remove unneeded fmt.Sprintf and indirections

Remove fmt.Sprintf and use string-concatenation instead to reduce
some allocations.

Before / After:

    BenchmarkNewDigestFromEncoded-10     8474174   128.4 ns/op   112 B/op  3 allocs/op
    BenchmarkNewDigestFromEncoded-10    37912695    31.55 ns/op   80 B/op  1 allocs/op

    BenchmarkNewDigestFromBytes-10       5087299    237.2 ns/op	 200 B/op  5 allocs/op
    BenchmarkNewDigestFromBytes-10       8416543    146.8 ns/op	 168 B/op  3 allocs/op

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

digest.go

@@ -47,19 +47,19 @@ func NewDigest(alg Algorithm, h hash.Hash) Digest {
 // functions. This is also useful for rebuilding digests from binary
 // serializations.
 func NewDigestFromBytes(alg Algorithm, p []byte) Digest {
-	return NewDigestFromEncoded(alg, alg.Encode(p))
+	return Digest(string(alg) + ":" + alg.Encode(p))
 }
 
 // NewDigestFromHex returns a Digest from alg and the hex encoded digest.
 //
 // Deprecated: use [NewDigestFromEncoded] instead.
 func NewDigestFromHex(alg, hex string) Digest {
-	return NewDigestFromEncoded(Algorithm(alg), hex)
+	return Digest(alg + ":" + hex)
 }
 
 // NewDigestFromEncoded returns a Digest from alg and the encoded digest.
 func NewDigestFromEncoded(alg Algorithm, encoded string) Digest {
-	return Digest(fmt.Sprintf("%s:%s", alg, encoded))
+	return Digest(string(alg) + ":" + encoded)
 }
 
 // DigestRegexp matches valid digest types.

digest_test.go

@@ -15,6 +15,7 @@
 package digest_test
 
 import (
+	"crypto/sha256"
 	"testing"
 
 	"github.com/opencontainers/go-digest"
@@ -118,3 +119,21 @@ func TestParseDigest(t *testing.T) {
 		})
 	}
 }
+
+func BenchmarkNewDigestFromEncoded(b *testing.B) {
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		_ = digest.NewDigestFromEncoded("sha256", "e58fcf7418d4390dec8e8fb69d88c06ec07039d651fedd3aa72af9972e7d046b")
+	}
+}
+
+func BenchmarkNewDigestFromBytes(b *testing.B) {
+	s := sha256.Sum256([]byte("hello world"))
+
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		_ = digest.NewDigestFromBytes("sha256", s[:])
+	}
+}