Address comments

Author: Tuyen

README.md

@@ -67,10 +67,7 @@ Options is an optional object with the following key-value pairs:
 
 * **`fallbackToFloodsub`**: boolean identifying whether the node should fallback to the floodsub protocol, if another connecting peer does not support gossipsub (defaults to **true**).
 * **`emitSelf`**: boolean identifying whether the node should emit to self on publish, in the event of the topic being subscribed (defaults to **false**).
-* **`msgIdFn`**: a function defining the message id given a message, this is optional. For example:
-```js
-const msgIdFn = (message) => Buffer.from(hash(message.data)).toString("base64")
-```
+* **`msgIdFn`**: a function with signature `(message) => string` defining the message id given a message, used internally to deduplicate gossip (defaults to `(message) => message.from + message.seqno.toString('hex')`)
 
 For the remaining API, see https://github.com/libp2p/js-libp2p-pubsub
 

src/index.js

@@ -23,6 +23,7 @@ class GossipSub extends BasicPubsub {
    * @param {bool} [options.gossipIncoming] if incoming messages on a subscribed topic should be automatically gossiped, defaults to true
    * @param {bool} [options.fallbackToFloodsub] if dial should fallback to floodsub, defaults to true
    * @param {function} [options.msgIdFn] override the default message id function
+   * @param {Object} [options.messageCache] override the default MessageCache
    * @constructor
    */
   constructor (peerInfo, registrar, options = {}) {
@@ -82,7 +83,7 @@ class GossipSub extends BasicPubsub {
      * A message cache that contains the messages for last few hearbeat ticks
      *
      */
-    this.messageCache = new MessageCache(constants.GossipSubHistoryGossip, constants.GossipSubHistoryLength, this._msgIdFn)
+    this.messageCache = options.messageCache || new MessageCache(constants.GossipSubHistoryGossip, constants.GossipSubHistoryLength, this._msgIdFn)
 
     /**
      * A heartbeat timer that maintains the mesh

src/messageCache.js

@@ -53,11 +53,20 @@ class MessageCache {
    * @returns {void}
    */
   put (msg) {
-    const msgID = this.msgIdFn(msg)
+    const msgID = this.getMsgId(msg)
     this.msgs.set(msgID, msg)
     this.history[0].push(new CacheEntry(msgID, msg.topicIDs))
   }
 
+  /**
+   * Get message id of message.
+   * @param {rpc.RPC.Message} msg
+   * @returns {string}
+   */
+  getMsgId (msg) {
+    return this.msgIdFn(msg)
+  }
+
   /**
    * Retrieves a message from the cache by its ID, if it is still present
    *