markdownShortcutPlugin,
  quotePlugin,
  thematicBreakPlugin,
  type RealmPlugin,
} from "@mdxeditor/editor";
import { cn } from "../lib/utils";

/* ---- Mention types ---- */

export interface MentionOption {
  id: string;
  name: string;
}

/* ---- Editor props ---- */

interface MarkdownEditorProps {
  value: string;
  onChange: (value: string) => void;
  placeholder?: string;
  className?: string;
  contentClassName?: string;
  onBlur?: () => void;
  imageUploadHandler?: (file: File) => Promise<string>;
  bordered?: boolean;
  /** List of mentionable users/agents. Enables @-mention autocomplete. */
  mentions?: MentionOption[];
  /** Called on Cmd/Ctrl+Enter */
  onSubmit?: () => void;
}

export interface MarkdownEditorRef {
  focus: () => void;
}

/* ---- Mention detection helpers ---- */

interface MentionState {
  query: string;
  top: number;
  left: number;
  textNode: Text;
  atPos: number;
  endPos: number;
}

function detectMention(container: HTMLElement): MentionState | null {
  const sel = window.getSelection();
  if (!sel || sel.rangeCount === 0 || !sel.isCollapsed) return null;

  const range = sel.getRangeAt(0);
  const textNode = range.startContainer;
  if (textNode.nodeType !== Node.TEXT_NODE) return null;
  if (!container.contains(textNode)) return null;

  const text = textNode.textContent ?? "";
  const offset = range.startOffset;

  // Walk backwards from cursor to find @
  let atPos = -1;
  for (let i = offset - 1; i >= 0; i--) {
    const ch = text[i];
    if (ch === "@") {
      if (i === 0 || /\s/.test(text[i - 1])) {
        atPos = i;
      }
      break;
    }
    if (/\s/.test(ch)) break;
  }

  if (atPos === -1) return null;

  const query = text.slice(atPos + 1, offset);

  // Get position relative to container
  const tempRange = document.createRange();
  tempRange.setStart(textNode, atPos);
  tempRange.setEnd(textNode, atPos + 1);
  const rect = tempRange.getBoundingClientRect();
  const containerRect = container.getBoundingClientRect();

  return {
    query,
    top: rect.bottom - containerRect.top,
    left: rect.left - containerRect.left,
    textNode: textNode as Text,
    atPos,
    endPos: offset,
  };
}

function insertMention(state: MentionState, option: MentionOption) {
  const range = document.createRange();
  range.setStart(state.textNode, state.atPos);
  range.setEnd(state.textNode, state.endPos);

  const sel = window.getSelection();
  sel?.removeAllRanges();
  sel?.addRange(range);

  // insertText preserves undo stack and triggers editor onChange
  document.execCommand("insertText", false, `@${option.name} `);
}

/* ---- Component ---- */

export const MarkdownEditor = forwardRef<MarkdownEditorRef, MarkdownEditorProps>(function MarkdownEditor({
  value,
  onChange,
  placeholder,
  className,
  contentClassName,
  onBlur,
  imageUploadHandler,
  bordered = true,
  mentions,
  onSubmit,
}: MarkdownEditorProps, forwardedRef) {
  const containerRef = useRef<HTMLDivElement>(null);
  const ref = useRef<MDXEditorMethods>(null);
  const latestValueRef = useRef(value);
  const [uploadError, setUploadError] = useState<string | null>(null);
  const [isDragOver, setIsDragOver] = useState(false);
  const dragDepthRef = useRef(0);

  // Mention state
  const [mentionState, setMentionState] = useState<MentionState | null>(null);
  const [mentionIndex, setMentionIndex] = useState(0);
  const mentionActive = mentionState !== null && mentions && mentions.length > 0;

  const filteredMentions = useMemo(() => {
    if (!mentionState || !mentions) return [];
    const q = mentionState.query.toLowerCase();
    return mentions.filter((m) => m.name.toLowerCase().includes(q)).slice(0, 8);
  }, [mentionState?.query, mentions]);

  useImperativeHandle(forwardedRef, () => ({
    focus: () => {
      ref.current?.focus(undefined, { defaultSelection: "rootEnd" });
    },
  }), []);

  const plugins = useMemo<RealmPlugin[]>(() => {
    const imageHandler = imageUploadHandler
      ? async (file: File) => {
          try {
            const src = await imageUploadHandler(file);
            setUploadError(null);
            return src;
          } catch (err) {
            const message = err instanceof Error ? err.message : "Image upload failed";
            setUploadError(message);
            throw err;
          }
        }
      : undefined;
    const all: RealmPlugin[] = [
      headingsPlugin(),
      listsPlugin(),
      quotePlugin(),
      linkPlugin(),
      linkDialogPlugin(),
      thematicBreakPlugin(),
      markdownShortcutPlugin(),
    ];
    if (imageHandler) {
      all.push(imagePlugin({ imageUploadHandler: imageHandler }));
    }
    return all;
  }, [imageUploadHandler]);

  useEffect(() => {
    if (value !== latestValueRef.current) {
      ref.current?.setMarkdown(value);
      latestValueRef.current = value;
    }
  }, [value]);

  // Mention detection: listen for selection changes and input events
  const checkMention = useCallback(() => {
    if (!mentions || mentions.length === 0 || !containerRef.current) {
      setMentionState(null);
      return;
    }
    const result = detectMention(containerRef.current);
    if (result) {
      setMentionState(result);
      setMentionIndex(0);
    } else {
      setMentionState(null);
    }
  }, [mentions]);

  useEffect(() => {
    if (!mentions || mentions.length === 0) return;

    document.addEventListener("selectionchange", checkMention);
    return () => document.removeEventListener("selectionchange", checkMention);
  }, [checkMention, mentions]);

  const selectMention = useCallback(
    (option: MentionOption) => {
      if (!mentionState) return;
      insertMention(mentionState, option);
      setMentionState(null);
    },
    [mentionState],
  );

  function hasFilePayload(evt: DragEvent<HTMLDivElement>) {
    return Array.from(evt.dataTransfer?.types ?? []).includes("Files");
  }

  const canDropImage = Boolean(imageUploadHandler);

  return (
    <div
      ref={containerRef}
      className={cn(
        "relative paperclip-mdxeditor-scope",
        bordered ? "rounded-md border border-border bg-transparent" : "bg-transparent",
        isDragOver && "ring-1 ring-primary/60 bg-accent/20",
        className,
      )}
      onKeyDownCapture={(e) => {
        // Cmd/Ctrl+Enter to submit
        if (onSubmit && e.key === "Enter" && (e.metaKey || e.ctrlKey)) {
          e.preventDefault();
          e.stopPropagation();
          onSubmit();
          return;
        }

        // Mention keyboard navigation
        if (mentionActive && filteredMentions.length > 0) {
          if (e.key === "ArrowDown") {
            e.preventDefault();
            e.stopPropagation();
            setMentionIndex((prev) => Math.min(prev + 1, filteredMentions.length - 1));
            return;
          }
          if (e.key === "ArrowUp") {
            e.preventDefault();
            e.stopPropagation();
            setMentionIndex((prev) => Math.max(prev - 1, 0));
            return;
          }
          if (e.key === "Enter" || e.key === "Tab") {
            e.preventDefault();
            e.stopPropagation();
            selectMention(filteredMentions[mentionIndex]);
            return;
          }
          if (e.key === "Escape") {
            e.preventDefault();
            e.stopPropagation();
            setMentionState(null);
            return;
          }
        }
      }}
      onDragEnter={(evt) => {
        if (!canDropImage || !hasFilePayload(evt)) return;
        dragDepthRef.current += 1;
        setIsDragOver(true);
      }}
      onDragOver={(evt) => {
        if (!canDropImage || !hasFilePayload(evt)) return;
        evt.preventDefault();
        evt.dataTransfer.dropEffect = "copy";
      }}
      onDragLeave={() => {
        if (!canDropImage) return;
        dragDepthRef.current = Math.max(0, dragDepthRef.current - 1);
        if (dragDepthRef.current === 0) setIsDragOver(false);
      }}
      onDrop={() => {
        dragDepthRef.current = 0;
        setIsDragOver(false);
      }}
    >
      <MDXEditor
        ref={ref}
        markdown={value}
        placeholder={placeholder}
        onChange={(next) => {
          latestValueRef.current = next;
          onChange(next);
        }}
        onBlur={() => onBlur?.()}
        className={cn("paperclip-mdxeditor", !bordered && "paperclip-mdxeditor--borderless")}
        contentEditableClassName={cn(
          "paperclip-mdxeditor-content focus:outline-none",
          contentClassName,
        )}
        overlayContainer={containerRef.current}
        plugins={plugins}
      />

      {/* Mention dropdown */}
      {mentionActive && filteredMentions.length > 0 && (
        <div
          className="absolute z-50 min-w-[180px] max-h-[200px] overflow-y-auto rounded-md border border-border bg-popover shadow-md"
          style={{ top: mentionState.top + 4, left: mentionState.left }}
        >
          {filteredMentions.map((option, i) => (
            <button
              key={option.id}
              className={cn(
                "flex items-center gap-2 w-full px-3 py-1.5 text-sm text-left hover:bg-accent/50 transition-colors",
                i === mentionIndex && "bg-accent",
              )}
              onMouseDown={(e) => {
                e.preventDefault(); // prevent blur
                selectMention(option);
              }}
              onMouseEnter={() => setMentionIndex(i)}
            >
              <span className="text-muted-foreground">@</span>
              <span>{option.name}</span>
            </button>
          ))}
        </div>
      )}

      {isDragOver && canDropImage && (
        <div
          className={cn(
            "pointer-events-none absolute inset-1 z-40 flex items-center justify-center rounded-md border border-dashed border-primary/80 bg-primary/10 text-xs font-medium text-primary",
            !bordered && "inset-0 rounded-sm",
          )}
        >
          Drop image to upload
        </div>
      )}
      {uploadError && (
        <p className="px-3 pb-2 text-xs text-destructive">{uploadError}</p>
      )}
    </div>
  );
});
