Add Inliner CID Builder.

inliner.go

+package cidutil
+
+import (
+	cid "github.com/ipfs/go-cid"
+	mhash "github.com/multiformats/go-multihash"
+)
+
+// Inliner is a cid.Builder that will use the id multihash when the
+// size of the content is no more than limit
+type Inliner struct {
+	cid.Builder
+	Limit int
+}
+
+// WithCodec implements the cid.Builder interface
+func (p Inliner) WithCodec(c uint64) cid.Builder {
+	return Inliner{p.Builder.WithCodec(c), p.Limit}
+}
+
+// Sum implements the cid.Builder interface
+func (p Inliner) Sum(data []byte) (*cid.Cid, error) {
+	if len(data) > p.Limit {
+		return p.Builder.Sum(data)
+	}
+	return cid.V1Builder{Codec: p.GetCodec(), MhType: mhash.ID}.Sum(data)
+}

inliner_test.go

+package cidutil
+
+import (
+	"math/rand"
+	"testing"
+
+	cid "github.com/ipfs/go-cid"
+	mhash "github.com/multiformats/go-multihash"
+)
+
+func TestInlinerSmallValue(t *testing.T) {
+	builder := Inliner{cid.V0Builder{}, 64}
+	c, err := builder.Sum([]byte("Hello World"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c.Prefix().MhType != mhash.ID {
+		t.Fatal("Inliner builder failed to use ID Multihash on small values")
+	}
+}
+
+func TestInlinerLargeValue(t *testing.T) {
+	builder := Inliner{cid.V0Builder{}, 64}
+	data := make([]byte, 512)
+	rand.Read(data)
+	c, err := builder.Sum(data)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if c.Prefix().MhType == mhash.ID {
+		t.Fatal("Inliner builder used ID Multihash on large values")
+	}
+}