[Minor] Expose GetPathBit() (#15)

## Overview

This PR exposes `GetPathBit()` for use outside of the SMT repo

proofs.go

@@ -167,7 +167,7 @@ func verifyProofWithUpdates(proof *SparseMerkleProof, root []byte, key []byte, v
 		node := make([]byte, hashSize(spec))
 		copy(node, proof.SideNodes[i])
 
-		if getPathBit(path, len(proof.SideNodes)-1-i) == left {
+		if GetPathBit(path, len(proof.SideNodes)-1-i) == left {
 			currentHash, currentData = digestNode(spec, currentHash, node)
 		} else {
 			currentHash, currentData = digestNode(spec, node, currentHash)
@@ -235,7 +235,7 @@ func DecompactProof(proof *SparseCompactMerkleProof, spec *TreeSpec) (*SparseMer
 	decompactedSideNodes := make([][]byte, proof.NumSideNodes)
 	position := 0
 	for i := 0; i < proof.NumSideNodes; i++ {
-		if getPathBit(proof.BitMask, i) == 1 {
+		if GetPathBit(proof.BitMask, i) == 1 {
 			decompactedSideNodes[i] = placeholder(spec)
 		} else {
 			decompactedSideNodes[i] = proof.SideNodes[position]

smt.go

@@ -114,7 +114,7 @@ func (smt *SMT) Get(key []byte) ([]byte, error) {
 			}
 		}
 		inner := (*node).(*innerNode)
-		if getPathBit(path, depth) == left {
+		if GetPathBit(path, depth) == left {
 			node = &inner.leftChild
 		} else {
 			node = &inner.rightChild
@@ -172,7 +172,7 @@ func (smt *SMT) update(
 			*last = &ext
 			last = &ext.child
 		}
-		if getPathBit(path, prefixlen) == left {
+		if GetPathBit(path, prefixlen) == left {
 			*last = &innerNode{leftChild: newLeaf, rightChild: leaf}
 		} else {
 			*last = &innerNode{leftChild: leaf, rightChild: newLeaf}
@@ -195,7 +195,7 @@ func (smt *SMT) update(
 
 	inner := node.(*innerNode)
 	var child *treeNode
-	if getPathBit(path, depth) == left {
+	if GetPathBit(path, depth) == left {
 		child = &inner.leftChild
 	} else {
 		child = &inner.rightChild
@@ -266,7 +266,7 @@ func (smt *SMT) delete(node treeNode, depth int, path []byte, orphans *orphanNod
 
 	inner := node.(*innerNode)
 	var child, sib *treeNode
-	if getPathBit(path, depth) == left {
+	if GetPathBit(path, depth) == left {
 		child, sib = &inner.leftChild, &inner.rightChild
 	} else {
 		child, sib = &inner.rightChild, &inner.leftChild
@@ -335,7 +335,7 @@ func (smt *SMT) Prove(key []byte) (proof *SparseMerkleProof, err error) {
 			}
 		}
 		inner := node.(*innerNode)
-		if getPathBit(path, depth) == left {
+		if GetPathBit(path, depth) == left {
 			node, sib = inner.leftChild, inner.rightChild
 		} else {
 			node, sib = inner.rightChild, inner.leftChild
@@ -558,7 +558,7 @@ func (ext *extensionNode) match(path []byte, depth int) (int, bool) {
 		panic("depth != path_begin")
 	}
 	for i := ext.pathStart(); i < ext.pathEnd(); i++ {
-		if getPathBit(ext.path, i) != getPathBit(path, i) {
+		if GetPathBit(ext.path, i) != GetPathBit(path, i) {
 			return i - ext.pathStart(), false
 		}
 	}
@@ -569,7 +569,7 @@ func (ext *extensionNode) match(path []byte, depth int) (int, bool) {
 func (ext *extensionNode) commonPrefix(path []byte) int {
 	count := 0
 	for i := ext.pathStart(); i < ext.pathEnd(); i++ {
-		if getPathBit(ext.path, i) != getPathBit(path, i) {
+		if GetPathBit(ext.path, i) != GetPathBit(path, i) {
 			break
 		}
 		count++
@@ -588,8 +588,8 @@ func (ext *extensionNode) split(path []byte, depth int) (treeNode, *treeNode, in
 	index := ext.pathStart()
 	var myBit, branchBit int
 	for ; index < ext.pathEnd(); index++ {
-		myBit = getPathBit(ext.path, index)
-		branchBit = getPathBit(path, index)
+		myBit = GetPathBit(ext.path, index)
+		branchBit = GetPathBit(path, index)
 		if myBit != branchBit {
 			break
 		}
@@ -640,7 +640,7 @@ func (ext *extensionNode) expand() treeNode {
 	last := ext.child
 	for i := ext.pathEnd() - 1; i >= ext.pathStart(); i-- {
 		var next innerNode
-		if getPathBit(ext.path, i) == left {
+		if GetPathBit(ext.path, i) == left {
 			next.leftChild = last
 		} else {
 			next.rightChild = last

utils.go

@@ -1,7 +1,18 @@
 package smt
 
-// getPathBit gets the bit at an offset from the most significant bit
-func getPathBit(data []byte, position int) int {
+// GetPathBit gets the bit at an offset from the most significant bit
+func GetPathBit(data []byte, position int) int {
+	// get the byte at the position and then left shift one by the offset of the position
+	// from the leftmost bit in the byte. Check if the bitwise AND is the same
+	// Path: []byte{ {0 1 0 1 1 0 1 0}, {0 1 1 0 1 1 0 1}, {1 0 0 1 0 0 1 0} } (length = 24 bits / 3 bytes)
+	// Position: 13 - 13/8=1
+	// Path[1] = {0 1 1 0 1 1 0 1}
+	// uint(13)%8 = 5, 8-1-5=2
+	// 00000001 << 2 = 00000100
+	//   {0 1 1 0 1 1 0 1}
+	// & {0 0 0 0 0 1 0 0}
+	// = {0 0 0 0 0 1 0 0}
+	// > 0 so Path is on the right at position 13
 	if int(data[position/8])&(1<<(8-1-uint(position)%8)) > 0 {
 		return 1
 	}
@@ -18,7 +29,7 @@ func setPathBit(data []byte, position int) {
 func countSetBits(data []byte) int {
 	count := 0
 	for i := 0; i < len(data)*8; i++ {
-		if getPathBit(data, i) == 1 {
+		if GetPathBit(data, i) == 1 {
 			count++
 		}
 	}
@@ -29,7 +40,7 @@ func countSetBits(data []byte) int {
 func countCommonPrefix(data1, data2 []byte, from int) int {
 	count := 0
 	for i := from; i < len(data1)*8; i++ {
-		if getPathBit(data1, i) == getPathBit(data2, i) {
+		if GetPathBit(data1, i) == GetPathBit(data2, i) {
 			count++
 		} else {
 			break