fix logic determining tips of the tangle

lib/feed-v1/index.js

@@ -91,7 +91,16 @@ function readDepth(msg, tangleId = null) {
     return msg.metadata.tangles?.[tangleId]?.depth ?? 0
   } else {
     return msg.metadata.depth
-  }}
+  }
+}
+
+function readPrev(msg, tangleId = null) {
+  if (tangleId) {
+    return msg.metadata.tangles?.[tangleId]?.prev ?? []
+  } else {
+    return msg.metadata.prev
+  }
+}
 
 function calculateDepth(existing, tangleId = null) {
   let max = -1
@@ -132,12 +141,33 @@ function lipmaa(n) {
   return n - po3
 }
 
+function determineTips(existing, tangleId = null) {
+  const tips = new Set()
+  for (const msg of existing.values()) {
+    tips.add(getMsgHash(msg))
+  }
+
+  for (const msg of existing.values()) {
+    const prev = readPrev(msg, tangleId)
+    for (const p of prev) {
+      tips.delete(p)
+    }
+  }
+  return tips
+}
+
 function calculatePrev(existing, depth, lipmaaDepth, tangleId = null) {
   const prev = []
+  const tips = determineTips(existing, tangleId)
   for (const msg of existing.values()) {
     const msgDepth = readDepth(msg, tangleId)
-    if (msgDepth === lipmaaDepth || msgDepth === depth - 1) {
+    const msgHash = getMsgHash(msg)
-      prev.push(getMsgHash(msg))
+    if (
+      msgDepth === depth - 1 ||
+      msgDepth === lipmaaDepth ||
+      tips.has(msgHash)
+    ) {
+      prev.push(msgHash)
     }
   }
   return prev

test/feed-v1-create.test.js

@@ -80,3 +80,72 @@ tape('FeedV1.create()', (t) => {
 
   t.end()
 })
+
+tape('create() handles DAG tips correctly', (t) => {
+  const keys = generateKeypair('alice')
+  const when = 1652037377204
+  const existing = new Map()
+
+  const msg1 = FeedV1.create({
+    keys,
+    content: { text: '1' },
+    type: 'post',
+    existing: new Map(),
+    when: when + 1,
+  })
+  const msgHash1 = FeedV1.getMsgHash(msg1)
+  t.deepEquals(msg1.metadata.prev, [], 'msg1.prev is empty')
+
+  existing.set(msgHash1, msg1)
+
+  const msg2A = FeedV1.create({
+    keys,
+    content: { text: '2A' },
+    type: 'post',
+    existing,
+    when: when + 2,
+  })
+  t.deepEquals(msg2A.metadata.prev, [msgHash1], 'msg2A.prev is msg1')
+
+  const msg2B = FeedV1.create({
+    keys,
+    content: { text: '2B' },
+    type: 'post',
+    existing,
+    when: when + 2,
+  })
+  const msgHash2B = FeedV1.getMsgHash(msg2B)
+  t.deepEquals(msg2B.metadata.prev, [msgHash1], 'msg2B.prev is msg1')
+
+  existing.set(msgHash2B, msg2B)
+
+  const msg3 = FeedV1.create({
+    keys,
+    content: { text: '3' },
+    type: 'post',
+    existing,
+    when: when + 3,
+  })
+  const msgHash3 = FeedV1.getMsgHash(msg3)
+  t.deepEquals(msg3.metadata.prev, [msgHash2B], 'msg3.prev is msg2B')
+  existing.set(msgHash3, msg3)
+
+  const msgHash2A = FeedV1.getMsgHash(msg2A)
+  existing.set(msgHash2A, msg2A)
+  t.pass('msg2A comes into awareness')
+
+  const msg4 = FeedV1.create({
+    keys,
+    content: { text: '4' },
+    type: 'post',
+    existing,
+    when: when + 4,
+  })
+  t.deepEquals(
+    msg4.metadata.prev,
+    [msgHash1, msgHash3, msgHash2A],
+    'msg4.prev is [msg1(lipmaa),msg3(previous),msg2A(old fork as tip)]'
+  )
+
+  t.end()
+})