From 0f09109d9f226bd0d6061715d81793c16890e42e Mon Sep 17 00:00:00 2001
From: Kevin Atkinson <k@kevina.org>
Date: Thu, 9 Aug 2018 02:37:09 -0400
Subject: [PATCH] Replace DecodeV2 with ExtractEncoding

---
 cid-fmt/main.go      |  7 ++++---
 cid-fmt/main_test.go |  4 ++--
 cid.go               | 41 +++++++++++++++++++++++++++--------------
 format_test.go       | 10 +++++-----
 4 files changed, 38 insertions(+), 24 deletions(-)

diff --git a/cid-fmt/main.go b/cid-fmt/main.go
index 8d7f53b..988126a 100644
--- a/cid-fmt/main.go
+++ b/cid-fmt/main.go
@@ -69,15 +69,16 @@ outer:
 		}
 	}
 	for _, cidStr := range args[1:] {
-		base, cid, err := c.DecodeV2(cidStr)
+		cid, err := c.Decode(cidStr)
 		if err != nil {
 			fmt.Fprintf(os.Stdout, "!INVALID_CID!\n")
 			errorMsg("%s: %v", cidStr, err)
 			// Don't abort on a bad cid
 			continue
 		}
-		if newBase != -1 {
-			base = newBase
+		base := newBase
+		if newBase == -1 {
+			base, _ = c.ExtractEncoding(cidStr)
 		}
 		if verConv != nil {
 			cid, err = verConv(cid)
diff --git a/cid-fmt/main_test.go b/cid-fmt/main_test.go
index 5161258..ce66c0a 100644
--- a/cid-fmt/main_test.go
+++ b/cid-fmt/main_test.go
@@ -10,7 +10,7 @@ import (
 func TestCidConv(t *testing.T) {
 	cidv0 := "QmUNLLsPACCz1vLxQVkXqqLX5R1X345qqfHbsf67hvA3Nn"
 	cidv1 := "zdj7WbTaiJT1fgatdet9Ei9iDB5hdCxkbVyhyh8YTUnXMiwYi"
-	_, cid, err := c.DecodeV2(cidv0)
+	cid, err := c.Decode(cidv0)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -34,7 +34,7 @@ func TestCidConv(t *testing.T) {
 func TestBadCidConv(t *testing.T) {
 	// this cid is a raw leaf and should not be able to convert to cidv0
 	cidv1 := "zb2rhhzX7uSKrtQ2ZZXFAabKiKFYZrJqKY2KE1cJ8yre2GSWZ"
-	_, cid, err := c.DecodeV2(cidv1)
+	cid, err := c.Decode(cidv1)
 	if err != nil {
 		t.Fatal(err)
 	}
diff --git a/cid.go b/cid.go
index 8992055..337e45f 100644
--- a/cid.go
+++ b/cid.go
@@ -213,34 +213,47 @@ func Parse(v interface{}) (*Cid, error) {
 // starting with "Qm" are considered CidV0 and treated directly
 // as B58-encoded multihashes.
 func Decode(v string) (*Cid, error) {
-	_, cid, err := DecodeV2(v)
-	return cid, err
-}
-
-// DecodeV2 is like Decide but also returns the Multibase encoding the
-// Cid was encoded in.  EXPERIMENTAL and interface may change at any time.
-func DecodeV2(v string) (mbase.Encoding, *Cid, error) {
 	if len(v) < 2 {
-		return 0, nil, ErrCidTooShort
+		return nil, ErrCidTooShort
 	}
 
 	if len(v) == 46 && v[:2] == "Qm" {
 		hash, err := mh.FromB58String(v)
 		if err != nil {
-			return 0, nil, err
+			return nil, err
 		}
 
-		return mbase.Base58BTC, NewCidV0(hash), nil
+		return NewCidV0(hash), nil
 	}
 
-	base, data, err := mbase.Decode(v)
+	_, data, err := mbase.Decode(v)
 	if err != nil {
-		return 0, nil, err
+		return nil, err
 	}
 
-	cid, err := Cast(data)
+	return Cast(data)
+}
 
-	return base, cid, err
+// Extract the encoding from a Cid.  If Decode on the same string did
+// not return an error neither will this function.
+func ExtractEncoding(v string) (mbase.Encoding, error) {
+	if len(v) < 2 {
+		return -1, ErrCidTooShort
+	}
+
+	if len(v) == 46 && v[:2] == "Qm" {
+		return mbase.Base58BTC, nil
+	}
+
+	encoding := mbase.Encoding(v[0])
+
+	// check encoding is valid
+	_, err := mbase.NewEncoder(encoding)
+	if err != nil {
+		return -1, err
+	}
+
+	return encoding, nil
 }
 
 func uvError(read int) error {
diff --git a/format_test.go b/format_test.go
index 7a92bb8..019d85c 100644
--- a/format_test.go
+++ b/format_test.go
@@ -55,13 +55,14 @@ func TestFmt(t *testing.T) {
 }
 
 func testFmt(t *testing.T, cidStr string, newBase mb.Encoding, fmtStr string, result string) {
-	base, cid, err := DecodeV2(cidStr)
-	if newBase != -1 {
-		base = newBase
-	}
+	cid, err := Decode(cidStr)
 	if err != nil {
 		t.Fatal(err)
 	}
+	base := newBase
+	if newBase == -1 {
+		base, _ = ExtractEncoding(cidStr)
+	}
 	str, err := Format(fmtStr, base, cid)
 	if err != nil {
 		t.Fatal(err)
@@ -70,4 +71,3 @@ func testFmt(t *testing.T, cidStr string, newBase mb.Encoding, fmtStr string, re
 		t.Error(fmt.Sprintf("expected: %s; but got: %s", result, str))
 	}
 }
-