UpgradeSequence() instead of seq.Upgrade() and correct behavior.

README.md

@@ -16,8 +16,8 @@ func main () {
     hash := sha256.Sum256([]byte{1,2,3,4,5,6})
     seq, _ := opentimestamps.Stamp(context.Background(), "https://alice.btc.calendar.opentimestamps.org/", hash)
 
-    // you can just call .Upgrade() to get the upgraded sequence (or an error if not yet available)
-    upgradedSeq, err := seq.Upgrade(context.Background(), hash[:])
+    // you can just call UpgradeSequence() to get the upgraded sequence (or an error if not yet available)
+    upgradedSeq, err := opentimestamps.UpgradeSequence(context.Background(), seq, hash[:])
     if err != nil {
         fmt.Println("wait more")
     }
@@ -43,7 +43,7 @@ func main () {
     fmt.Println(hex.EncodeToString(seq[2].Argument)) // "c40fe258f9b828a0b5a7"
 
     // all these instructions can be executed in order, starting from the initial hash
-    result := seq.Compute(hash) // this is the value we send to the calendar server in order to get the upgraded sequence on .Upgrade()
+    result := seq.Compute(hash) // this is the value we send to the calendar server in order to get the upgraded sequence
     finalResult := upgradedSeq.Compute(hash) // this should be the merkle root of a bitcoin block if this sequence is upgraded
 
     // each sequence always ends in an "attestation"

ots.go

@@ -71,6 +71,17 @@ type Instruction struct {
 
 type Sequence []Instruction
 
+func (seq Sequence) GetAttestation() Attestation {
+	if len(seq) == 0 {
+		return Attestation{}
+	}
+	att := seq[len(seq)-1]
+	if att.Attestation == nil {
+		return Attestation{}
+	}
+	return *att.Attestation
+}
+
 func (seq Sequence) Compute(initial []byte) []byte {
 	current := initial
 	for _, inst := range seq {

stamp.go

@@ -41,9 +41,9 @@ func ReadFromFile(data []byte) (*File, error) {
 	return parseOTSFile(newBuffer(data))
 }
 
-func (seq Sequence) Upgrade(ctx context.Context, initial []byte) (Sequence, error) {
+func UpgradeSequence(ctx context.Context, seq Sequence, initial []byte) (Sequence, error) {
 	result := seq.Compute(initial)
-	attestation := seq[len(seq)-1]
+	attestation := seq.GetAttestation()
 
 	url := fmt.Sprintf("%s/timestamp/%x", normalizeUrl(attestation.CalendarServerURL), result)
 	req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
@@ -69,10 +69,14 @@ func (seq Sequence) Upgrade(ctx context.Context, initial []byte) (Sequence, erro
 	}
 	resp.Body.Close()
 
-	newSeq, err := parseCalendarServerResponse(newBuffer(body))
+	tail, err := parseCalendarServerResponse(newBuffer(body))
 	if err != nil {
 		return nil, fmt.Errorf("failed to parse response from '%s': %w", attestation.CalendarServerURL, err)
 	}
 
+	newSeq := make(Sequence, len(seq)+len(tail)-1)
+	copy(newSeq, seq[0:len(seq)-1])
+	copy(newSeq[len(seq)-1:], tail)
+
 	return newSeq, nil
 }