blob: add Ref.EqualString method

Adds allocation-free way to check if a blob ref is equal to its
stringified form.

For #972 (doesn't fix it, as that bug is about a pending CL)

Change-Id: I49c6dee162698d38bb12314623b1507ee7bb246e

pkg/blob/ref.go

@@ -65,13 +65,13 @@ type digestType interface {
 	bytes() []byte
 	digestName() string
 	newHash() hash.Hash
+	equalString(string) bool
 }
 
 func (r Ref) String() string {
 	if r.digest == nil {
 		return "<invalid-blob.Ref>"
 	}
-	// TODO: maybe memoize this.
 	dname := r.digest.digestName()
 	bs := r.digest.bytes()
 	buf := getBuf(len(dname) + 1 + len(bs)*2)[:0]
@@ -84,7 +84,6 @@ func (r Ref) StringMinusOne() string {
 	if r.digest == nil {
 		return "<invalid-blob.Ref>"
 	}
-	// TODO: maybe memoize this.
 	dname := r.digest.digestName()
 	bs := r.digest.bytes()
 	buf := getBuf(len(dname) + 1 + len(bs)*2)[:0]
@@ -94,6 +93,10 @@ func (r Ref) StringMinusOne() string {
 	return string(buf)
 }
 
+// EqualString reports whether r.String() is equal to s.
+// It does not allocate.
+func (r Ref) EqualString(s string) bool { return r.digest.equalString(s) }
+
 func (r Ref) appendString(buf []byte) []byte {
 	dname := r.digest.digestName()
 	bs := r.digest.bytes()
@@ -403,9 +406,24 @@ func SHA1FromBytes(b []byte) Ref {
 
 type sha1Digest [20]byte
 
-func (s sha1Digest) digestName() string { return "sha1" }
+func (d sha1Digest) digestName() string { return "sha1" }
-func (s sha1Digest) bytes() []byte      { return s[:] }
+func (d sha1Digest) bytes() []byte      { return d[:] }
-func (s sha1Digest) newHash() hash.Hash { return sha1.New() }
+func (d sha1Digest) newHash() hash.Hash { return sha1.New() }
+func (d sha1Digest) equalString(s string) bool {
+	if len(s) != 45 {
+		return false
+	}
+	if !strings.HasPrefix(s, "sha1-") {
+		return false
+	}
+	s = s[len("sha1-"):]
+	for i, b := range d[:] {
+		if s[i*2] != hexDigit[b>>4] || s[i*2+1] != hexDigit[b&0xf] {
+			return false
+		}
+	}
+	return true
+}
 
 const maxOtherDigestLen = 128
 
@@ -419,6 +437,28 @@ type otherDigest struct {
 func (d otherDigest) digestName() string { return d.name }
 func (d otherDigest) bytes() []byte      { return d.sum[:d.sumLen] }
 func (d otherDigest) newHash() hash.Hash { return nil }
+func (d otherDigest) equalString(s string) bool {
+	wantLen := len(d.name) + len("-") + 2*d.sumLen
+	if d.odd {
+		wantLen--
+	}
+	if len(s) != wantLen || !strings.HasPrefix(s, d.name) || s[len(d.name)] != '-' {
+		return false
+	}
+	s = s[len(d.name)+1:]
+	for i, b := range d.sum[:d.sumLen] {
+		if s[i*2] != hexDigit[b>>4] {
+			return false
+		}
+		if i == d.sumLen-1 && d.odd {
+			break
+		}
+		if s[i*2+1] != hexDigit[b&0xf] {
+			return false
+		}
+	}
+	return true
+}
 
 var sha1Meta = &digestMeta{
 	ctor:  sha1FromBinary,

pkg/blob/ref_test.go

@@ -283,3 +283,54 @@ func TestJSONMarshalSized(t *testing.T) {
 		t.Fatalf("got %q, want %q", g, e)
 	}
 }
+
+var equalStringTests = []struct {
+	ref  Ref
+	str  string
+	want bool
+}{
+	{MustParse("sha1-ce284c167558a9ef22df04390c87a6d0c9ed9659"), "sha1-ce284c167558a9ef22df04390c87a6d0c9ed9659", true},
+	// last digit wrong:
+	{MustParse("sha1-ce284c167558a9ef22df04390c87a6d0c9ed9659"), "sha1-ce284c167558a9ef22df04390c87a6d0c9ed9658", false},
+	// second to last digit wrong:
+	{MustParse("sha1-ce284c167558a9ef22df04390c87a6d0c9ed9659"), "sha1-ce284c167558a9ef22df04390c87a6d0c9ed9669", false},
+	// hyphen wrong:
+	{MustParse("sha1-ce284c167558a9ef22df04390c87a6d0c9ed9659"), "sha1xce284c167558a9ef22df04390c87a6d0c9ed9659", false},
+	// truncated:
+	{MustParse("sha1-ce284c167558a9ef22df04390c87a6d0c9ed9659"), "sha1-", false},
+	{MustParse("sha1-ce284c167558a9ef22df04390c87a6d0c9ed9659"), "sha1", false},
+	{MustParse("sha1-ce284c167558a9ef22df04390c87a6d0c9ed9659"), "", false},
+	// right length, wrong hash:
+	{MustParse("sha1-ce284c167558a9ef22df04390c87a6d0c9ed9659"), "sha2-ce284c167558a9ef22df04390c87a6d0c9ed9659", false},
+
+	// Other hashes:
+	{MustParse("foo-cafe"), "foo-cafe", true},
+	{MustParse("foo-caf"), "foo-caf", true},
+
+	{MustParse("foo-cafe"), "foo-beef", false},
+	{MustParse("foo-cafe"), "bar-cafe", false},
+	{MustParse("foo-cafe"), "fooxbeef", false},
+	{MustParse("foo-caf"), "foo-cae", false},
+	{MustParse("foo-caf"), "foo-ca", false},
+}
+
+func TestEqualString(t *testing.T) {
+	for _, tt := range equalStringTests {
+		got := tt.ref.EqualString(tt.str)
+		if got != tt.want {
+			t.Errorf("ref %q EqualString(%q) = %v; want %v", tt.ref, tt.str, got, tt.want)
+		}
+	}
+}
+
+func BenchmarkEqualString(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		for _, tt := range equalStringTests {
+			got := tt.ref.EqualString(tt.str)
+			if got != tt.want {
+				b.Fatalf("ref %q EqualString(%q) = %v; want %v", tt.ref, tt.str, got, tt.want)
+			}
+		}
+	}
+}