Implement basic 'cid-fmt' utility.

Currently only implements printing the Cid prefix.

cid-fmt/main.go

+package main
+
+import (
+	"fmt"
+	"os"
+
+	c "github.com/ipfs/go-cid"
+
+	mh "github.com/multiformats/go-multihash"
+)
+
+func main() {
+	if len(os.Args) < 2 {
+		fmt.Fprintf(os.Stderr, "usage: %s prefix ...\n", os.Args[0])
+		os.Exit(1)
+	}
+	switch os.Args[1] {
+	case "prefix":
+		err := prefixCmd(os.Args[2:])
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "%v\n", err)
+			os.Exit(1)
+		}
+	default:
+		fmt.Fprintf(os.Stderr, "usage: %s prefix ...\n")
+		os.Exit(1)
+	}
+}
+
+func prefixCmd(args []string) error {
+	for _, cid := range args {
+		p, err := prefix(cid)
+		if err != nil {
+			return err
+		}
+		fmt.Fprintf(os.Stdout, "%s\n", p)
+	}
+	return nil
+}
+
+func prefix(str string) (string, error) {
+	cid, err := c.Decode(str)
+	if err != nil {
+		return "", err
+	}
+	p := cid.Prefix()
+	return fmt.Sprintf("cidv%d-%s-%s-%d",
+		p.Version,
+		codecToStr(p.Codec),
+		mhToStr(p.MhType),
+		p.MhLength,
+	), nil
+}
+
+func codecToStr(num uint64) string {
+	name, ok := c.CodecToStr[num]
+	if !ok {
+		return fmt.Sprintf("c?%d", num)
+	}
+	return name
+}
+
+func mhToStr(num uint64) string {
+	name, ok := mh.Codes[num]
+	if !ok {
+		return fmt.Sprintf("h?%d", num)
+	}
+	return name
+}

cid.go

@@ -101,6 +101,27 @@ var Codecs = map[string]uint64{
 	"zcash-tx":             ZcashTx,
 }
 
+// CodecToStr maps the numeric codec to its name
+var CodecToStr = map[uint64]string{
+	Raw:                "raw",
+	DagProtobuf:        "protobuf",
+	DagCBOR:            "cbor",
+	GitRaw:             "git-raw",
+	EthBlock:           "eth-block",
+	EthBlockList:       "eth-block-list",
+	EthTxTrie:          "eth-tx-trie",
+	EthTx:              "eth-tx",
+	EthTxReceiptTrie:   "eth-tx-receipt-trie",
+	EthTxReceipt:       "eth-tx-receipt",
+	EthStateTrie:       "eth-state-trie",
+	EthAccountSnapshot: "eth-account-snapshot",
+	EthStorageTrie:     "eth-storage-trie",
+	BitcoinBlock:       "bitcoin-block",
+	BitcoinTx:          "bitcoin-tx",
+	ZcashBlock:         "zcash-block",
+	ZcashTx:            "zcash-tx",
+}
+
 // NewCidV0 returns a Cid-wrapped multihash.
 // They exist to allow IPFS to work with Cids while keeping
 // compatibility with the plain-multihash format used used in IPFS.