feat(marshal): cheap capData stringify and parse

packages/marshal/src/stringify-capdata.js

@@ -0,0 +1,57 @@
+/** @template S @typedef {import('./types.js').CapData<S>} CapData */
+
+const { Fail, quote: q } = assert;
+
+/** @typedef {string} SimpleString */
+
+/**
+ * A SimpleString is one where JSON.strigify-ing the string just
+ * puts quotes around the contents, and it contains no close square
+ * brackets. Many slot encodings use slot strings that
+ * obey these rules, which is why they are useful.
+ *
+ * @param {unknown} str
+ * @returns {asserts str is SimpleString}
+ */
+export const assertSimpleString = str => {
+  assert(typeof str === 'string');
+  const stringified = JSON.stringify(str);
+  const expected = `"${str}"`;
+  stringified === expected ||
+    Fail`Expected to stringify to ${q(expected)}, not ${q(stringified)}}`;
+  assert(str.indexOf(']') === -1);
+};
+harden(assertSimpleString);
+
+/**
+ * @param {CapData<SimpleString>} capData
+ * @returns {string}
+ */
+export const stringifyCapData = capData => {
+  const {
+    body,
+    slots: [...slots],
+  } = capData;
+  assert(typeof body === 'string');
+  assert(body.startsWith('#'));
+  const scBody = body.slice(1);
+  JSON.parse(scBody); // Just asserts that it parses as JSON
+  slots.forEach(assertSimpleString);
+  return `{"slots":${JSON.stringify(slots)},"#body":${scBody}}`;
+};
+harden(stringifyCapData);
+
+const DSL = /^\{"slots":(\[[^\]]*\]),"#body":(.*)\}$/;
+
+export const parseCapData = str => {
+  assert(typeof str === 'string');
+  const matches = DSL.exec(str);
+  assert(matches && matches.length === 3);
+  const slots = JSON.parse(matches[1]);
+  slots.forEach(assertSimpleString);
+  const scBody = matches[2];
+  JSON.parse(scBody); // Just asserts that it parses as JSON
+  const body = `#${scBody}`;
+  return harden({ body, slots });
+};
+harden(parseCapData);

packages/marshal/test/test-stringify-capdata.js

@@ -0,0 +1,26 @@
+import { test } from './prepare-test-env-ava.js';
+
+import {
+  assertSimpleString,
+  parseCapData,
+  stringifyCapData,
+} from '../src/stringify-capdata.js';
+
+test('test assertSimpleString', t => {
+  t.notThrows(() => assertSimpleString('x'));
+  t.throws(() => assertSimpleString('x"'), {
+    message: 'Expected to stringify to "\\"x\\"\\"", not "\\"x\\\\\\"\\""}',
+  });
+  t.throws(() => assertSimpleString('[x]'), {
+    message: 'Check failed',
+  });
+});
+
+const roundTrips = (t, capData, str) => {
+  t.is(stringifyCapData(capData), str);
+  t.deepEqual(parseCapData(str), capData);
+};
+
+test('test capData roundTrips', t => {
+  roundTrips(t, { body: '#[]', slots: ['a'] }, '{"slots":["a"],"#body":[]}');
+});