ContainerRegistry: Make digest(of:Data) an ImageReference.Digest constructor (#147)

Motivation
----------

The `digest(of:)` function calculates an image digest from a `Data`
object. Previously the digest was a string and the function stood alone,
but after the introduction of the `ImageReference.Digest` type it is
logical for it to be an alternative constructor.

Modifications
-------------

`digest(of:)` is now `ImageReference.Digest(of:)`

Result
------

Refactoring.   No functional change.

Test Plan
---------

All existing tests continue to pass.

Author: euanh

Sources/ContainerRegistry/Blobs.swift

@@ -97,7 +97,7 @@ public extension RegistryClient {
         // Append the digest to the upload location, as the specification requires.
         // The server's URL is arbitrary and might already contain query items which we must not overwrite.
         // The URL could even point to a different host.
-        let digest = digest(of: data)
+        let digest = ImageReference.Digest(of: data)
         let uploadURL = location.appending(queryItems: [.init(name: "digest", value: "\(digest)")])
 
         let httpResponse = try await executeRequestThrowing(

Sources/ContainerRegistry/ImageReference+Digest.swift

@@ -0,0 +1,49 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the SwiftContainerPlugin open source project
+//
+// Copyright (c) 2025 Apple Inc. and the SwiftContainerPlugin project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of SwiftContainerPlugin project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import Foundation
+import HTTPTypes
+import struct Crypto.SHA256
+import struct Crypto.SHA512
+
+// The distribution spec says that Docker- prefix headers are no longer to be used,
+// but also specifies that the registry digest is returned in this header.
+// https://github.com/opencontainers/distribution-spec/blob/main/spec.md#pulling-manifests
+extension HTTPField.Name {
+    static let dockerContentDigest = Self("Docker-Content-Digest")!
+}
+
+extension ImageReference.Digest {
+    /// Calculate the digest of a blob of data.
+    /// - Parameters:
+    ///   - data: Blob of data to digest.
+    ///   - algorithm: Digest algorithm to use.
+    public init(
+        of data: any DataProtocol,
+        algorithm: ImageReference.Digest.Algorithm = .sha256
+    ) {
+        // SHA256 is required; some registries might also support SHA512
+        switch algorithm {
+        case .sha256:
+            let hash = SHA256.hash(data: data)
+            let digest = hash.compactMap { String(format: "%02x", $0) }.joined()
+            try! self.init("sha256:" + digest)
+
+        case .sha512:
+            let hash = SHA512.hash(data: data)
+            let digest = hash.compactMap { String(format: "%02x", $0) }.joined()
+            try! self.init("sha512:" + digest)
+        }
+    }
+}

Sources/ContainerRegistry/Manifests.swift

@@ -21,7 +21,7 @@ public extension RegistryClient {
         // See https://github.com/opencontainers/distribution-spec/blob/main/spec.md#pushing-manifests
 
         let encoded = try encoder.encode(manifest)
-        let digest = digest(of: encoded)
+        let digest = ImageReference.Digest(of: encoded)
         let mediaType = manifest.mediaType ?? "application/vnd.oci.image.manifest.v1+json"
 
         let _ = try await executeRequestThrowing(
@@ -62,7 +62,7 @@ public extension RegistryClient {
             try decoder.decode(ImageManifest.self, from: data),
             ContentDescriptor(
                 mediaType: response.headerFields[.contentType] ?? "application/vnd.oci.image.manifest.v1+json",
-                digest: "\(digest(of: data))",
+                digest: "\(ImageReference.Digest(of: data))",
                 size: Int64(data.count)
             )
         )

Sources/ContainerRegistry/RegistryClient+Digest.swift

@@ -52,7 +52,7 @@ extension RegistryClient {
         mediaType: String = "application/vnd.oci.image.layer.v1.tar+gzip"
     ) async throws -> ImageLayer {
         // The diffID is the hash of the unzipped layer tarball
-        let diffID = digest(of: contents)
+        let diffID = ImageReference.Digest(of: contents)
         // The layer blob is the gzipped tarball;  the descriptor is the hash of this gzipped blob
         let blob = Data(gzip(contents))
         let descriptor = try await putBlob(repository: repository, mediaType: mediaType, data: blob)