From 3464114b88af8e0e6f69460333215a1ece09ec0a Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Mon, 13 Apr 2020 08:54:35 -0700
Subject: [PATCH] logtail: add ParsePublicID that doesn't allocate

---
 logtail/id.go      | 17 +++++++++++++++++
 logtail/id_test.go |  5 +++++
 2 files changed, 22 insertions(+)

diff --git a/logtail/id.go b/logtail/id.go
index 164995c42..2c0b72517 100644
--- a/logtail/id.go
+++ b/logtail/id.go
@@ -92,6 +92,23 @@ func (id PrivateID) Public() (pub PublicID) {
 // The public ID value is a SHA-256 hash of a private ID.
 type PublicID [sha256.Size]byte
 
+// ParsePublicID returns a PublicID from its hex (String) representation.
+func ParsePublicID(s string) (PublicID, error) {
+	if len(s) != sha256.Size*2 {
+		return PublicID{}, errors.New("invalid length")
+	}
+	var p PublicID
+	for i := range p {
+		a, ok1 := fromHexChar(s[i*2+0])
+		b, ok2 := fromHexChar(s[i*2+1])
+		if !ok1 || !ok2 {
+			return PublicID{}, errors.New("invalid hex character")
+		}
+		p[i] = (a << 4) | b
+	}
+	return p, nil
+}
+
 func (id PublicID) MarshalText() ([]byte, error) {
 	b := make([]byte, hex.EncodedLen(len(id)))
 	if i := hex.Encode(b, id[:]); i != len(b) {
diff --git a/logtail/id_test.go b/logtail/id_test.go
index b335562d6..1a7798dc6 100644
--- a/logtail/id_test.go
+++ b/logtail/id_test.go
@@ -51,6 +51,11 @@ func TestIDs(t *testing.T) {
 	if id1.String() != id3.String() {
 		t.Fatalf("id1.String()=%v does not match id3.String()=%v", id1.String(), id3.String())
 	}
+	if id3, err := ParsePublicID(id1.Public().String()); err != nil {
+		t.Errorf("ParsePublicID: %v", err)
+	} else if id1.Public() != id3 {
+		t.Errorf("ParsePublicID mismatch")
+	}
 
 	id4, err := ParsePrivateID(id1.String())
 	if err != nil {