feat: add AutoIncrementing contract (#1023)

* feat: add AutoIncrementing contract

* feat: allow multiple counters per instance

* fix: some linting errors

* feat: use recommended implementaiton

* fix: remove .only in tests

* fix: PR notes

* fix: add note about incrementing counter

test/AutoIncrementing.test.js

@@ -0,0 +1,41 @@
+const { hashMessage } = require('./helpers/sign');
+
+const AutoIncrementing = artifacts.require('AutoIncrementingImpl');
+
+require('chai')
+  .use(require('chai-bignumber')(web3.BigNumber))
+  .should();
+
+const EXPECTED = [1, 2, 3, 4];
+const KEY1 = hashMessage('key1');
+const KEY2 = hashMessage('key2');
+
+contract('AutoIncrementing', function ([_, owner]) {
+  beforeEach(async function () {
+    this.mock = await AutoIncrementing.new({ from: owner });
+  });
+
+  context('custom key', async function () {
+    it('should return expected values', async function () {
+      for (let expectedId of EXPECTED) {
+        await this.mock.doThing(KEY1, { from: owner });
+        const actualId = await this.mock.theId();
+        actualId.should.be.bignumber.eq(expectedId);
+      }
+    });
+  });
+
+  context('parallel keys', async function () {
+    it('should return expected values for each counter', async function () {
+      for (let expectedId of EXPECTED) {
+        await this.mock.doThing(KEY1, { from: owner });
+        let actualId = await this.mock.theId();
+        actualId.should.be.bignumber.eq(expectedId);
+
+        await this.mock.doThing(KEY2, { from: owner });
+        actualId = await this.mock.theId();
+        actualId.should.be.bignumber.eq(expectedId);
+      }
+    });
+  });
+});