fix: GetTrieKey and GetTrieEntry leafindex calc (#20)

Use the new (safe) mmr.LeafIndex method.

Also add comments in places where we use LeafCount directly indicating
why it is safe to do so

AB#9551

Co-authored-by: Robin Bryce <robin.bryce@datatrails.ai>

massifs/go.mod

@@ -3,7 +3,7 @@ module github.com/datatrails/go-datatrails-merklelog/massifs
 go 1.22
 
 require (
-	github.com/datatrails/go-datatrails-merklelog/mmr v0.0.1
+	github.com/datatrails/go-datatrails-merklelog/mmr v0.0.2
 	github.com/datatrails/go-datatrails-merklelog/mmrtesting v0.0.1
 )
 

massifs/go.sum

@@ -49,8 +49,8 @@ github.com/coreos/go-systemd/v22 v22.5.0 h1:RrqgGjYQKalulkV8NGVIfkXQf6YYmOyiJKk8
 github.com/coreos/go-systemd/v22 v22.5.0/go.mod h1:Y58oyj3AT4RCenI/lSvhwexgC+NSVTIJ3seZv2GcEnc=
 github.com/datatrails/go-datatrails-common v0.15.1 h1:wu3Gs6v7TkMLltzavPY2aHPniJabEiuqSJSHW79bX+4=
 github.com/datatrails/go-datatrails-common v0.15.1/go.mod h1:lVLYVw5o+Wj+z8sn8bJBzp9qBCdYQ0DUX91+R5Gn73Q=
-github.com/datatrails/go-datatrails-merklelog/mmr v0.0.1 h1:XggMVejsJ72cAL/msjPhQUSQNeElSh/O1zZcvX4d4JU=
-github.com/datatrails/go-datatrails-merklelog/mmr v0.0.1/go.mod h1:+Oz8O6bns0rF6gr03xJzKTBzUzyskZ8Gics8/qeNzYk=
+github.com/datatrails/go-datatrails-merklelog/mmr v0.0.2 h1:Jxov4/onoFiCISLQNSPy/nyt3USAEvUZpEjlScHJYKI=
+github.com/datatrails/go-datatrails-merklelog/mmr v0.0.2/go.mod h1:+Oz8O6bns0rF6gr03xJzKTBzUzyskZ8Gics8/qeNzYk=
 github.com/datatrails/go-datatrails-merklelog/mmrtesting v0.0.1 h1:sIyXWKTadqmVEsPj66RlKwRKzNQ7hK9SH1fRjZFDCa8=
 github.com/datatrails/go-datatrails-merklelog/mmrtesting v0.0.1/go.mod h1:KGdkOtamWG48EN4AXtTHPv6C0jJKrj840IMSkrD+egk=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=

massifs/massifcontext.go

@@ -243,7 +243,8 @@ func (mc *MassifContext) Get(i uint64) ([]byte, error) {
 // GetTrieEntry gets the trie entry given the mmrIndex of its corresponding leaf node.
 func (mc MassifContext) GetTrieEntry(mmrIndex uint64) ([]byte, error) {
 
-	trieIndex := mmr.LeafCount(mmrIndex+1) - 1
+	// Note: mmrIndex identifies an arbitrary node, so LeafIndex is necessary
+	trieIndex := mmr.LeafIndex(mmrIndex)
 
 	massifTrieIndex, err := mc.GetMassifTrieIndex(trieIndex)
 	if err != nil {
@@ -256,7 +257,8 @@ func (mc MassifContext) GetTrieEntry(mmrIndex uint64) ([]byte, error) {
 // GetTrieKey gets the trie key given the mmrIndex of the trie entries corresponding leaf node.
 func (mc MassifContext) GetTrieKey(mmrIndex uint64) ([]byte, error) {
 
-	trieIndex := mmr.LeafCount(mmrIndex+1) - 1
+	// Note: mmrIndex identifies an arbitrary node, so LeafIndex is necessary
+	trieIndex := mmr.LeafIndex(mmrIndex)
 
 	massifTrieIndex, err := mc.GetMassifTrieIndex(trieIndex)
 	if err != nil {
@@ -458,6 +460,9 @@ func (mc MassifContext) LastCommitUnixMS(idTimestampEpoch uint8) (int64, error)
 
 // GetMassifLeafIndex returns the leafIndex into the whole log relative to the start of the massif leaf index.
 func (mc MassifContext) GetMassifLeafIndex(leafIndex uint64) (uint64, error) {
+
+	// Note: FirstIndex is also the MMRSize at the end of the previous massif, so LeafCount is used.
+	// similarly RangeCount (below) will always return a valid MMRSize
 	firstLeafIndex := mmr.LeafCount(mc.Start.FirstIndex)
 	if leafIndex < firstLeafIndex {
 		return 0, fmt.Errorf("index %d: %w", leafIndex, ErrBeforeFirstLeaf)
@@ -490,10 +495,11 @@ func (mc MassifContext) GetLastIdTimestamp() uint64 {
 // the number of leaves in the entire mmr call mmr.LeafCount directly)
 func (mc MassifContext) MassifLeafCount() uint64 {
 
-	// Get the count of leaves in the entire mmr
+	// Get the count of leaves in the entire mmr, RangeCount always returns a valid MMRSize
 	count := mmr.LeafCount(mc.RangeCount())
-	// Subtract the number of leaves in the mmr defined by the end of the last blob
-	// to get the count of leaves in the current blob
+	// Subtract the number of leaves in the mmr defined by the end of the last
+	// blob to get the count of leaves in the current blob. FirstIndex is the
+	// valid MMRSize of the end of the preceding massif.
 	return count - mmr.LeafCount(mc.Start.FirstIndex)
 }