TheAlgorithms/Go · error

the size of "chars" must be even

Error message

the size of "chars" must be even

What it means

decipher expects exactly two runes (a row char and a column char); being handed 0 or 1 runes means the ciphertext had odd length or the slicing window was wrong, so it reports the pair-size requirement.

Source

Thrown at cipher/polybius/polybius.go:91

		decryptedText += decryptedChar
	}
	return decryptedText, nil
}

func (p *Polybius) encipher(char rune) (string, error) {
	index := strings.IndexRune(p.key, char)
	if index < 0 {
		return "", fmt.Errorf("%q does not exist in keys", char)
	}
	row := index / p.size
	col := index % p.size
	chars := []rune(p.characters)
	return string([]rune{chars[row], chars[col]}), nil
}

func (p *Polybius) decipher(chars []rune) (string, error) {
	if len(chars) != 2 {
		return "", fmt.Errorf("the size of \"chars\" must be even")
	}
	row := strings.IndexRune(p.characters, chars[0])
	if row < 0 {
		return "", fmt.Errorf("%c does not exist in characters", chars[0])
	}
	col := strings.IndexRune(p.characters, chars[1])
	if col < 0 {
		return "", fmt.Errorf("%c does not exist in characters", chars[1])
	}
	return string(p.key[row*p.size+col]), nil
}

View on GitHub (pinned to 5ba447ec5f)

Solutions

1. Check len(text)%2==0 before decrypting and reject odd-length ciphertext
2. Ensure ciphertext was produced by this cipher (pairs of characters)
3. Handle the trailing leftover rune explicitly instead of calling decipher with one rune

When it happens

Trigger: Thrown at cipher/polybius/polybius.go:91 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/a8b0b4d4892c2b0e. Report an issue: GitHub.