Move tarsum utilities out of common package

In preparation for removing the common package, the tarsum utilities are being
moved to the more relevant digest package. This functionality will probably go
away in the future, but it's maintained here for the time being.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

digest/tarsum_test.go

@@ -0,0 +1,79 @@
+package digest
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestParseTarSumComponents(t *testing.T) {
+	for _, testcase := range []struct {
+		input    string
+		expected TarSumInfo
+		err      error
+	}{
+		{
+			input: "tarsum.v1+sha256:220a60ecd4a3c32c282622a625a54db9ba0ff55b5ba9c29c7064a2bc358b6a3e",
+			expected: TarSumInfo{
+				Version:   "v1",
+				Algorithm: "sha256",
+				Digest:    "220a60ecd4a3c32c282622a625a54db9ba0ff55b5ba9c29c7064a2bc358b6a3e",
+			},
+		},
+		{
+			input: "",
+			err:   InvalidTarSumError{},
+		},
+		{
+			input: "purejunk",
+			err:   InvalidTarSumError{TarSum: "purejunk"},
+		},
+		{
+			input: "tarsum.v23+test:12341234123412341effefefe",
+			expected: TarSumInfo{
+				Version:   "v23",
+				Algorithm: "test",
+				Digest:    "12341234123412341effefefe",
+			},
+		},
+
+		// The following test cases are ported from docker core
+		{
+			// Version 0 tarsum
+			input: "tarsum+sha256:e58fcf7418d4390dec8e8fb69d88c06ec07039d651fedd3aa72af9972e7d046b",
+			expected: TarSumInfo{
+				Algorithm: "sha256",
+				Digest:    "e58fcf7418d4390dec8e8fb69d88c06ec07039d651fedd3aa72af9972e7d046b",
+			},
+		},
+		{
+			// Dev version tarsum
+			input: "tarsum.dev+sha256:e58fcf7418d4390dec8e8fb69d88c06ec07039d651fedd3aa72af9972e7d046b",
+			expected: TarSumInfo{
+				Version:   "dev",
+				Algorithm: "sha256",
+				Digest:    "e58fcf7418d4390dec8e8fb69d88c06ec07039d651fedd3aa72af9972e7d046b",
+			},
+		},
+	} {
+		tsi, err := ParseTarSum(testcase.input)
+		if err != nil {
+			if testcase.err != nil && err == testcase.err {
+				continue // passes
+			}
+
+			t.Fatalf("unexpected error parsing tarsum: %v", err)
+		}
+
+		if testcase.err != nil {
+			t.Fatalf("expected error not encountered on %q: %v", testcase.input, testcase.err)
+		}
+
+		if !reflect.DeepEqual(tsi, testcase.expected) {
+			t.Fatalf("expected tarsum info: %v != %v", tsi, testcase.expected)
+		}
+
+		if testcase.input != tsi.String() {
+			t.Fatalf("input should equal output: %q != %q", tsi.String(), testcase.input)
+		}
+	}
+}