add() supports inferring tangleID

lib/index.js

@@ -451,9 +451,30 @@ function initDB(peer, config) {
     return null
   }
 
+  /**
+   * @param {Pick<RecPresent, 'id' | 'msg'>} rec
+   * @returns {MsgID}
+   */
+  function inferTangleID(rec) {
+    if (MsgV3.isRoot(rec.msg)) return rec.id
+    let tangleID = /**@type {string | null}*/ (null)
+    for (const id in rec.msg.metadata.tangles) {
+      if (tangleID) {
+        // prettier-ignore
+        throw new Error('Cannot infer tangleID in msg because it has more than one tangle', { cause: JSON.stringify(rec.msg) })
+      } else {
+        tangleID = id
+      }
+    }
+    if (!tangleID) {
+      throw new Error('Cannot infer tangleID in msg because it has no tangles')
+    }
+    return tangleID
+  }
+
   /**
    * @param {Msg} msg
-   * @param {MsgID} tangleID
+   * @param {MsgID | null} tangleID
    * @param {CB<RecPresent>} cb
    */
   function add(msg, tangleID, cb) {
@@ -465,6 +486,8 @@ function initDB(peer, config) {
     if ((rec = getRecord(msgID))) return cb(null, rec)
     else rec = { msg, id: msgID }
 
+    tangleID ??= inferTangleID(rec)
+
     let err
     if ((err = verifyRec(rec, tangleID))) {
       return cb(new Error('add() failed to verify msg', { cause: err }))

test/add.test.js

@@ -45,7 +45,7 @@ test('add()', async (t) => {
     },
   })
 
-  const rec = await p(peer.db.add)(inputMsg, rootID)
+  const rec = await p(peer.db.add)(inputMsg, null) // tangleID implicit
   assert.equal(rec.msg.data.text, 'This is the first post!')
 
   const stats = await p(peer.db.log.stats)()