huffman coding: HuffTree : calling method with empty list of

Error message

huffman coding: HuffTree : calling method with empty list of symbol-frequency pairs

What it means

Raised by HuffTree when listfreq contains fewer than one SymbolFreq entry; building a Huffman tree requires at least one symbol, so an empty (or nil) slice cannot produce a root node. It is a generic input-size guard at the top of the method.

Source

Thrown at compression/huffmancoding.go:37

type Node struct {
	left   *Node
	right  *Node
	symbol rune
	weight int
}

// A SymbolFreq is a pair of a symbol and its associated frequency.
type SymbolFreq struct {
	Symbol rune
	Freq   int
}

// HuffTree returns the root Node of the Huffman tree by compressing listfreq.
// The compression produces the most optimal code lengths, provided listfreq is ordered,
// i.e.: listfreq[i] <= listfreq[j], whenever i < j.
func HuffTree(listfreq []SymbolFreq) (*Node, error) {
	if len(listfreq) < 1 {
		return nil, fmt.Errorf("huffman coding: HuffTree : calling method with empty list of symbol-frequency pairs")
	}
	q1 := make([]Node, len(listfreq))
	q2 := make([]Node, 0, len(listfreq))
	for i, x := range listfreq { // after the loop, q1 is a slice of leaf nodes representing listfreq
		q1[i] = Node{left: nil, right: nil, symbol: x.Symbol, weight: x.Freq}
	}
	//loop invariant: q1, q2 are ordered by increasing weights
	for len(q1)+len(q2) > 1 {
		var node1, node2 Node
		node1, q1, q2 = least(q1, q2)
		node2, q1, q2 = least(q1, q2)
		node := Node{left: &node1, right: &node2,
			symbol: -1, weight: node1.weight + node2.weight}
		q2 = append(q2, node)
	}
	if len(q1) == 1 { // returns the remaining node in q1, q2
		return &q1[0], nil
	}

View on GitHub (pinned to 5ba447ec5f)

Solutions

Pass a non-empty []SymbolFreq slice to HuffTree
Check len(listfreq) > 0 before calling and construct a trivial single-node tree for one symbol

Defensive patterns

Strategy: validation

When it happens

Trigger: Thrown at compression/huffmancoding.go:37 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of TheAlgorithms/Go@5ba447ec5f (2026-09-02). Data as JSON: /api/errors/acfc266fb8afc84d. Report an issue: GitHub.