Benchmark existing ways to check for IDENTITY CIDs

Add benchmarks that compare two ways of checking for
`multihash.IDENTITY` code:
1. `Cid.Prefix().MhType`
2. Decode of `Cid.Has()`

Relates to #133

benchmark_test.go

@@ -0,0 +1,69 @@
+package cid_test
+
+import (
+	"github.com/ipfs/go-cid"
+	"github.com/multiformats/go-multihash"
+	"math/rand"
+	"testing"
+)
+
+// randSourceSeed represents the static RNG seed, fixed for reproducibility.
+const randSourceSeed = 1413
+
+// BenchmarkCidV1IdentityCheckUsingPrefix benchmarks checking whether a CIDv1 has multihash.IDENTITY
+// code via. Cid.Prefix().
+func BenchmarkCidV1IdentityCheckUsingPrefix(b *testing.B) {
+	rng := rand.New(rand.NewSource(randSourceSeed))
+	cv1 := generateCidV1(b, rng, multihash.IDENTITY)
+	benchmarkCid(b, cv1, func(pb *testing.PB) {
+		for pb.Next() {
+			if cv1.Prefix().MhType != multihash.IDENTITY {
+				b.Fatal("expected IDENTITY CID")
+			}
+		}
+	})
+}
+
+// BenchmarkCidV1IdentityCheckUsingMultihashDecode benchmarks checking whether a CIDv1 has multihash.IDENTITY
+// code via. decoding the Cid.Hash().
+func BenchmarkCidV1IdentityCheckUsingMultihashDecode(b *testing.B) {
+	rng := rand.New(rand.NewSource(randSourceSeed))
+	cv1 := generateCidV1(b, rng, multihash.IDENTITY)
+	benchmarkCid(b, cv1, func(pb *testing.PB) {
+		for pb.Next() {
+			dmh, err := multihash.Decode(cv1.Hash())
+			if err != nil {
+				b.Fatal(err)
+			}
+			if dmh.Code != multihash.IDENTITY {
+				b.Fatal("expected IDENTITY CID")
+			}
+		}
+	})
+}
+
+func benchmarkCid(b *testing.B, target cid.Cid, pb func(pb *testing.PB)) {
+	b.SetBytes(int64(target.ByteLen()))
+	b.ReportAllocs()
+	b.ResetTimer()
+	b.RunParallel(pb)
+}
+
+func generateMultihash(b *testing.B, rng *rand.Rand, mhCode uint64) multihash.Multihash {
+	// Generate random data to hash.
+	data := make([]byte, rng.Intn(100)+1024)
+	if _, err := rng.Read(data); err != nil {
+		b.Fatal(err)
+	}
+	// Generate multihash from data.
+	mh, err := multihash.Sum(data, mhCode, -1)
+	if err != nil {
+		b.Fatal(err)
+	}
+	return mh
+}
+
+func generateCidV1(b *testing.B, rng *rand.Rand, mhCode uint64) cid.Cid {
+	mh := generateMultihash(b, rng, mhCode)
+	return cid.NewCidV1(cid.Raw, mh)
+}