refactor: Move prefix code from rust-cid to here

Using prefixes for creating CIDs is tightly coupled to Bitswap. Hence moving
this functionality from rust-cid [1] directly into bitswap.

[1]: https://github.com/multiformats/rust-cid/blob/fcb8ea3f48eb4eb03c85a373d3e55ba57ce49817/src/prefix.rs

bitswap/Cargo.toml

@@ -17,5 +17,7 @@ libipld = "0.1.0"
 libp2p-core = "0.16.0"
 libp2p-swarm = "0.16.1"
 log = "0.4.8"
+multihash = "0.10.1"
 prost = "0.6.1"
 thiserror = "1.0.11"
+unsigned-varint = "0.3.2"

bitswap/src/ledger.rs

@@ -1,9 +1,10 @@
 use crate::bitswap_pb;
 use crate::block::Block;
 use crate::error::BitswapError;
+use crate::prefix::Prefix;
 use core::convert::TryFrom;
 use core::marker::PhantomData;
-use libipld::cid::{Cid, Prefix};
+use libipld::cid::Cid;
 use prost::Message as ProstMessage;
 use std::collections::HashMap;
 
@@ -168,7 +169,7 @@ impl Into<Vec<u8>> for &Message<O> {
         }
         for block in self.blocks() {
             let mut payload = bitswap_pb::message::Block::default();
-            payload.prefix = block.cid().prefix().as_bytes();
+            payload.prefix = Prefix::from(block.cid()).to_bytes();
             payload.data = block.data().to_vec();
             proto.payload.push(payload);
         }
@@ -204,8 +205,8 @@ impl TryFrom<&[u8]> for Message<I> {
             }
         }
         for payload in proto.payload {
-            let prefix = Prefix::new_from_bytes(&payload.prefix)?;
-            let cid = Cid::new_from_prefix(&prefix, &payload.data);
+            let prefix = Prefix::new(&payload.prefix)?;
+            let cid = prefix.to_cid(&payload.data)?;
             let block = Block {
                 cid,
                 data: payload.data.to_vec().into_boxed_slice(),

bitswap/src/lib.rs

@@ -6,6 +6,7 @@ mod behaviour;
 mod block;
 mod error;
 mod ledger;
+mod prefix;
 mod protocol;
 mod strategy;
 

bitswap/src/prefix.rs

@@ -0,0 +1,84 @@
+use std::convert::TryFrom;
+
+use libipld::cid::{self, Cid, Codec, Version};
+use multihash::Code;
+use unsigned_varint::{decode as varint_decode, encode as varint_encode};
+
+/// Prefix represents all metadata of a CID, without the actual content.
+#[derive(PartialEq, Eq, Clone, Debug)]
+pub struct Prefix {
+    /// The version of CID.
+    pub version: Version,
+    /// The codec of CID.
+    pub codec: Codec,
+    /// The multihash type of CID.
+    pub mh_type: Code,
+    /// The multihash length of CID.
+    pub mh_len: usize,
+}
+
+impl Prefix {
+    /// Create a new prefix from encoded bytes.
+    pub fn new(data: &[u8]) -> Result<Prefix, cid::Error> {
+        let (raw_version, remain) = varint_decode::u64(data)?;
+        let version = Version::try_from(raw_version)?;
+
+        let (raw_codec, remain) = varint_decode::u64(remain)?;
+        let codec = Codec::try_from(raw_codec)?;
+
+        let (raw_mh_type, remain) = varint_decode::u64(remain)?;
+        let mh_type = match multihash::Code::from_u64(raw_mh_type) {
+            multihash::Code::Custom(_) => return Err(cid::Error::UnknownCodec),
+            code => code,
+        };
+
+        let (mh_len, _remain) = varint_decode::usize(remain)?;
+
+        Ok(Prefix {
+            version,
+            codec,
+            mh_type,
+            mh_len,
+        })
+    }
+
+    /// Convert the prefix to encoded bytes.
+    pub fn to_bytes(&self) -> Vec<u8> {
+        let mut res = Vec::with_capacity(4);
+
+        let mut buf = varint_encode::u64_buffer();
+        let version = varint_encode::u64(self.version.into(), &mut buf);
+        res.extend_from_slice(version);
+        let mut buf = varint_encode::u64_buffer();
+        let codec = varint_encode::u64(self.codec.into(), &mut buf);
+        res.extend_from_slice(codec);
+        let mut buf = varint_encode::u64_buffer();
+        let mh_type = varint_encode::u64(self.mh_type.to_u64(), &mut buf);
+        res.extend_from_slice(mh_type);
+        let mut buf = varint_encode::u64_buffer();
+        let mh_len = varint_encode::u64(self.mh_len as u64, &mut buf);
+        res.extend_from_slice(mh_len);
+
+        res
+    }
+
+    /// Create a CID out of the prefix and some data that will be hashed
+    pub fn to_cid(&self, data: &[u8]) -> Result<Cid, cid::Error> {
+        let mut hash = self.mh_type.hasher().unwrap().digest(data);
+        if self.mh_len < hash.digest().len() {
+            hash = multihash::wrap(hash.algorithm(), &hash.digest()[..self.mh_len]);
+        }
+        Cid::new(self.version, self.codec, hash)
+    }
+}
+
+impl From<&Cid> for Prefix {
+    fn from(cid: &Cid) -> Self {
+        Self {
+            version: cid.version(),
+            codec: cid.codec(),
+            mh_type: cid.hash().algorithm(),
+            mh_len: cid.hash().digest().len(),
+        }
+    }
+}