From 70ed2947c0e7a62a0a9b396b1df9c4b7576ef788 Mon Sep 17 00:00:00 2001
From: "Mark S. Miller" <erights@gmail.com>
Date: Fri, 7 Jul 2023 11:02:28 -0700
Subject: [PATCH] feat(zoe): first transferable

---
 .../src/contractSupport/transferableKit.js    | 94 +++++++++++++++++++
 .../zoe/src/contracts/transferable-counter.js | 89 ++++++++++++++++++
 2 files changed, 183 insertions(+)
 create mode 100644 packages/zoe/src/contractSupport/transferableKit.js
 create mode 100644 packages/zoe/src/contracts/transferable-counter.js

diff --git a/packages/zoe/src/contractSupport/transferableKit.js b/packages/zoe/src/contractSupport/transferableKit.js
new file mode 100644
index 000000000000..ff3d76ef97df
--- /dev/null
+++ b/packages/zoe/src/contractSupport/transferableKit.js
@@ -0,0 +1,94 @@
+import { M } from '@endo/patterns';
+import { prepareExo } from '@agoric/vat-data';
+import { OfferHandlerI } from '../typeGuards.js';
+
+/** @typedef {import('@agoric/vat-data').Baggage} Baggage */
+
+const { apply } = Reflect;
+
+const TransferProposalShape = harden({
+  give: {},
+  want: {},
+  exit: {
+    onDemand: {},
+  },
+});
+
+const defaultGetSelfFromThis = {
+  getSelfFromThis() {
+    const { self } = this;
+    return self;
+  },
+}.getSelfFromThis;
+
+export const makeTransferablesKit = (
+  zcf,
+  baggage,
+  detailsShape,
+  makeUseObject,
+  getSelfFromThis = defaultGetSelfFromThis,
+) => {
+  const UseObjectMethodGuards = harden({
+    incr: M.call().returns(M.bigint()),
+    getCustomDetails: M.call().returns(detailsShape),
+    makeTransferInvitation: M.call().returns(M.promise()),
+  });
+
+  let revokeTransferHandler;
+
+  const transferHandler = prepareExo(
+    baggage,
+    'TransferHandler',
+    OfferHandlerI,
+    {
+      handle(seat) {
+        // @ts-expect-error Usual self-typing problem
+        const { self } = this;
+        // TODO implement *Seat.getDetails()
+        const { customDetails } = seat.getDetails();
+        seat.exit();
+        revokeTransferHandler(self);
+        return makeUseObject(customDetails);
+      },
+    },
+    {
+      receiveRevoker(revoke) {
+        revokeTransferHandler = revoke;
+      },
+    },
+  );
+
+  let revokeUseObject;
+
+  const useObjectMethods = harden({
+    makeTransferInvitation() {
+      const self = apply(getSelfFromThis, this, []);
+      const invitation = zcf.makeInvitation(
+        // eslint-disable-next-line no-use-before-define
+        transferHandler,
+        'transfer',
+        self.getCustomDetails(),
+        TransferProposalShape,
+      );
+      revokeUseObject(self);
+      return invitation;
+    },
+  });
+
+  const useObjectOptions = harden({
+    receiveRevoker(revoke) {
+      revokeUseObject = revoke;
+    },
+  });
+
+  return harden({
+    // note: includes getCustomDetails
+    UseObjectMethodGuards,
+    // note: does not include getCustomDetails,
+    // so getCustomDetails is effectively an abstract method that must be
+    // concretely implemented.
+    useObjectMethods,
+    useObjectOptions,
+  });
+};
+harden(makeTransferablesKit);
diff --git a/packages/zoe/src/contracts/transferable-counter.js b/packages/zoe/src/contracts/transferable-counter.js
new file mode 100644
index 000000000000..d6f6fcca980f
--- /dev/null
+++ b/packages/zoe/src/contracts/transferable-counter.js
@@ -0,0 +1,89 @@
+import { M } from '@endo/patterns';
+import { prepareExo, prepareExoClass } from '@agoric/vat-data';
+import { makeTransferablesKit } from '../contractSupport/transferableKit.js';
+
+/** @typedef {import('@agoric/vat-data').Baggage} Baggage */
+
+const CounterDetailsShape = harden({
+  count: M.bigint(),
+});
+
+/**
+ * @param {ZCF} zcf
+ * @param {{ count: bigint}} privateArgs
+ * @param {Baggage} instanceBaggage
+ */
+export const start = async (zcf, privateArgs, instanceBaggage) => {
+  const { count: startCount = 0n } = privateArgs;
+  assert.typeof(startCount, 'bigint');
+
+  // for use by upgraded versions.
+  const firstTime = !instanceBaggage.has('count');
+  if (firstTime) {
+    instanceBaggage.init('count', startCount);
+  }
+
+  const makeForwardUseCounter = customDetails =>
+    // eslint-disable-next-line no-use-before-define
+    makeUseCounter(customDetails);
+
+  const { UseObjectMethodGuards, useObjectMethods, useObjectOptions } =
+    makeTransferablesKit(
+      zcf,
+      instanceBaggage,
+      CounterDetailsShape,
+      makeForwardUseCounter,
+    );
+
+  const UseCounterI = M.interface('UseCounter', {
+    ...UseObjectMethodGuards,
+    incr: M.call().returns(M.bigint()),
+  });
+
+  const makeUseCounter = prepareExoClass(
+    instanceBaggage,
+    'UseCounter',
+    UseCounterI,
+    customDetails => {
+      const { count } = customDetails;
+      assert(count === instanceBaggage.get('count'));
+      return harden({});
+    },
+    {
+      ...useObjectMethods,
+
+      incr() {
+        const count = instanceBaggage.get('count') + 1n;
+        instanceBaggage.set('count', count);
+        return count;
+      },
+
+      // note: abstract method must be concretely implemented
+      getCustomDetails() {
+        return harden({
+          count: instanceBaggage.get('count'),
+        });
+      },
+    },
+
+    {
+      ...useObjectOptions,
+    },
+  );
+
+  const ViewCounterI = M.interface('ViewCounter', {
+    view: M.call().returns(M.bigint()),
+  });
+
+  const viewCounter = prepareExo(instanceBaggage, 'ViewCounter', ViewCounterI, {
+    view() {
+      return instanceBaggage.get('count');
+    },
+  });
+
+  return harden({
+    creatorFacet: makeUseCounter(startCount),
+    publicFacet: viewCounter,
+  });
+};
+harden(start);