fixing up tests to work with for non-tarsum future

Signed-off-by: David Lawrence <david.lawrence@docker.com> (github: endophage)

digest/digest.go

@@ -16,6 +16,8 @@ import (
 const (
 	// DigestTarSumV1EmptyTar is the digest for the empty tar file.
 	DigestTarSumV1EmptyTar = "tarsum.v1+sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
+	// DigestSha256EmptyTar is the canonical sha256 digest of empty data
+	DigestSha256EmptyTar = "sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
 )
 
 // Digest allows simple protection of hex formatted digest strings, prefixed

digest/digester.go

@@ -0,0 +1,44 @@
+package digest
+
+import (
+	"crypto/sha256"
+	"hash"
+)
+
+// Digester calculates the digest of written data. It is functionally
+// equivalent to hash.Hash but provides methods for returning the Digest type
+// rather than raw bytes.
+type Digester struct {
+	alg  string
+	hash hash.Hash
+}
+
+// NewDigester create a new Digester with the given hashing algorithm and instance
+// of that algo's hasher.
+func NewDigester(alg string, h hash.Hash) Digester {
+	return Digester{
+		alg:  alg,
+		hash: h,
+	}
+}
+
+// NewCanonicalDigester is a convenience function to create a new Digester with
+// out default settings.
+func NewCanonicalDigester() Digester {
+	return NewDigester("sha256", sha256.New())
+}
+
+// Write data to the digester. These writes cannot fail.
+func (d *Digester) Write(p []byte) (n int, err error) {
+	return d.hash.Write(p)
+}
+
+// Digest returns the current digest for this digester.
+func (d *Digester) Digest() Digest {
+	return NewDigest(d.alg, d.hash)
+}
+
+// Reset the state of the digester.
+func (d *Digester) Reset() {
+	d.hash.Reset()
+}