TheAlgorithms/Go · critical

Must be >= 3 keys

Error message

Must be >= 3 keys

What it means

NewBTree panics when maxKeys <= 2 because a B-tree with fewer than 3 max keys cannot satisfy the minimum-key invariants of internal nodes (minKeys = (maxKeys-1)/2 leaves no room to split or borrow). The library rejects structurally impossible tree parameters at construction time.

Source

Thrown at structure/tree/btree.go:37

func minKeys(maxKeys int) int {
	return (maxKeys - 1) / 2
}

func NewBTreeNode[T constraints.Ordered](maxKeys int, isLeaf bool) *BTreeNode[T] {
	if maxKeys <= 0 {
		panic("BTree maxKeys cannot be zero")
	}
	return &BTreeNode[T]{
		keys:     make([]T, maxKeys),
		children: make([]*BTreeNode[T], maxKeys+1),
		isLeaf:   isLeaf,
	}
}

func NewBTree[T constraints.Ordered](maxKeys int) *BTree[T] {
	if maxKeys <= 2 {
		panic("Must be >= 3 keys")
	}
	return &BTree[T]{
		root:    nil,
		maxKeys: maxKeys,
	}
}

func (node *BTreeNode[T]) Verify(tree *BTree[T]) {
	minKeys := minKeys(tree.maxKeys)
	if node != tree.root && node.numKeys < minKeys {
		panic("node has too few keys")
	} else if node.numKeys > tree.maxKeys {
		panic("node has too many keys")
	}
}

func (node *BTreeNode[T]) IsFull(maxKeys int) bool {
	return node.numKeys == maxKeys

View on GitHub (pinned to 5ba447ec5f)

Solutions

1. Call NewBTree with maxKeys >= 3 (4 is a common even choice).
2. Clamp or validate configuration values before constructing the tree.
3. If you need a smaller branching factor, B-trees are not the right structure; use a BST/2-3 tree implementation.

Example fix

// before
tree := NewBTree[int](2)
// after
tree := NewBTree[int](4)

func NewBTreeSafe[T constraints.Ordered](maxKeys int) *BTree[T] {
    if maxKeys < 3 { maxKeys = 4 }
    return NewBTree[T](maxKeys)
}

defer func() {
    if r := recover(); r != nil {
        log.Fatalf("invalid BTree maxKeys: %v", r)
    }
}()

When it happens

Trigger: Calling NewBTree with 0, 1, or 2, e.g. NewBTree[int](2), often from a misconfigured degree setting or a default zero value of an int variable.

Common situations: Zero-valued struct field used as maxKeys; misunderstanding whether the parameter is 'order' vs 'max keys'; hardcoded small values in tests.

Related errors

• BTree maxKeys cannot be zero
• node has too few keys
• node has too many keys
• Called InsertNonFull() with a full node
• deleting out of bounds key

AI-assisted analysis of TheAlgorithms/Go@5ba447ec5f (2026-09-02). Data as JSON: /api/errors/9083257767f06227. Report an issue: GitHub.