diff --git a/lib/tangle.js b/lib/tangle.js
new file mode 100644
index 0000000..3b97bd2
--- /dev/null
+++ b/lib/tangle.js
@@ -0,0 +1,166 @@
+/**
+ * @typedef {import("./plugin").Rec} Rec
+ */
+
+function lipmaa(n) {
+  let m = 1
+  let po3 = 3
+  let u = n
+
+  // find k such that (3^k - 1)/2 >= n
+  while (m < n) {
+    po3 *= 3
+    m = (po3 - 1) / 2
+  }
+
+  // find longest possible backjump
+  po3 /= 3
+  if (m !== n) {
+    while (u !== 0) {
+      m = (po3 - 1) / 2
+      po3 /= 3
+      u %= m
+    }
+
+    if (m !== po3) {
+      po3 = m
+    }
+  }
+
+  return n - po3
+}
+
+/**
+ * @param {string} a
+ * @param {string} b
+ * @returns number
+ */
+function compareMsgHashes(a, b) {
+  return a.localeCompare(b)
+}
+
+class Tangle {
+  /**
+   * @type {Set<string>}
+   */
+  #tips = new Set()
+
+  /**
+   * @type {Map<string, Array<string>>}
+   */
+  #prev = new Map()
+
+  /**
+   * @type {Map<string, number>}
+   */
+  #depth = new Map()
+
+  /**
+   * @type {Map<number, Array<string>>}
+   */
+  #perDepth = new Map()
+
+  /**
+   *
+   * @param {string} rootHash
+   * @param {Iterable<Rec>} recordsIter
+   */
+  constructor(rootHash, recordsIter) {
+    for (const rec of recordsIter) {
+      const msgHash = rec.hash
+      const tangles = rec.msg.metadata.tangles
+      if (msgHash === rootHash) {
+        this.#tips.add(msgHash)
+        this.#perDepth.set(0, [msgHash])
+        this.#depth.set(msgHash, 0)
+      } else if (tangles[rootHash]) {
+        this.#tips.add(msgHash)
+        const prev = tangles[rootHash].prev
+        for (const p of prev) {
+          this.#tips.delete(p)
+        }
+        this.#prev.set(msgHash, prev)
+        const depth = tangles[rootHash].depth
+        this.#depth.set(msgHash, depth)
+        const atDepth = this.#perDepth.get(depth) ?? []
+        atDepth.push(msgHash)
+        atDepth.sort(compareMsgHashes)
+        this.#perDepth.set(depth, atDepth)
+      }
+    }
+  }
+
+  /**
+   * @param {number} depth
+   * @returns {Array<string>}
+   */
+  #getAllAtDepth(depth) {
+    return this.#perDepth.get(depth) ?? []
+  }
+
+  /**
+   * @returns {Array<string>}
+   */
+  topoSort() {
+    const sorted = []
+    for (let i = 0; i < 1e9; i++) {
+      const atDepth = this.#getAllAtDepth(i)
+      if (atDepth.length === 0) break
+      for (const msgHash of atDepth) {
+        sorted.push(msgHash)
+      }
+    }
+    return sorted
+  }
+
+  /**
+   * @returns {Array<string>}
+   */
+  getTips() {
+    return [...this.#tips]
+  }
+
+  /**
+   * @param {number} depth
+   * @returns {Array<string>}
+   */
+  getLipmaa(depth) {
+    const lipmaaDepth = lipmaa(depth + 1) - 1
+    return this.#getAllAtDepth(lipmaaDepth)
+  }
+
+  #shortestPathToRoot(msgHash) {
+    const path = []
+    let current = msgHash
+    while (true) {
+      const prev = this.#prev.get(current)
+      if (!prev) break
+      let minDepth = this.#depth.get(current)
+      let min = current
+      for (const p of prev) {
+        const d = this.#depth.get(p)
+        if (d < minDepth) {
+          minDepth = d
+          min = p
+        } else if (d === minDepth && compareMsgHashes(p, min) < 0) {
+          min = p
+        }
+      }
+      path.push(min)
+      current = min
+    }
+    return path
+  }
+
+  getDeletablesAndEmptyables(msgHash) {
+    const emptyables = this.#shortestPathToRoot(msgHash)
+    const sorted = this.topoSort()
+    const index = sorted.indexOf(msgHash)
+    const deletables = sorted.filter(
+      (msgHash, i) => i < index && !emptyables.includes(msgHash)
+    )
+    return { deletables, emptyables }
+  }
+}
+
+module.exports = Tangle
diff --git a/test/tangle.test.js b/test/tangle.test.js
new file mode 100644
index 0000000..c635e52
--- /dev/null
+++ b/test/tangle.test.js
@@ -0,0 +1,145 @@
+const test = require('tape')
+const path = require('path')
+const os = require('os')
+const rimraf = require('rimraf')
+const SecretStack = require('secret-stack')
+const caps = require('ssb-caps')
+const p = require('util').promisify
+const { generateKeypair } = require('./util')
+const Tangle = require('../lib/tangle')
+
+const DIR = path.join(os.tmpdir(), 'ppppp-db-tangle')
+rimraf.sync(DIR)
+
+let peer
+let rootPost, reply1Lo, reply1Hi, reply2A, reply3Lo, reply3Hi
+test('setup', async (t) => {
+  const keysA = generateKeypair('alice')
+  const keysB = generateKeypair('bob')
+  const keysC = generateKeypair('carol')
+
+  peer = SecretStack({ appKey: caps.shs })
+    .use(require('../'))
+    .use(require('ssb-box'))
+    .call(null, { keys: keysA, path: DIR })
+
+  await peer.db.loaded()
+
+  // Slow down append so that we can create msgs in parallel
+  const originalAppend = peer.db._getLog().append
+  peer.db._getLog().append = function (...args) {
+    setTimeout(originalAppend, 20, ...args)
+  }
+
+  rootPost = (
+    await p(peer.db.create)({
+      keys: keysA,
+      type: 'comment',
+      content: { text: 'root' },
+    })
+  ).hash
+
+  const [{ hash: reply1B }, { hash: reply1C }] = await Promise.all([
+    p(peer.db.create)({
+      keys: keysB,
+      type: 'comment',
+      content: { text: 'reply 1' },
+      tangles: [rootPost],
+    }),
+    p(peer.db.create)({
+      keys: keysC,
+      type: 'comment',
+      content: { text: 'reply 1' },
+      tangles: [rootPost],
+    }),
+  ])
+  reply1Lo = reply1B.localeCompare(reply1C) < 0 ? reply1B : reply1C
+  reply1Hi = reply1B.localeCompare(reply1C) < 0 ? reply1C : reply1B
+
+  reply2A = (
+    await p(peer.db.create)({
+      keys: keysA,
+      type: 'comment',
+      content: { text: 'reply 2' },
+      tangles: [rootPost],
+    })
+  ).hash
+
+  const [{ hash: reply3B }, { hash: reply3C }] = await Promise.all([
+    p(peer.db.create)({
+      keys: keysB,
+      type: 'comment',
+      content: { text: 'reply 3' },
+      tangles: [rootPost],
+    }),
+    p(peer.db.create)({
+      keys: keysC,
+      type: 'comment',
+      content: { text: 'reply 3' },
+      tangles: [rootPost],
+    }),
+  ])
+  reply3Lo = reply3B.localeCompare(reply3C) < 0 ? reply3B : reply3C
+  reply3Hi = reply3B.localeCompare(reply3C) < 0 ? reply3C : reply3B
+})
+
+test('Tangle.topoSort', (t) => {
+  const tangle = new Tangle(rootPost, peer.db.records())
+  const sorted = tangle.topoSort()
+
+  t.deepEquals(sorted, [
+    rootPost,
+    reply1Lo,
+    reply1Hi,
+    reply2A,
+    reply3Lo,
+    reply3Hi,
+  ])
+  console.log(sorted);
+  t.end()
+})
+
+test('Tangle.getTips', (t) => {
+  const tangle = new Tangle(rootPost, peer.db.records())
+  const tips = tangle.getTips()
+
+  t.equals(tips.length, 2, 'there are 2 tips')
+  t.true(tips.includes(reply3Lo), 'tips contains reply3Lo')
+  t.true(tips.includes(reply3Hi), 'tips contains reply3Hi')
+  t.end()
+})
+
+test('Tangle.getLipmaa', (t) => {
+  const tangle = new Tangle(rootPost, peer.db.records())
+  t.deepEquals(tangle.getLipmaa(0), [], 'lipmaa 0 (empty)')
+  t.deepEquals(tangle.getLipmaa(1), [rootPost], 'lipmaa 1 (-1)')
+  t.deepEquals(tangle.getLipmaa(2), [reply1Lo, reply1Hi], 'lipmaa 2 (-1)')
+  t.deepEquals(tangle.getLipmaa(3), [rootPost], 'lipmaa 3 (leap!)')
+  t.deepEquals(tangle.getLipmaa(4), [reply3Lo, reply3Hi], 'lipmaa 4 (-1)')
+  t.deepEquals(tangle.getLipmaa(5), [], 'lipmaa 5 (empty)')
+
+  t.end()
+})
+
+test('Tangle.getDeletablesAndEmptyables basic', t => {
+  const tangle = new Tangle(rootPost, peer.db.records())
+  const { deletables, emptyables } = tangle.getDeletablesAndEmptyables(reply2A)
+
+  t.deepEquals(deletables, [reply1Hi], 'deletables')
+  t.deepEquals(emptyables, [reply1Lo, rootPost], 'emptyables')
+  t.end()
+})
+
+
+test('Tangle.getDeletablesAndEmptyables with lipmaa', t => {
+  const tangle = new Tangle(rootPost, peer.db.records())
+  const { deletables, emptyables } = tangle.getDeletablesAndEmptyables(reply3Lo)
+
+  t.deepEquals(deletables, [reply1Lo, reply1Hi, reply2A], 'deletables')
+  t.deepEquals(emptyables, [rootPost], 'emptyables')
+  t.end()
+})
+
+test('teardown', async (t) => {
+  await p(peer.close)(true)
+})