From f7cb4c91eb6f0815b6dbfa9bbefbf59597af1e32 Mon Sep 17 00:00:00 2001
From: Steven Allen <steven@stebalien.com>
Date: Wed, 22 Jul 2020 19:45:20 -0700
Subject: [PATCH] feat: optimize cid.Prefix

We call this frequently and having to allocate here is really unfortunate.
---
 cid.go | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/cid.go b/cid.go
index f59c76c..1268f39 100644
--- a/cid.go
+++ b/cid.go
@@ -518,12 +518,29 @@ func (c Cid) Loggable() map[string]interface{} {
 
 // Prefix builds and returns a Prefix out of a Cid.
 func (c Cid) Prefix() Prefix {
-	dec, _ := mh.Decode(c.Hash()) // assuming we got a valid multiaddr, this will not error
+	if c.Version() == 0 {
+		return Prefix{
+			MhType:   mh.SHA2_256,
+			MhLength: 32,
+			Version:  0,
+			Codec:    DagProtobuf,
+		}
+	}
+
+	offset := 0
+	version, n, _ := uvarint(c.str[offset:])
+	offset += n
+	codec, n, _ := uvarint(c.str[offset:])
+	offset += n
+	mhtype, n, _ := uvarint(c.str[offset:])
+	offset += n
+	mhlen, _, _ := uvarint(c.str[offset:])
+
 	return Prefix{
-		MhType:   dec.Code,
-		MhLength: dec.Length,
-		Version:  c.Version(),
-		Codec:    c.Type(),
+		MhType:   mhtype,
+		MhLength: int(mhlen),
+		Version:  version,
+		Codec:    codec,
 	}
 }
 
-- 
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