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// Copyright 2019, 2020 OCI Contributors
// Copyright 2017 Docker, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// 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
// 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
//
// The "algorithm" portion defines both the hashing algorithm used to calculate
// the digest and the encoding of the resulting digest, which defaults to "hex"
// if not otherwise specified. Currently, all supported algorithms have their
// digests encoded in hex strings.
//
// In the example above, the string "sha256" is the algorithm and the hex bytes
// are the "digest".
//
// 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
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