[Misc] Integration fixes after the v0.11.0 release (#49)

- Exposing `NewTrieSpec` publically
- Adding back `Sum` function to the `MerkleRoot` type
- Adding a new `Count` function to the `MerkleRoot` type
- Adding back `PathHasherSize`

---

Signed-off-by: Daniel Olshansky <olshansky.daniel@gmail.com>
Co-authored-by: Redouane Lakrache <r3d0ne@gmail.com>

root.go

@@ -0,0 +1,39 @@
+package smt
+
+import "encoding/binary"
+
+const (
+	// These are intentionally exposed to allow for for testing and custom
+	// implementations of downstream applications.
+	SmtRootSizeBytes  = 32
+	SmstRootSizeBytes = SmtRootSizeBytes + sumSizeBytes + countSizeBytes
+)
+
+// Sum returns the uint64 sum of the merkle root, it checks the length of the
+// merkle root and if it is no the same as the size of the SMST's expected
+// root hash it will panic.
+func (r MerkleRoot) Sum() uint64 {
+	if len(r)%SmtRootSizeBytes == 0 {
+		panic("root#sum: not a merkle sum trie")
+	}
+
+	firstSumByteIdx, firstCountByteIdx := getFirstMetaByteIdx([]byte(r))
+
+	var sumBz [sumSizeBytes]byte
+	copy(sumBz[:], []byte(r)[firstSumByteIdx:firstCountByteIdx])
+	return binary.BigEndian.Uint64(sumBz[:])
+}
+
+// Count returns the uint64 count of the merkle root, a cryptographically secure
+// count of the number of non-empty leafs in the tree.
+func (r MerkleRoot) Count() uint64 {
+	if len(r)%SmtRootSizeBytes == 0 {
+		panic("root#sum: not a merkle sum trie")
+	}
+
+	_, firstCountByteIdx := getFirstMetaByteIdx([]byte(r))
+
+	var countBz [countSizeBytes]byte
+	copy(countBz[:], []byte(r)[firstCountByteIdx:])
+	return binary.BigEndian.Uint64(countBz[:])
+}

smst.go

@@ -38,7 +38,7 @@ func NewSparseMerkleSumTrie(
 	hasher hash.Hash,
 	options ...TrieSpecOption,
 ) *SMST {
-	trieSpec := newTrieSpec(hasher, true)
+	trieSpec := NewTrieSpec(hasher, true)
 	for _, option := range options {
 		option(&trieSpec)
 	}

smt.go

@@ -34,7 +34,7 @@ func NewSparseMerkleTrie(
 	options ...TrieSpecOption,
 ) *SMT {
 	smt := SMT{
-		TrieSpec: newTrieSpec(hasher, false),
+		TrieSpec: NewTrieSpec(hasher, false),
 		nodes:    nodes,
 	}
 	for _, option := range options {

trie_spec.go

@@ -14,12 +14,21 @@ type TrieSpec struct {
 	sumTrie bool
 }
 
-// newTrieSpec returns a new TrieSpec with the given hasher and sumTrie flag
-func newTrieSpec(hasher hash.Hash, sumTrie bool) TrieSpec {
+// NewTrieSpec returns a new TrieSpec with the given hasher and sumTrie flag
+func NewTrieSpec(
+	hasher hash.Hash,
+	sumTrie bool,
+	opts ...TrieSpecOption,
+) TrieSpec {
 	spec := TrieSpec{th: *NewTrieHasher(hasher)}
 	spec.ph = &pathHasher{spec.th}
 	spec.vh = &valueHasher{spec.th}
 	spec.sumTrie = sumTrie
+
+	for _, opt := range opts {
+		opt(&spec)
+	}
+
 	return spec
 }
 
@@ -28,6 +37,10 @@ func (spec *TrieSpec) Spec() *TrieSpec {
 	return spec
 }
 
+// PathHasherSize returns the length (in bytes) of digests produced by the
+// path hasher
+func (spec *TrieSpec) PathHasherSize() int { return spec.ph.PathSize() }
+
 // placeholder returns the default placeholder value depending on the trie type
 func (spec *TrieSpec) placeholder() []byte {
 	if spec.sumTrie {