Store the multihash length as an uint32.

.gx/lastpubver

@@ -1 +1 @@
-0.7.17: QmTprEaAA2A9bst5XH7exuyi5KzNMK3SEDNN8rBDnKWcUS
+0.8.0: QmRwpQU6ysjKZoTFtZ2PhajRatNVwTBR6TbfLkaGMb5xDf

cid.go

@@ -25,6 +25,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"math"
 	"strings"
 
 	mbase "github.com/multiformats/go-multibase"
@@ -123,6 +124,24 @@ func NewCidV1(codecType uint64, mhash mh.Multihash) *Cid {
 	}
 }
 
+func NewPrefixV0(mhType uint64) Prefix {
+	return Prefix{
+		MhType:   mhType,
+		MhLength: uint32(mh.DefaultLengths[mhType]),
+		Version:  0,
+		Codec:    DagProtobuf,
+	}
+}
+
+func NewPrefixV1(codecType uint64, mhType uint64) Prefix {
+	return Prefix{
+		MhType:   mhType,
+		MhLength: uint32(mh.DefaultLengths[mhType]),
+		Version:  1,
+		Codec:    codecType,
+	}
+}
+
 // Cid represents a self-describing content adressed
 // identifier. It is formed by a Version, a Codec (which indicates
 // a multicodec-packed content type) and a Multihash.
@@ -390,7 +409,7 @@ func (c *Cid) Prefix() Prefix {
 	dec, _ := mh.Decode(c.hash) // assuming we got a valid multiaddr, this will not error
 	return Prefix{
 		MhType:   dec.Code,
-		MhLength: dec.Length,
+		MhLength: uint32(dec.Length),
 		Version:  c.version,
 		Codec:    c.codec,
 	}
@@ -404,13 +423,13 @@ type Prefix struct {
 	Version  uint64
 	Codec    uint64
 	MhType   uint64
-	MhLength int
+	MhLength uint32
 }
 
 // Sum uses the information in a prefix to perform a multihash.Sum()
 // and return a newly constructed Cid with the resulting multihash.
 func (p Prefix) Sum(data []byte) (*Cid, error) {
-	hash, err := mh.Sum(data, p.MhType, p.MhLength)
+	hash, err := mh.Sum(data, p.MhType, int(p.MhLength))
 	if err != nil {
 		return nil, err
 	}
@@ -461,10 +480,14 @@ func PrefixFromBytes(buf []byte) (Prefix, error) {
 		return Prefix{}, err
 	}
 
+	if mhlen > math.MaxInt32 {
+		return Prefix{}, errors.New("digest too long, supporting only <= 2^31-1")
+	}
+
 	return Prefix{
 		Version:  vers,
 		Codec:    codec,
 		MhType:   mhtype,
-		MhLength: int(mhlen),
+		MhLength: uint32(mhlen),
 	}, nil
 }

cid_test.go

@@ -189,6 +189,64 @@ func TestV0ErrorCases(t *testing.T) {
 	}
 }
 
+func TestNewPrefixV1(t *testing.T) {
+	data := []byte("this is some test content")
+
+	// Construct c1
+	prefix := NewPrefixV1(DagCBOR, mh.SHA2_256)
+	c1, err := prefix.Sum(data)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if c1.Prefix() != prefix {
+		t.Fatal("prefix not preserved")
+	}
+
+	// Construct c2
+	hash, err := mh.Sum(data, mh.SHA2_256, -1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	c2 := NewCidV1(DagCBOR, hash)
+
+	if !c1.Equals(c2) {
+		t.Fatal("cids mismatch")
+	}
+	if c1.Prefix() != c2.Prefix() {
+		t.Fatal("prefixes mismatch")
+	}
+}
+
+func TestNewPrefixV0(t *testing.T) {
+	data := []byte("this is some test content")
+
+	// Construct c1
+	prefix := NewPrefixV0(mh.SHA2_256)
+	c1, err := prefix.Sum(data)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if c1.Prefix() != prefix {
+		t.Fatal("prefix not preserved")
+	}
+
+	// Construct c2
+	hash, err := mh.Sum(data, mh.SHA2_256, -1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	c2 := NewCidV0(hash)
+
+	if !c1.Equals(c2) {
+		t.Fatal("cids mismatch")
+	}
+	if c1.Prefix() != c2.Prefix() {
+		t.Fatal("prefixes mismatch")
+	}
+}
+
 func TestPrefixRoundtrip(t *testing.T) {
 	data := []byte("this is some test content")
 	hash, _ := mh.Sum(data, mh.SHA2_256, -1)

package.json

@@ -25,6 +25,6 @@
   "license": "MIT",
   "name": "go-cid",
   "releaseCmd": "git commit -a -m \"gx publish $VERSION\"",
-  "version": "0.7.17"
+  "version": "0.8.0"
 }