Use a build flag to disable resumable digests.

Signed-off-by: Andy Goldstein <agoldste@redhat.com>

digest/digester_resumable.go

@@ -0,0 +1,52 @@
+// +build !noresumabledigest
+
+package digest
+
+import (
+	"fmt"
+
+	"github.com/jlhawn/go-crypto"
+	// For ResumableHash
+	_ "github.com/jlhawn/go-crypto/sha256" // For Resumable SHA256
+	_ "github.com/jlhawn/go-crypto/sha512" // For Resumable SHA384, SHA512
+)
+
+// resumableDigester implements ResumableDigester.
+type resumableDigester struct {
+	alg string
+	crypto.ResumableHash
+}
+
+var resumableHashAlgs = map[string]crypto.Hash{
+	"sha256": crypto.SHA256,
+	"sha384": crypto.SHA384,
+	"sha512": crypto.SHA512,
+}
+
+// NewResumableDigester creates a new ResumableDigester with the given hashing
+// algorithm.
+func NewResumableDigester(alg string) (ResumableDigester, error) {
+	hash, supported := resumableHashAlgs[alg]
+	if !supported {
+		return resumableDigester{}, fmt.Errorf("unsupported resumable hash algorithm: %s", alg)
+	}
+
+	return resumableDigester{
+		alg:           alg,
+		ResumableHash: hash.New(),
+	}, nil
+}
+
+// NewCanonicalResumableDigester creates a ResumableDigester using the default
+// digest algorithm.
+func NewCanonicalResumableDigester() ResumableDigester {
+	return resumableDigester{
+		alg:           "sha256",
+		ResumableHash: crypto.SHA256.New(),
+	}
+}
+
+// Digest returns the current digest for this resumable digester.
+func (d resumableDigester) Digest() Digest {
+	return NewDigest(d.alg, d.ResumableHash)
+}