Add explicit FIX message build API

field_map.go

@@ -302,6 +302,10 @@ func (m *FieldMap) clearNoLock() {
 }
 
 // CopyInto overwrites the given FieldMap with this one.
+//
+// Warning: CopyInto preserves legacy behavior for repeated TagValues under the
+// same tag, such as repeating group contents. Only the first TagValue for each
+// tag is copied. Use CopyIntoDeep to preserve complete field slices.
 func (m *FieldMap) CopyInto(to *FieldMap) {
 	m.rwLock.RLock()
 	defer m.rwLock.RUnlock()
@@ -317,6 +321,33 @@ func (m *FieldMap) CopyInto(to *FieldMap) {
 	to.compare = m.compare
 }
 
+// CopyIntoDeep overwrites the given FieldMap with this one, preserving every
+// TagValue stored for each tag.
+func (m *FieldMap) CopyIntoDeep(to *FieldMap) {
+	m.rwLock.RLock()
+	defer m.rwLock.RUnlock()
+
+	to.tagLookup = make(map[Tag]field)
+	for tag, f := range m.tagLookup {
+		clone := make(field, len(f))
+		for i := range f {
+			clone[i] = cloneTagValue(f[i])
+		}
+		to.tagLookup[tag] = clone
+	}
+	to.tags = make([]Tag, len(m.tags))
+	copy(to.tags, m.tags)
+	to.compare = m.compare
+}
+
+func cloneTagValue(tv TagValue) TagValue {
+	return TagValue{
+		tag:   tv.tag,
+		value: append([]byte(nil), tv.value...),
+		bytes: append([]byte(nil), tv.bytes...),
+	}
+}
+
 func (m *FieldMap) add(f field) {
 	t := fieldTag(f)
 	if _, ok := m.tagLookup[t]; !ok {

field_map_test.go

@@ -191,6 +191,41 @@ func TestFieldMap_CopyInto(t *testing.T) {
 	assert.Equal(t, "a", s)
 }
 
+func TestFieldMap_CopyIntoDeep(t *testing.T) {
+	group := NewRepeatingGroup(453, GroupTemplate{
+		GroupElement(448),
+		GroupElement(447),
+		GroupElement(452),
+	})
+	group.Add().
+		SetString(448, "party1").
+		SetString(447, "D").
+		SetString(452, "28")
+	group.Add().
+		SetString(448, "party2").
+		SetString(447, "D").
+		SetString(452, "28")
+
+	var fMapA FieldMap
+	fMapA.init()
+	fMapA.SetGroup(group)
+
+	var fMapB FieldMap
+	fMapB.init()
+	fMapA.CopyIntoDeep(&fMapB)
+
+	assert.Equal(t, len(fMapA.tagLookup[453]), len(fMapB.tagLookup[453]))
+
+	var copied bytes.Buffer
+	writeField(fMapB.tagLookup[453], &copied)
+	assert.Equal(t, "453=2
448=party1
447=D
452=28
448=party2
447=D
452=28
", copied.String())
+
+	fMapA.tagLookup[453][1].value[0] = 'X'
+	fMapA.tagLookup[453][1].bytes[4] = 'X'
+	assert.Equal(t, []byte("party1"), fMapB.tagLookup[453][1].value)
+	assert.Equal(t, "448=party1
", string(fMapB.tagLookup[453][1].bytes))
+}
+
 func TestFieldMap_Remove(t *testing.T) {
 	var fMap FieldMap
 	fMap.init()

message.go

@@ -139,8 +139,11 @@ func NewMessage() *Message {
 	return m
 }
 
-// CopyInto erases the dest messages and copies the currency message content
-// into it.
+// CopyInto erases the dest message and copies this message content into it.
+//
+// Warning: CopyInto preserves legacy FieldMap.CopyInto behavior, so repeated
+// TagValues under the same tag, such as repeating group contents, are not fully
+// copied. Use CopyIntoDeep to preserve complete field slices.
 func (m *Message) CopyInto(to *Message) {
 	m.Header.CopyInto(&to.Header.FieldMap)
 	m.Body.CopyInto(&to.Body.FieldMap)
@@ -155,6 +158,23 @@ func (m *Message) CopyInto(to *Message) {
 	}
 }
 
+// CopyIntoDeep erases the dest message and copies this message content into it,
+// preserving every TagValue stored for each tag. As with CopyInto, rawMessage is
+// not copied, so Bytes and String on the destination rebuild from field maps.
+func (m *Message) CopyIntoDeep(to *Message) {
+	m.Header.CopyIntoDeep(&to.Header.FieldMap)
+	m.Body.CopyIntoDeep(&to.Body.FieldMap)
+	m.Trailer.CopyIntoDeep(&to.Trailer.FieldMap)
+
+	to.ReceiveTime = m.ReceiveTime
+	to.bodyBytes = make([]byte, len(m.bodyBytes))
+	copy(to.bodyBytes, m.bodyBytes)
+	to.fields = make([]TagValue, len(m.fields))
+	for i := range to.fields {
+		to.fields[i] = cloneTagValue(m.fields[i])
+	}
+}
+
 // ParseMessage constructs a Message from a byte slice wrapping a FIX message.
 func ParseMessage(msg *Message, rawMessage *bytes.Buffer) (err error) {
 	return ParseMessageWithDataDictionary(msg, rawMessage, nil, nil)
@@ -574,6 +594,13 @@ func (m *Message) Bytes() []byte {
 	return m.build()
 }
 
+// Build constructs bytes from the current Header, Body, and Trailer fields.
+// It ignores any raw message buffer captured during parsing and recalculates
+// BodyLength and CheckSum.
+func (m *Message) Build() []byte {
+	return m.build()
+}
+
 func (m *Message) String() string {
 	if m.rawMessage != nil {
 		return m.rawMessage.String()

message_test.go

@@ -126,6 +126,40 @@ func (s *MessageSuite) TestBuild() {
 	s.True(bytes.Equal(expectedBytes, result), "Unexpected bytes, got %s", string(result))
 }
 
+func (s *MessageSuite) TestBuildIgnoresRawMessage() {
+	rawMsg := bytes.NewBufferString("8=FIX.4.2
9=104
35=D
34=2
49=TW
52=20140515-19:49:56.659
56=ISLD
11=100
21=1
40=1
54=1
55=TSLA
60=00010101-00:00:00.000
10=039
")
+
+	s.Nil(ParseMessage(s.msg, rawMsg))
+	s.msg.Body.SetString(11, "upstream-id")
+
+	built := s.msg.Build()
+	s.Equal(rawMsg.String(), string(s.msg.Bytes()))
+	s.NotEqual(rawMsg.String(), string(built))
+	s.Contains(string(built), "11=upstream-id
")
+	s.NotContains(string(built), "11=100
")
+
+	parsedBuilt := NewMessage()
+	s.Nil(ParseMessage(parsedBuilt, bytes.NewBuffer(built)))
+
+	bodyLength, err := parsedBuilt.Header.GetInt(tagBodyLength)
+	s.Nil(err)
+	bodyStart := bytes.Index(built, []byte("35="))
+	checkSumStart := bytes.LastIndex(built, []byte("10="))
+	s.Equal(checkSumStart-bodyStart, bodyLength)
+
+	checkSum, err := parsedBuilt.Trailer.GetString(tagCheckSum)
+	s.Nil(err)
+	s.Equal(formatCheckSum(byteSum(built[:checkSumStart])%256), checkSum)
+}
+
+func byteSum(b []byte) int {
+	total := 0
+	for _, c := range b {
+		total += int(c)
+	}
+	return total
+}
+
 func (s *MessageSuite) TestReBuild() {
 	rawMsg := bytes.NewBufferString("8=FIX.4.2
9=104
35=D
34=2
49=TW
52=20140515-19:49:56.659
56=ISLD
11=100
21=1
40=1
54=1
55=TSLA
60=00010101-00:00:00.000
10=039
")
 
@@ -505,6 +539,25 @@ func (s *MessageSuite) TestCopyIntoMessage() {
 	s.Equal(string(dest.Bytes()), renderedString)
 }
 
+func (s *MessageSuite) TestCopyIntoDeepMessage() {
+	dict, dictErr := datadictionary.Parse("spec/FIX44.xml")
+	s.Nil(dictErr)
+
+	msgString := "8=FIX.4.4
9=187
35=D
34=2
49=01001
50=01001a
52=20231231-20:19:41
56=TEST
" +
+		"1=acct1
11=13976
21=1
38=1
40=2
44=12
54=1
55=SYMABC
59=0
60=20231231-20:19:41
453=2
448=4501
447=D
452=28
448=4502
447=D
452=28
" +
+		"10=044
"
+	msgBuf := bytes.NewBufferString(msgString)
+	s.Nil(ParseMessageWithDataDictionary(s.msg, msgBuf, dict, dict))
+
+	dest := NewMessage()
+	s.msg.CopyIntoDeep(dest)
+
+	s.Equal(msgString, string(dest.Build()))
+
+	msgBuf.Reset()
+	s.Equal(msgString, string(dest.Build()))
+}
+
 func checkFieldInt(s *MessageSuite, fields FieldMap, tag, expected int) {
 	toCheck, _ := fields.GetInt(Tag(tag))
 	s.Equal(expected, toCheck)