import { useEffect, useRef, useState } from 'react';
import type { Key } from 'react';
import Tree from 'antd/es/tree';
import Button from 'antd/es/button';
import Input from 'antd/es/input';
import Modal from 'antd/es/modal';
import Select from 'antd/es/select';
import Space from 'antd/es/space';
import Typography from 'antd/es/typography';
import Empty from 'antd/es/empty';
import Spin from 'antd/es/spin';
import message from 'antd/es/message';
import type { DataNode, TreeProps } from 'antd/es/tree';
import {
  Category,
  fetchAdminCategories,
  createCategory,
  updateCategory,
  deleteCategory
} from '../api';
import { buildCategoryTree, categoryPath, CategoryNode } from '../filters';

const { Title, Text } = Typography;

// Counts the whole branch, so the delete confirmation can say what a parent
// takes down with it rather than only naming itself.
function branchTotals(node: CategoryNode): { categories: number; items: number } {
  return node.children.reduce(
    (acc, child) => {
      const nested = branchTotals(child);
      return { categories: acc.categories + nested.categories, items: acc.items + nested.items };
    },
    { categories: 1, items: node.item_count }
  );
}

function findNode(nodes: CategoryNode[], id: number): CategoryNode | null {
  for (const node of nodes) {
    if (node.id === id) return node;
    const nested = findNode(node.children, id);
    if (nested) return nested;
  }
  return null;
}

export default function Categories() {
  const [categories, setCategories] = useState<Category[]>([]);
  const [loading, setLoading] = useState(true);
  const [modalOpen, setModalOpen] = useState(false);
  const [editing, setEditing] = useState<Category | null>(null);
  const [name, setName] = useState('');
  const [parentId, setParentId] = useState<number | null>(null);
  // The tree stays mounted across reloads, so `defaultExpandAll` would only
  // ever apply to the categories present on first render — a branch added
  // afterwards would render collapsed and its children be unreachable.
  const [expandedKeys, setExpandedKeys] = useState<Key[]>([]);
  const expansionInitialized = useRef(false);

  const tree = buildCategoryTree(categories);

  function expand(id: number) {
    setExpandedKeys((keys) => (keys.includes(id) ? keys : [...keys, id]));
  }

  function load() {
    setLoading(true);
    return fetchAdminCategories()
      .then((list) => {
        setCategories(list);
        // Start fully expanded, then leave expansion to the user — reloading
        // must not silently re-open branches they collapsed.
        if (!expansionInitialized.current) {
          setExpandedKeys(list.map((category) => category.id));
          expansionInitialized.current = true;
        }
      })
      .finally(() => setLoading(false));
  }

  useEffect(() => { load(); }, []);

  function openNew(parent: number | null) {
    setEditing(null);
    setName('');
    setParentId(parent);
    setModalOpen(true);
  }

  function openEdit(category: Category) {
    setEditing(category);
    setName(category.name);
    setParentId(category.parent_id);
    setModalOpen(true);
  }

  async function handleSave() {
    const trimmed = name.trim();
    if (!trimmed) {
      message.error('Name is required');
      return;
    }
    try {
      if (editing) {
        await updateCategory(editing.id, { name: trimmed, parent_id: parentId });
        message.success('Category updated');
      } else {
        await createCategory(trimmed, parentId);
        message.success('Category added');
      }
      // Reveal where the category just landed instead of filing it out of sight.
      if (parentId !== null) expand(parentId);
      setModalOpen(false);
      load();
    } catch (err) {
      message.error((err as Error).message);
    }
  }

  function handleDelete(category: Category) {
    const node = findNode(tree, category.id);
    const totals = node ? branchTotals(node) : { categories: 1, items: category.item_count };
    const subcategories = totals.categories - 1;

    Modal.confirm({
      title: `Delete "${category.name}"?`,
      content: (
        <span>
          This deletes {subcategories === 0 ? 'no subcategories' : `${subcategories} subcategor${subcategories === 1 ? 'y' : 'ies'}`}
          {' '}and uncategorizes {totals.items} item{totals.items === 1 ? '' : 's'}. The items themselves are kept.
        </span>
      ),
      okText: 'Delete',
      okButtonProps: { danger: true },
      onOk: async () => {
        const result = await deleteCategory(category.id);
        message.success(
          `Deleted ${result.deleted_categories} categor${result.deleted_categories === 1 ? 'y' : 'ies'}, ` +
          `uncategorized ${result.uncategorized_items} item${result.uncategorized_items === 1 ? '' : 's'}`
        );
        load();
      }
    });
  }

  // Dragging a node onto another reparents it. The server rejects a move that
  // would put a node beneath its own descendant, so a refused drop just
  // reloads the unchanged tree.
  const handleDrop: TreeProps['onDrop'] = async (info) => {
    const dragId = Number(info.dragNode.key);
    const dropId = Number(info.node.key);
    const dropToGap = !info.dropToGap ? dropId : findNode(tree, dropId)?.parent_id ?? null;

    try {
      await updateCategory(dragId, { parent_id: dropToGap });
      if (dropToGap !== null) expand(dropToGap);
      message.success('Category moved');
    } catch (err) {
      message.error((err as Error).message);
    }
    load();
  };

  function toTreeData(nodes: CategoryNode[]): DataNode[] {
    return nodes.map((node) => ({
      key: node.id,
      title: (
        <span className="admin-category-node">
          <span>{node.name}</span>
          <Text type="secondary" style={{ fontSize: 12 }}>
            {node.item_count} item{node.item_count === 1 ? '' : 's'}
          </Text>
          <Space size={4} onClick={(event) => event.stopPropagation()}>
            <Button size="small" type="link" onClick={() => openNew(node.id)}>Add child</Button>
            <Button size="small" type="link" onClick={() => openEdit(node)}>Rename</Button>
            <Button size="small" type="link" danger onClick={() => handleDelete(node)}>Delete</Button>
          </Space>
        </span>
      ),
      children: node.children.length ? toTreeData(node.children) : undefined
    }));
  }

  // A category may not be moved beneath itself or its own descendants, so those
  // options are withheld from the parent picker too.
  const forbidden = new Set<number>();
  if (editing) {
    const collect = (node: CategoryNode) => {
      forbidden.add(node.id);
      node.children.forEach(collect);
    };
    const node = findNode(tree, editing.id);
    if (node) collect(node);
  }

  const parentOptions = [
    { value: null as number | null, label: 'No parent (top level)' },
    ...categories
      .filter((category) => !forbidden.has(category.id))
      .map((category) => ({ value: category.id as number | null, label: categoryPath(categories, category.id) }))
  ];

  return (
    <div>
      <div style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center', marginBottom: 16, flexWrap: 'wrap', gap: 8 }}>
        <Title level={4} style={{ margin: 0 }}>Categories</Title>
        <Button type="primary" onClick={() => openNew(null)}>Add Category</Button>
      </div>

      {loading && !categories.length ? <Spin /> : null}
      {!loading && !categories.length ? (
        <Empty description="No categories yet. Add one to start organizing items." />
      ) : (
        <Tree
          treeData={toTreeData(tree)}
          draggable
          blockNode
          expandedKeys={expandedKeys}
          onExpand={setExpandedKeys}
          selectable={false}
          onDrop={handleDrop}
        />
      )}

      <Modal
        title={editing ? 'Edit Category' : 'Add Category'}
        open={modalOpen}
        onOk={handleSave}
        onCancel={() => setModalOpen(false)}
        destroyOnHidden
      >
        <Space direction="vertical" style={{ width: '100%' }}>
          <label htmlFor="category-name">Name</label>
          <Input
            id="category-name"
            value={name}
            onChange={(event) => setName(event.target.value)}
            onPressEnter={handleSave}
          />
          <label htmlFor="category-parent">Parent</label>
          <Select
            id="category-parent"
            style={{ width: '100%' }}
            value={parentId}
            onChange={setParentId}
            options={parentOptions}
          />
        </Space>
      </Modal>
    </div>
  );
}