feat: add Valid, and Decode methods, and create CID struct

cid/cid.go

@@ -1,16 +1,68 @@
 package cid
 
 import (
+	"bytes"
 	"encoding/binary"
+	"errors"
 	"github.com/multiformats/go-multibase"
 )
 
+var MAGIC_BYTES = []byte{0x26, 0x1f}
+
+type CID struct {
+	Hash [32]byte
+	Size uint64
+}
+
 func Encode(hash [32]byte, size uint64) (string, error) {
 	sizeBytes := make([]byte, 8)
 	binary.LittleEndian.PutUint64(sizeBytes, size)
 
-	prefixedHash := append([]byte{0x26, 0x1f}, hash[:]...)
+	prefixedHash := append(MAGIC_BYTES, hash[:]...)
 	prefixedHash = append(prefixedHash, sizeBytes...)
 
 	return multibase.Encode(multibase.Base58BTC, prefixedHash)
 }
+
+func Valid(cid string) (bool, error) {
+	_, err := maybeDecode(cid)
+	if err != nil {
+		return false, err
+	}
+
+	return true, nil
+
+}
+
+func Decode(cid string) (*CID, error) {
+	data, err := maybeDecode(cid)
+	if err != nil {
+		return &CID{}, err
+	}
+
+	data = data[len(MAGIC_BYTES):]
+	var hash [32]byte
+	copy(hash[:], data[:])
+	size := binary.LittleEndian.Uint64(data[32:])
+
+	return &CID{Hash: hash, Size: size}, nil
+}
+
+func maybeDecode(cid string) ([]byte, error) {
+	_, data, err := multibase.Decode(cid)
+	if err != nil {
+		return nil, err
+	}
+
+	if bytes.Compare(data[0:len(MAGIC_BYTES)], MAGIC_BYTES) != 0 {
+		return nil, errors.New("CID magic bytes missing or invalid")
+	}
+
+	size := binary.LittleEndian.Uint64(data[len(MAGIC_BYTES)+32:])
+
+	if size == 0 {
+		return nil, errors.New("missing or empty size")
+	}
+
+	return data, nil
+}