gitmost/apps/client/src/features/page/tree/model/tree-model.test.ts

import { describe, it, expect } from 'vitest';
import { treeModel } from './tree-model';
import type { TreeNode } from './tree-model.types';

type N = TreeNode<{ name: string }>;

const fixture: N[] = [
  {
    id: 'a',
    name: 'A',
    children: [
      { id: 'a1', name: 'A1', children: [{ id: 'a1a', name: 'A1a' }] },
      { id: 'a2', name: 'A2' },
    ],
  },
  { id: 'b', name: 'B' },
];

describe('treeModel.find', () => {
  it('finds a root node', () => {
    expect(treeModel.find(fixture, 'a')?.name).toBe('A');
  });
  it('finds a deeply nested node', () => {
    expect(treeModel.find(fixture, 'a1a')?.name).toBe('A1a');
  });
  it('returns null for unknown id', () => {
    expect(treeModel.find(fixture, 'zzz')).toBeNull();
  });
});

describe('treeModel.path', () => {
  it('returns root-to-leaf path for nested id', () => {
    const p = treeModel.path(fixture, 'a1a');
    expect(p?.map((n) => n.id)).toEqual(['a', 'a1', 'a1a']);
  });
  it('returns [node] for root-level id', () => {
    expect(treeModel.path(fixture, 'b')?.map((n) => n.id)).toEqual(['b']);
  });
  it('returns null for unknown id', () => {
    expect(treeModel.path(fixture, 'zzz')).toBeNull();
  });
});

describe('treeModel.siblingsOf', () => {
  it('returns siblings + parent + index for a child', () => {
    const info = treeModel.siblingsOf(fixture, 'a2');
    expect(info?.parentId).toBe('a');
    expect(info?.siblings.map((n) => n.id)).toEqual(['a1', 'a2']);
    expect(info?.index).toBe(1);
  });
  it('returns parentId null + root siblings for a root id', () => {
    const info = treeModel.siblingsOf(fixture, 'b');
    expect(info?.parentId).toBeNull();
    expect(info?.siblings.map((n) => n.id)).toEqual(['a', 'b']);
    expect(info?.index).toBe(1);
  });
  it('returns null for unknown id', () => {
    expect(treeModel.siblingsOf(fixture, 'zzz')).toBeNull();
  });
});

describe('treeModel.isDescendant', () => {
  it('returns true when descendantId is nested under ancestorId', () => {
    expect(treeModel.isDescendant(fixture, 'a', 'a1a')).toBe(true);
  });
  it('returns false when ids are siblings', () => {
    expect(treeModel.isDescendant(fixture, 'a1', 'a2')).toBe(false);
  });
  it('returns false when ancestorId is the same as descendantId', () => {
    expect(treeModel.isDescendant(fixture, 'a', 'a')).toBe(false);
  });
  it('returns false for unknown ids', () => {
    expect(treeModel.isDescendant(fixture, 'zzz', 'a')).toBe(false);
  });
});

describe('treeModel.visible', () => {
  it('returns only root nodes when no openIds', () => {
    const v = treeModel.visible(fixture, new Set());
    expect(v.map((n) => n.id)).toEqual(['a', 'b']);
  });
  it('includes children of open ids in DFS order', () => {
    const v = treeModel.visible(fixture, new Set(['a']));
    expect(v.map((n) => n.id)).toEqual(['a', 'a1', 'a2', 'b']);
  });
  it('recursively descends through chains of open ids', () => {
    const v = treeModel.visible(fixture, new Set(['a', 'a1']));
    expect(v.map((n) => n.id)).toEqual(['a', 'a1', 'a1a', 'a2', 'b']);
  });
  it('ignores openIds that are not in the tree', () => {
    const v = treeModel.visible(fixture, new Set(['ghost']));
    expect(v.map((n) => n.id)).toEqual(['a', 'b']);
  });
});

describe('treeModel.insert', () => {
  const leaf = (id: string): N => ({ id, name: id.toUpperCase() });

  it('inserts at end when index is undefined', () => {
    const t = treeModel.insert(fixture, 'a', leaf('a3'));
    expect(treeModel.siblingsOf(t, 'a3')?.siblings.map((n) => n.id)).toEqual([
      'a1', 'a2', 'a3',
    ]);
  });
  it('inserts at index 0', () => {
    const t = treeModel.insert(fixture, 'a', leaf('a0'), 0);
    expect(treeModel.siblingsOf(t, 'a0')?.siblings.map((n) => n.id)).toEqual([
      'a0', 'a1', 'a2',
    ]);
  });
  it('inserts in the middle', () => {
    const t = treeModel.insert(fixture, 'a', leaf('a1half'), 1);
    expect(
      treeModel.siblingsOf(t, 'a1half')?.siblings.map((n) => n.id),
    ).toEqual(['a1', 'a1half', 'a2']);
  });
  it('inserts at root when parentId is null', () => {
    const t = treeModel.insert(fixture, null, leaf('c'));
    expect(t.map((n) => n.id)).toEqual(['a', 'b', 'c']);
  });
  it('returns same array reference for unknown parentId', () => {
    const t = treeModel.insert(fixture, 'ghost', leaf('zz'));
    expect(t).toBe(fixture);
  });
  it('initializes children array when parent had no children', () => {
    const t = treeModel.insert(fixture, 'b', leaf('b1'));
    expect(treeModel.find(t, 'b')?.children?.map((n) => n.id)).toEqual(['b1']);
  });
});

describe('treeModel.insertByPosition', () => {
  // Server-authoritative broadcasts ship the node's fractional `position`; the
  // receiver inserts among already-loaded siblings ordered by `position`.
  type P = TreeNode<{ name: string; position?: string }>;

  const roots: P[] = [
    { id: 'a', name: 'A', position: 'a0' },
    { id: 'b', name: 'B', position: 'a2' },
    { id: 'c', name: 'C', position: 'a4' },
  ];

  it('inserts a root node in position order (middle)', () => {
    const node: P = { id: 'x', name: 'X', position: 'a3' };
    const t = treeModel.insertByPosition(roots, null, node);
    expect(t.map((n) => n.id)).toEqual(['a', 'b', 'x', 'c']);
  });

  it('inserts a root node at the front when its position sorts first', () => {
    const node: P = { id: 'x', name: 'X', position: 'a-' };
    const t = treeModel.insertByPosition(roots, null, node);
    expect(t.map((n) => n.id)).toEqual(['x', 'a', 'b', 'c']);
  });

  it('appends a root node when its position sorts last', () => {
    const node: P = { id: 'x', name: 'X', position: 'a9' };
    const t = treeModel.insertByPosition(roots, null, node);
    expect(t.map((n) => n.id)).toEqual(['a', 'b', 'c', 'x']);
  });

  it('produces the same order regardless of which siblings are loaded', () => {
    // Client 1 loaded all siblings; client 2 only loaded a subset. The inserted
    // node lands in a consistent relative position for both.
    const full: P[] = roots;
    const partial: P[] = [roots[0], roots[2]]; // a, c (b not loaded)
    const node: P = { id: 'x', name: 'X', position: 'a3' };

    expect(
      treeModel.insertByPosition(full, null, node).map((n) => n.id),
    ).toEqual(['a', 'b', 'x', 'c']);
    expect(
      treeModel.insertByPosition(partial, null, node).map((n) => n.id),
    ).toEqual(['a', 'x', 'c']);
  });

  it('inserts a child in position order under the parent', () => {
    const tree: P[] = [
      {
        id: 'p',
        name: 'P',
        position: 'a0',
        children: [
          { id: 'p1', name: 'P1', position: 'a0' },
          { id: 'p2', name: 'P2', position: 'a2' },
        ],
      },
    ];
    const node: P = { id: 'p15', name: 'P1.5', position: 'a1' };
    const t = treeModel.insertByPosition(tree, 'p', node);
    expect(treeModel.find(t, 'p')?.children?.map((n) => n.id)).toEqual([
      'p1', 'p15', 'p2',
    ]);
  });

  it('appends when the new node has no position', () => {
    const node: P = { id: 'x', name: 'X' };
    const t = treeModel.insertByPosition(roots, null, node);
    expect(t.map((n) => n.id)).toEqual(['a', 'b', 'c', 'x']);
  });

  it('tie-break: a node whose position EQUALS a sibling lands deterministically (strict >)', () => {
    // The insertion index is the first sibling whose position sorts STRICTLY
    // after the new node's. An equal sibling is not strictly after, so it is
    // skipped — the new node lands immediately AFTER every equal-position
    // sibling and before the first strictly-greater one. This is deterministic:
    // a tie always resolves the same way on every client.
    const node: P = { id: 'x', name: 'X', position: 'a2' }; // equals b's position
    const t = treeModel.insertByPosition(roots, null, node);
    expect(t.map((n) => n.id)).toEqual(['a', 'b', 'x', 'c']);
  });
});

// addTreeNode idempotency: the receiver early-returns when the node id already
// exists, so re-delivery (or the author's optimistic node) is never duplicated.
// This guards the find-then-skip contract insertByPosition relies on.
describe('addTreeNode idempotency (find-then-skip)', () => {
  type P = TreeNode<{ name: string; position?: string }>;

  const applyAddTreeNode = (tree: P[], node: P): P[] => {
    if (treeModel.find(tree, node.id)) return tree;
    return treeModel.insertByPosition(tree, null, node);
  };

  it('does not insert a duplicate when the id already exists', () => {
    const tree: P[] = [{ id: 'a', name: 'A', position: 'a0' }];
    const node: P = { id: 'a', name: 'A again', position: 'a5' };
    const t1 = applyAddTreeNode(tree, node);
    expect(t1).toBe(tree);
    expect(t1.map((n) => n.id)).toEqual(['a']);
  });

  it('inserts once, then is a no-op on repeat delivery', () => {
    let tree: P[] = [{ id: 'a', name: 'A', position: 'a0' }];
    const node: P = { id: 'x', name: 'X', position: 'a5' };
    tree = applyAddTreeNode(tree, node);
    expect(tree.map((n) => n.id)).toEqual(['a', 'x']);
    const again = applyAddTreeNode(tree, node);
    expect(again).toBe(tree);
    expect(again.filter((n) => n.id === 'x')).toHaveLength(1);
  });
});

// handleCreate optimistic-insert idempotency: the author's optimistic insert is
// now guarded by `treeModel.find` (same contract as the addTreeNode socket
// handler) because the server's broadcast can win the race and insert the node
// first. Whichever runs first inserts; the second is a no-op. Exactly one row.
describe('handleCreate optimistic-insert idempotency (find-then-skip)', () => {
  // Mirrors the guarded optimistic insert in use-tree-mutation handleCreate.
  const applyOptimisticInsert = (
    tree: N[],
    parentId: string | null,
    node: N,
    index: number,
  ): N[] => {
    if (treeModel.find(tree, node.id)) return tree;
    return treeModel.insert(tree, parentId, node, index);
  };

  // Mirrors the addTreeNode socket handler guard.
  const applyAddTreeNode = (tree: N[], parentId: string | null, node: N): N[] => {
    if (treeModel.find(tree, node.id)) return tree;
    return treeModel.insert(tree, parentId, node);
  };

  const created: N = { id: 'new', name: '' };

  it('optimistic insert is a no-op when server addTreeNode already inserted it', () => {
    // Reverse-of-reverse race: server wins.
    const afterServer = applyAddTreeNode(fixture, null, created);
    expect(afterServer.filter((n) => n.id === 'new')).toHaveLength(1);
    const afterOptimistic = applyOptimisticInsert(
      afterServer,
      null,
      created,
      afterServer.length,
    );
    expect(afterOptimistic).toBe(afterServer); // skipped
    expect(afterOptimistic.filter((n) => n.id === 'new')).toHaveLength(1);
  });

  it('server addTreeNode is a no-op when optimistic insert already ran (optimistic-first)', () => {
    const afterOptimistic = applyOptimisticInsert(fixture, null, created, fixture.length);
    expect(afterOptimistic.filter((n) => n.id === 'new')).toHaveLength(1);
    const afterServer = applyAddTreeNode(afterOptimistic, null, created);
    expect(afterServer).toBe(afterOptimistic); // skipped
    expect(afterServer.filter((n) => n.id === 'new')).toHaveLength(1);
  });

  it('inserts exactly once when only the optimistic path runs', () => {
    const t = applyOptimisticInsert(fixture, 'a', { id: 'a3', name: '' }, 2);
    expect(treeModel.find(t, 'a')?.children?.filter((n) => n.id === 'a3')).toHaveLength(1);
  });
});

// moveTreeNode socket-handler semantics: the receiver must place the moved node
// by `position` (NOT index 0) and apply the `pageData` the payload carries so a
// moved node's title/icon/chevron stay correct. This mirrors the reducer in
// use-tree-socket.ts so the contract is unit-tested without rendering the hook.
describe('moveTreeNode handler (place by position + apply pageData)', () => {
  type P = TreeNode<{
    name: string;
    position?: string;
    icon?: string;
    hasChildren?: boolean;
    parentPageId?: string | null;
  }>;

  const applyMoveTreeNode = (
    tree: P[],
    payload: {
      id: string;
      parentId: string | null;
      position: string;
      pageData?: { title?: string | null; icon?: string | null; hasChildren?: boolean };
    },
  ): P[] => {
    if (!treeModel.find(tree, payload.id)) return tree;
    const placed = treeModel.placeByPosition(tree, payload.id, {
      parentId: payload.parentId,
      position: payload.position,
    });
    if (placed === tree) return treeModel.remove(tree, payload.id);
    const patch: Partial<P> = {
      position: payload.position,
      parentPageId: payload.parentId,
    } as Partial<P>;
    const pd = payload.pageData;
    if (pd) {
      if (pd.title !== undefined) (patch as { name?: string }).name = pd.title ?? '';
      if (pd.icon !== undefined) (patch as { icon?: string }).icon = pd.icon ?? undefined;
      if (pd.hasChildren !== undefined)
        (patch as { hasChildren?: boolean }).hasChildren = pd.hasChildren;
    }
    return treeModel.update(placed, payload.id, patch);
  };

  const tree: P[] = [
    {
      id: 'dst',
      name: 'DST',
      position: 'a0',
      children: [
        { id: 'c1', name: 'C1', position: 'a1' },
        { id: 'c2', name: 'C2', position: 'a3' },
        { id: 'c3', name: 'C3', position: 'a5' },
      ],
    },
    { id: 'src', name: 'SRC', position: 'a9' },
  ];

  it('lands the moved node in the correct MIDDLE slot, not at index 0', () => {
    const t = applyMoveTreeNode(tree, {
      id: 'src',
      parentId: 'dst',
      position: 'a4',
    });
    expect(treeModel.find(t, 'dst')?.children?.map((n) => n.id)).toEqual([
      'c1', 'c2', 'src', 'c3',
    ]);
  });

  it('lands the moved node at the END when position sorts last', () => {
    const t = applyMoveTreeNode(tree, {
      id: 'src',
      parentId: 'dst',
      position: 'a8',
    });
    expect(treeModel.find(t, 'dst')?.children?.map((n) => n.id)).toEqual([
      'c1', 'c2', 'c3', 'src',
    ]);
  });

  it('applies pageData (title/icon/hasChildren) to the moved node', () => {
    const t = applyMoveTreeNode(tree, {
      id: 'src',
      parentId: 'dst',
      position: 'a4',
      pageData: { title: 'Renamed', icon: '🔥', hasChildren: true },
    });
    const moved = treeModel.find(t, 'src');
    expect(moved?.name).toBe('Renamed');
    expect(moved?.icon).toBe('🔥');
    expect(moved?.hasChildren).toBe(true);
    expect(moved?.position).toBe('a4');
  });

  it('falls back to removing the node when the destination parent is not loaded', () => {
    const t = applyMoveTreeNode(tree, {
      id: 'src',
      parentId: 'not-loaded',
      position: 'a4',
    });
    expect(treeModel.find(t, 'src')).toBeNull();
  });
});

describe('treeModel.remove', () => {
  it('removes a leaf', () => {
    const t = treeModel.remove(fixture, 'a2');
    expect(treeModel.find(t, 'a2')).toBeNull();
  });
  it('removes a subtree', () => {
    const t = treeModel.remove(fixture, 'a1');
    expect(treeModel.find(t, 'a1')).toBeNull();
    expect(treeModel.find(t, 'a1a')).toBeNull();
  });
  it('removes a root node', () => {
    const t = treeModel.remove(fixture, 'b');
    expect(t.map((n) => n.id)).toEqual(['a']);
  });
  it('returns same array reference for unknown id', () => {
    expect(treeModel.remove(fixture, 'ghost')).toBe(fixture);
  });
});

describe('treeModel.update', () => {
  it('shallow-merges a patch on the matching node', () => {
    const t = treeModel.update(fixture, 'a1', { name: 'A1-renamed' });
    expect(treeModel.find(t, 'a1')?.name).toBe('A1-renamed');
  });
  it('returns same array reference for unknown id', () => {
    expect(treeModel.update(fixture, 'ghost', { name: 'x' })).toBe(fixture);
  });
  it("preserves children when patching parent's own fields", () => {
    const t = treeModel.update(fixture, 'a', { name: 'A-renamed' });
    expect(treeModel.find(t, 'a')?.children?.map((n) => n.id)).toEqual([
      'a1', 'a2',
    ]);
  });
  it('preserves reference identity of unrelated subtrees', () => {
    const t = treeModel.update(fixture, 'a1', { name: 'X' });
    expect(t[1]).toBe(fixture[1]);
  });
});

describe('treeModel.appendChildren', () => {
  const kid = (id: string): N => ({ id, name: id });

  it('appends to existing children', () => {
    const t = treeModel.appendChildren(fixture, 'a', [kid('a3'), kid('a4')]);
    expect(treeModel.find(t, 'a')?.children?.map((n) => n.id)).toEqual([
      'a1', 'a2', 'a3', 'a4',
    ]);
  });
  it('initializes children when parent had none', () => {
    const t = treeModel.appendChildren(fixture, 'b', [kid('b1')]);
    expect(treeModel.find(t, 'b')?.children?.map((n) => n.id)).toEqual(['b1']);
  });
  it('returns same array reference for unknown parentId', () => {
    expect(treeModel.appendChildren(fixture, 'ghost', [kid('zz')])).toBe(
      fixture,
    );
  });

  // Regression: lazy-load + auto-expand can race and call appendChildren with
  // children that overlap what's already there. React then crashes on duplicate
  // keys. Defensive dedup at the model level.
  it('dedups against existing children by id', () => {
    const t1 = treeModel.appendChildren(fixture, 'a', [
      kid('a3'),
      kid('a4'),
    ]);
    const t2 = treeModel.appendChildren(t1, 'a', [
      kid('a3'),
      kid('a4'),
      kid('a5'),
    ]);
    expect(treeModel.find(t2, 'a')?.children?.map((n) => n.id)).toEqual([
      'a1', 'a2', 'a3', 'a4', 'a5',
    ]);
  });

  it('returns same array reference when every child is a duplicate', () => {
    const t1 = treeModel.appendChildren(fixture, 'a', [kid('a3')]);
    const t2 = treeModel.appendChildren(t1, 'a', [kid('a3')]);
    expect(t2).toBe(t1);
  });
});

describe('treeModel.place', () => {
  it('moves a node to a new parent at a given index', () => {
    const t = treeModel.place(fixture, 'a2', { parentId: 'b', index: 0 });
    expect(treeModel.find(t, 'a')?.children?.map((n) => n.id)).toEqual(['a1']);
    expect(treeModel.find(t, 'b')?.children?.map((n) => n.id)).toEqual(['a2']);
  });
  it('moves a node to root', () => {
    const t = treeModel.place(fixture, 'a1', { parentId: null, index: 0 });
    expect(t.map((n) => n.id)).toEqual(['a1', 'a', 'b']);
    expect(treeModel.find(t, 'a')?.children?.map((n) => n.id)).toEqual(['a2']);
  });
  it('reorders within the same parent', () => {
    const t = treeModel.place(fixture, 'a2', { parentId: 'a', index: 0 });
    expect(treeModel.find(t, 'a')?.children?.map((n) => n.id)).toEqual([
      'a2', 'a1',
    ]);
  });
  it('returns same array reference for unknown source', () => {
    expect(
      treeModel.place(fixture, 'ghost', { parentId: 'a', index: 0 }),
    ).toBe(fixture);
  });
  it('returns same array reference for unknown destination parent', () => {
    expect(
      treeModel.place(fixture, 'a1', { parentId: 'ghost', index: 0 }),
    ).toBe(fixture);
  });
});

describe('treeModel.placeByPosition', () => {
  // Server-authoritative `moveTreeNode` ships the moved node's fractional
  // `position`; the receiver must sort it into the correct slot among the new
  // siblings — NOT drop it at index 0.
  type P = TreeNode<{ name: string; position?: string }>;

  const tree: P[] = [
    {
      id: 'dst',
      name: 'DST',
      position: 'a0',
      children: [
        { id: 'c1', name: 'C1', position: 'a1' },
        { id: 'c2', name: 'C2', position: 'a3' },
        { id: 'c3', name: 'C3', position: 'a5' },
      ],
    },
    { id: 'src', name: 'SRC', position: 'a9' },
  ];

  it('places the moved node in the MIDDLE of new siblings by position', () => {
    const t = treeModel.placeByPosition(tree, 'src', {
      parentId: 'dst',
      position: 'a4',
    });
    expect(treeModel.find(t, 'dst')?.children?.map((n) => n.id)).toEqual([
      'c1', 'c2', 'src', 'c3',
    ]);
  });

  it('places the moved node at the END when its position sorts last', () => {
    const t = treeModel.placeByPosition(tree, 'src', {
      parentId: 'dst',
      position: 'a8',
    });
    expect(treeModel.find(t, 'dst')?.children?.map((n) => n.id)).toEqual([
      'c1', 'c2', 'c3', 'src',
    ]);
  });

  it('places the moved node at the FRONT only when its position sorts first', () => {
    const t = treeModel.placeByPosition(tree, 'src', {
      parentId: 'dst',
      position: 'a0',
    });
    expect(treeModel.find(t, 'dst')?.children?.map((n) => n.id)).toEqual([
      'src', 'c1', 'c2', 'c3',
    ]);
  });

  it('stamps the authoritative position onto the moved node', () => {
    const t = treeModel.placeByPosition(tree, 'src', {
      parentId: 'dst',
      position: 'a4',
    });
    expect(treeModel.find(t, 'src')?.position).toBe('a4');
  });

  it('reorders within the same parent by position (not to index 0)', () => {
    const same: P[] = [
      {
        id: 'p',
        name: 'P',
        position: 'a0',
        children: [
          { id: 'x', name: 'X', position: 'a1' },
          { id: 'y', name: 'Y', position: 'a2' },
          { id: 'z', name: 'Z', position: 'a3' },
        ],
      },
    ];
    // Move x to between y and z.
    const t = treeModel.placeByPosition(same, 'x', {
      parentId: 'p',
      position: 'a25',
    });
    expect(treeModel.find(t, 'p')?.children?.map((n) => n.id)).toEqual([
      'y', 'x', 'z',
    ]);
  });

  it('returns same array reference for unknown source', () => {
    expect(
      treeModel.placeByPosition(tree, 'ghost', { parentId: 'dst', position: 'a4' }),
    ).toBe(tree);
  });

  it('returns same array reference when destination parent is not loaded', () => {
    expect(
      treeModel.placeByPosition(tree, 'src', { parentId: 'ghost', position: 'a4' }),
    ).toBe(tree);
  });

  it('moves a node to root by position', () => {
    const roots: P[] = [
      { id: 'r1', name: 'R1', position: 'a1' },
      { id: 'r2', name: 'R2', position: 'a5' },
      {
        id: 'rp',
        name: 'RP',
        position: 'a7',
        children: [{ id: 'child', name: 'CHILD', position: 'a1' }],
      },
    ];
    const t = treeModel.placeByPosition(roots, 'child', {
      parentId: null,
      position: 'a3',
    });
    expect(t.map((n) => n.id)).toEqual(['r1', 'child', 'r2', 'rp']);
  });
});

describe('treeModel.move', () => {
  it('reorder-before within same parent: moves source to target index', () => {
    const { tree: t, result } = treeModel.move(fixture, 'a2', {
      kind: 'reorder-before',
      targetId: 'a1',
    });
    expect(treeModel.find(t, 'a')?.children?.map((n) => n.id)).toEqual([
      'a2', 'a1',
    ]);
    expect(result).toEqual({ parentId: 'a', index: 0 });
  });
  it('reorder-after within same parent', () => {
    const { tree: t, result } = treeModel.move(fixture, 'a1', {
      kind: 'reorder-after',
      targetId: 'a2',
    });
    expect(treeModel.find(t, 'a')?.children?.map((n) => n.id)).toEqual([
      'a2', 'a1',
    ]);
    expect(result).toEqual({ parentId: 'a', index: 1 });
  });
  it('make-child appends at end of target children', () => {
    const { tree: t, result } = treeModel.move(fixture, 'b', {
      kind: 'make-child',
      targetId: 'a',
    });
    expect(treeModel.find(t, 'a')?.children?.map((n) => n.id)).toEqual([
      'a1', 'a2', 'b',
    ]);
    expect(result).toEqual({ parentId: 'a', index: 2 });
  });
  it('make-child initializes children when target had none', () => {
    const { tree: t, result } = treeModel.move(fixture, 'a2', {
      kind: 'make-child',
      targetId: 'b',
    });
    expect(treeModel.find(t, 'b')?.children?.map((n) => n.id)).toEqual(['a2']);
    expect(result).toEqual({ parentId: 'b', index: 0 });
  });
  it('reorder-before across parents', () => {
    const { tree: t, result } = treeModel.move(fixture, 'b', {
      kind: 'reorder-before',
      targetId: 'a1',
    });
    expect(treeModel.find(t, 'a')?.children?.map((n) => n.id)).toEqual([
      'b', 'a1', 'a2',
    ]);
    expect(result).toEqual({ parentId: 'a', index: 0 });
  });
  it('reorder-after to root', () => {
    const { tree: t, result } = treeModel.move(fixture, 'a1', {
      kind: 'reorder-after',
      targetId: 'a',
    });
    expect(t.map((n) => n.id)).toEqual(['a', 'a1', 'b']);
    expect(treeModel.find(t, 'a')?.children?.map((n) => n.id)).toEqual(['a2']);
    expect(result).toEqual({ parentId: null, index: 1 });
  });
  it('no-op when sourceId === targetId', () => {
    const out = treeModel.move(fixture, 'a', {
      kind: 'make-child',
      targetId: 'a',
    });
    expect(out.tree).toBe(fixture);
  });
  it('no-op when target is descendant of source', () => {
    const out = treeModel.move(fixture, 'a', {
      kind: 'make-child',
      targetId: 'a1a',
    });
    expect(out.tree).toBe(fixture);
  });
  it('no-op when source is unknown', () => {
    const out = treeModel.move(fixture, 'ghost', {
      kind: 'reorder-before',
      targetId: 'a',
    });
    expect(out.tree).toBe(fixture);
  });
  it('no-op when target is unknown', () => {
    const out = treeModel.move(fixture, 'a1', {
      kind: 'reorder-before',
      targetId: 'ghost',
    });
    expect(out.tree).toBe(fixture);
  });

  it('cross-parent move does NOT apply the same-parent adjust (no off-by-one)', () => {
    // Source `x3` sits at index 2 in parent `x`; target `y1` sits at index 0 in
    // parent `y`. sourceInfo.index (2) > info.index (0) AND the parents differ,
    // so the `sameParent && source.index < info.index` adjust must be 0 — the
    // node must land at index 0 in `y`, not at index -1 (which would silently
    // drop it at a wrong slot / off-by-one).
    const crossFixture: N[] = [
      {
        id: 'x',
        name: 'X',
        children: [
          { id: 'x1', name: 'X1' },
          { id: 'x2', name: 'X2' },
          { id: 'x3', name: 'X3' },
        ],
      },
      {
        id: 'y',
        name: 'Y',
        children: [
          { id: 'y1', name: 'Y1' },
          { id: 'y2', name: 'Y2' },
        ],
      },
    ];
    const { tree: t, result } = treeModel.move(crossFixture, 'x3', {
      kind: 'reorder-before',
      targetId: 'y1',
    });
    expect(result).toEqual({ parentId: 'y', index: 0 });
    expect(treeModel.find(t, 'y')?.children?.map((n) => n.id)).toEqual([
      'x3',
      'y1',
      'y2',
    ]);
    expect(treeModel.find(t, 'x')?.children?.map((n) => n.id)).toEqual([
      'x1',
      'x2',
    ]);
  });
});