// the game is: walk the tree from the root to the target leaf
func (t *Tree) membershipProof(target, index, layer, version int, proof *MembershipProof) (err error) {
if layer == 0 {
return
}
// the number of events to the left of the node
n := index + util.Pow(2, layer-1)
if target < n {
// go left, but should we save right first? We need to save right if there are any leaf nodes
// fixed by the right node (otherwise we know it's hash is nil), dictated by the version of the
// tree we are generating
if version >= n {
p := new(ProofPosition)
p.Index = n
p.Layer = layer - 1
p.Hash, err = t.getHashedNode(p.Index, p.Layer, version, false)
if err != nil {
return
}
proof.Nodes = append(proof.Nodes, *p)
}
return t.membershipProof(target, index, layer-1, version, proof)
}
// go right, once we have saved the left node
p := new(ProofPosition)
p.Index = index
p.Layer = layer - 1
p.Hash, err = t.getHashedNode(p.Index, p.Layer, version, false)
if err != nil {
return
}
proof.Nodes = append(proof.Nodes, *p)
return t.membershipProof(target, n, layer-1, version, proof)
}
// layeR, Index, Version
// See Figure 5 in "Efficient Data Structures for Tamper-Evident Logging"
func (t *Tree) getHashedNode(index, layer, version int, proofMode bool) (value []byte, err error) {
// always prefer frozen hashes, if we have calculated them
if proofMode || version >= index+util.Pow(2, layer)-1 {
if t.isFrozenHash(index, layer) {
return t.getFrozenHash(index, layer)
}
}
// special case for child nodes
if layer == 0 && version >= index {
event, err := t.getEvent(index)
if err != nil {
return nil, errors.New("no event with the provided index")
}
value = util.Hash(prefixZero, event)
// have version determine if the right node is there or not
} else if version >= index+util.Pow(2, layer-1) {
a1, err := t.getHashedNode(index, layer-1, version, proofMode)
if err != nil {
return nil, errors.New("failed to get internal node with index " + strconv.Itoa(index))
}
a2, err := t.getHashedNode(index+util.Pow(2, layer-1), layer-1, version, proofMode)
if err != nil {
return nil, errors.New("failed to get internal node with index " + strconv.Itoa(index))
}
value = util.Hash(prefixOne, a1, a2)
} else {
a, err := t.getHashedNode(index, layer-1, version, proofMode)
if err != nil {
return nil, errors.New("failed to get internal node with index " + strconv.Itoa(index))
}
value = util.Hash(prefixOne, a)
}
// should we add this to the frozen hash cache?
if version >= index+util.Pow(2, layer)-1 {
t.setFrozenHash(index, layer, value)
}
return
}