repro a text mangling bug in automerge

repro-text-bug.cjs

const odiff = require("odiff");

/**
 * changeText uses odiff to write a minimal string diff to an automerge.Text.
 */
const changeText = (base, newValue) => {
  const diff = odiff([...base.toString()], [...newValue]);
  for (const patch of diff) {
    switch (patch.type) {
      case "add":
        base?.insertAt(patch.index, ...patch.vals);
        break;
      case "rm":
        base?.deleteAt(patch.index, patch.num);
        break;
      case "set":
        base?.set(patch.path[0], patch.val);
        break;
      default:
        throw new Error(`unexpected unset patch in text diff: ${patch}`);
    }
  }
};

const Automerge = require("@automerge/automerge");

// Make a doc and make some changes...
let original = Automerge.from({ t: new Automerge.Text("") });
original = Automerge.change(original, (doc) =>
  changeText(doc.t, `Billy Wald "You’re great"`)
);
original = Automerge.change(original, (doc) => changeText(doc.t, `Unknown`));
original = Automerge.change(original, (doc) =>
  changeText(doc.t, `billy culinary dropout`)
);

// Case 1: original output (ok)
// Output:
// original: billy culinary dropout
console.log("original:", original.t.toString());

// Case 2: using applyChanges (fails)
const changes = Automerge.getAllChanges(original);
const [replayed] = Automerge.applyChanges(Automerge.init(), changes);
// Output:
// replayed: billy culiknary dropowt
console.log("replayed using applyChanges:", replayed.t.toString());

// Case 3: using loadIncremental (fails)
let incrementalReplay = Automerge.init();
for (const change of changes) {
  incrementalReplay = Automerge.loadIncremental(incrementalReplay, change);
}

console.log("replayed using loadIncremental:", incrementalReplay.t.toString());

// Case 4: using sync (fails)
let msg;
let peerASync = Automerge.initSyncState();
let peerBSync = Automerge.initSyncState();

let peerA = Automerge.clone(original);
let peerB = Automerge.init();

do {
  if (msg) {
    [peerA, peerASync, msg] = Automerge.receiveSyncMessage(
      peerA,
      peerASync,
      msg
    );
  }

  [peerASync, msg] = Automerge.generateSyncMessage(peerA, peerASync);
  if (!msg) {
    break;
  }

  [peerB, peerBSync] = Automerge.receiveSyncMessage(peerB, peerBSync, msg);
  [peerBSync, msg] = Automerge.generateSyncMessage(peerB, peerBSync);
} while (msg);

console.log("replayed using sync:", peerB.t.toString());