From 8fa78dd067d274c165f16d50dc18593fcd7f25e4 Mon Sep 17 00:00:00 2001
From: SimonLab <simon.labondance@gmail.com>
Date: Tue, 22 Jan 2019 16:22:28 +0000
Subject: [PATCH] decode cid v0, #13 #15

---
 lib/cid.ex        | 39 +++++++++++++++++++++++++++++++++++++++
 test/cid_test.exs |  8 ++++++++
 2 files changed, 47 insertions(+)

diff --git a/lib/cid.ex b/lib/cid.ex
index 2ac2bb3..f96c6fe 100644
--- a/lib/cid.ex
+++ b/lib/cid.ex
@@ -35,4 +35,43 @@ defmodule Cid do
     |> Enum.map(fn (x) -> Map.get(input_map, x) end)
     |> Enum.join("")
   end
+
+  # check for cid v0. It needs to be a binary ie a string with length 46
+  def decode(cid) when is_binary(cid) do
+    if  String.length(cid) == 46  do # why 46? we could also use byte_size as
+      case cid do
+        <<"Qm", _rest :: binary >> -> {:ok, "cidv0"}
+        _ -> multibase_decode(cid)
+      end
+
+    else
+      multibase_decode(cid)
+    end
+  end
+
+  def multibase_decode(cid) do
+    # TODO cid follow v1 format, find base then get binary
+    {:ok, "cidv1"}
+  end
+
+    #1 - If it's a string (ASCII/UTF-8):
+    #
+    # If it is 46 characters long and starts with Qm..., it's a CIDv0. Decode it as base58btc and continue to step 2.
+    # Otherwise, decode it according to the multibase spec and:
+    #     If the first decoded byte is 0x12, return an error. CIDv0 CIDs may not be multibase encoded and there will be no CIDv18 (0x12 = 18) to prevent ambiguity with decoded CIDv0s.
+    #     Otherwise, you now have a binary CID. Continue to step 2.
+    #
+    #2 - Given a (binary) CID (cid):
+    #     If it's 34 bytes long with the leading bytes [0x12, 0x20, ...], it's a CIDv0.
+    #         The CID's multihash is cid.
+    #         The CID's multicodec is DagProtobuf
+    #         The CID's version is 0.
+    #     Otherwise, let N be the first varint in cid. This is the CID's version.
+    #         If N == 1 (CIDv1):
+    #             The CID's multicodec is the second varint in cid
+    #             The CID's multihash is the rest of the cid (after the second varint).
+    #             The CID's version is 1.
+    #         If N <= 0, the CID is malformed.
+    #         If N > 1, the CID version is reserved.
+
 end
diff --git a/test/cid_test.exs b/test/cid_test.exs
index ab6c192..3a7a43a 100644
--- a/test/cid_test.exs
+++ b/test/cid_test.exs
@@ -15,4 +15,12 @@ defmodule CidTest do
     assert Cid.make("hello world") == "MJ7MSJwS1utMxA9QyQLytNDtd5RGnx6m"
   end
 
+  test "Cid.decode cidv0 format" do
+    assert Cid.decode("QmPZ9gcCEpqKTo6aq61g2nXGUhM4iCL3ewB6LDXZCtioEB") == {:ok, "cidv0"}
+  end
+
+  test "Cid.decode cidv1 format" do
+    assert Cid.decode("wrongcid") == {:ok, "cidv1"}
+  end
+
 end