// Retrieve an object in a Merkle tree,
// validating the entire path in the process.
// Returns a slice of a buffer obtained from HashGet.Get(),
// which might be shared and should be considered read-only.
func MerkleGet(suite abstract.Suite, root []byte, path MerklePath,
ctx HashGet) ([]byte, error) {
// Follow pointers through intermediate levels
blob := root
for i := range path.Ptr {
beg := path.Ptr[i]
end := beg + suite.HashLen()
if end > len(blob) {
return nil, errors.New("bad Merkle tree pointer offset")
}
id := HashId(blob[beg:end])
b, e := ctx.Get(id) // Lookup the next-level blob
if e != nil {
return nil, e
}
blob = b
}
// Validate and extract the actual object
beg := path.Ofs
end := beg + path.Len
if end > len(blob) {
return nil, errors.New("bad Merkle tree object offset/length")
}
return blob[beg:end], nil
}