func Open()

in sumdb/note/note.go [520:611]
70 lines of code
23 McCabe index (conditional complexity)

func Open(msg []byte, known Verifiers) (*Note, error) {
	if known == nil {
		// Treat nil Verifiers as empty list, to produce useful error instead of crash.
		known = VerifierList()
	}

	// Must have valid UTF-8 with no non-newline ASCII control characters.
	for i := 0; i < len(msg); {
		r, size := utf8.DecodeRune(msg[i:])
		if r < 0x20 && r != '\n' || r == utf8.RuneError && size == 1 {
			return nil, errMalformedNote
		}
		i += size
	}

	// Must end with signature block preceded by blank line.
	split := bytes.LastIndex(msg, sigSplit)
	if split < 0 {
		return nil, errMalformedNote
	}
	text, sigs := msg[:split+1], msg[split+2:]
	if len(sigs) == 0 || sigs[len(sigs)-1] != '\n' {
		return nil, errMalformedNote
	}

	n := &Note{
		Text: string(text),
	}

	// Parse and verify signatures.
	// Ignore duplicate signatures.
	seen := make(map[nameHash]bool)
	seenUnverified := make(map[string]bool)
	numSig := 0
	for len(sigs) > 0 {
		// Pull out next signature line.
		// We know sigs[len(sigs)-1] == '\n', so IndexByte always finds one.
		i := bytes.IndexByte(sigs, '\n')
		line := sigs[:i]
		sigs = sigs[i+1:]

		if !bytes.HasPrefix(line, sigPrefix) {
			return nil, errMalformedNote
		}
		line = line[len(sigPrefix):]
		name, b64 := chop(string(line), " ")
		sig, err := base64.StdEncoding.DecodeString(b64)
		if err != nil || !isValidName(name) || b64 == "" || len(sig) < 5 {
			return nil, errMalformedNote
		}
		hash := binary.BigEndian.Uint32(sig[0:4])
		sig = sig[4:]

		if numSig++; numSig > 100 {
			// Avoid spending forever parsing a note with many signatures.
			return nil, errMalformedNote
		}

		v, err := known.Verifier(name, hash)
		if _, ok := err.(*UnknownVerifierError); ok {
			// Drop repeated identical unverified signatures.
			if seenUnverified[string(line)] {
				continue
			}
			seenUnverified[string(line)] = true
			n.UnverifiedSigs = append(n.UnverifiedSigs, Signature{Name: name, Hash: hash, Base64: b64})
			continue
		}
		if err != nil {
			return nil, err
		}

		// Drop repeated signatures by a single verifier.
		if seen[nameHash{name, hash}] {
			continue
		}
		seen[nameHash{name, hash}] = true

		ok := v.Verify(text, sig)
		if !ok {
			return nil, &InvalidSignatureError{name, hash}
		}

		n.Sigs = append(n.Sigs, Signature{Name: name, Hash: hash, Base64: b64})
	}

	// Parsed and verified all the signatures.
	if len(n.Sigs) == 0 {
		return nil, &UnverifiedNoteError{n}
	}
	return n, nil
}