move dependencies to vendor

vendor/github.com/docker/distribution/digest/doc.go

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+// Package digest provides a generalized type to opaquely represent message
+// digests and their operations within the registry. The Digest type is
+// designed to serve as a flexible identifier in a content-addressable system.
+// More importantly, it provides tools and wrappers to work with tarsums and
+// hash.Hash-based digests with little effort.
+//
+// Basics
+//
+// The format of a digest is simply a string with two parts, dubbed the
+// "algorithm" and the "digest", separated by a colon:
+//
+// 	<algorithm>:<digest>
+//
+// An example of a sha256 digest representation follows:
+//
+// 	sha256:7173b809ca12ec5dee4506cd86be934c4596dd234ee82c0662eac04a8c2c71dc
+//
+// In this case, the string "sha256" is the algorithm and the hex bytes are
+// the "digest". A tarsum example will be more illustrative of the use case
+// involved in the registry:
+//
+// 	tarsum+sha256:e58fcf7418d4390dec8e8fb69d88c06ec07039d651fedd3aa72af9972e7d046b
+//
+// For this, we consider the algorithm to be "tarsum+sha256". Prudent
+// applications will favor the ParseDigest function to verify the format over
+// using simple type casts. However, a normal string can be cast as a digest
+// with a simple type conversion:
+//
+// 	Digest("tarsum+sha256:e58fcf7418d4390dec8e8fb69d88c06ec07039d651fedd3aa72af9972e7d046b")
+//
+// Because the Digest type is simply a string, once a valid Digest is
+// obtained, comparisons are cheap, quick and simple to express with the
+// standard equality operator.
+//
+// Verification
+//
+// The main benefit of using the Digest type is simple verification against a
+// given digest. The Verifier interface, modeled after the stdlib hash.Hash
+// interface, provides a common write sink for digest verification. After
+// writing is complete, calling the Verifier.Verified method will indicate
+// whether or not the stream of bytes matches the target digest.
+//
+// Missing Features
+//
+// In addition to the above, we intend to add the following features to this
+// package:
+//
+// 1. A Digester type that supports write sink digest calculation.
+//
+// 2. Suspend and resume of ongoing digest calculations to support efficient digest verification in the registry.
+//
+package digest