feat(endo): Add assemble

The compartment assembler takes a compartment map and builds the
corresponding compartment instances, feeding the modules exported by
each dependency into the dependee compartment's module map.

packages/endo/src/assemble.js

@@ -0,0 +1,68 @@
+/* global Compartment */
+
+import { resolve } from "./node-module-specifier.js";
+
+const { entries } = Object;
+
+const defaultCompartment = Compartment;
+
+// q, as in quote, for strings in error messages.
+const q = JSON.stringify;
+
+// Assemble a DAG of compartments as declared in a compartment map starting at
+// the named compartment and building all compartments that it depends upon,
+// recursively threading the modules exported by one compartment into the
+// compartment that imports them.
+// Returns the root of the compartment DAG.
+// Does not load or execute any modules.
+// Uses makeImportHook with the given "root" string of each compartment in the
+// DAG.
+// Passes the given endowments and external modules into the root compartment
+// only.
+export const assemble = ({
+  name,
+  compartments,
+  makeImportHook,
+  parents = [],
+  loaded = {},
+  endowments = {},
+  modules = {},
+  Compartment = defaultCompartment
+}) => {
+  const descriptor = compartments[name];
+  if (descriptor === undefined) {
+    throw new Error(
+      `Cannot assemble compartment graph with missing compartment descriptor named ${q(
+        name
+      )}, needed by ${parents.map(q).join(", ")}`
+    );
+  }
+  const result = loaded[name];
+  if (result !== undefined) {
+    return result;
+  }
+  if (parents.includes(name)) {
+    throw new Error(`Cannot assemble compartment graph that includes a cycle`);
+  }
+
+  for (const [inner, outer] of entries(descriptor.modules || {})) {
+    const { compartment: compartmentName, module: moduleSpecifier } = outer;
+    const compartment = assemble({
+      name: compartmentName,
+      compartments,
+      makeImportHook,
+      parents: [...parents, name],
+      loaded,
+      Compartment
+    });
+    modules[inner] = compartment.module(moduleSpecifier);
+  }
+
+  const compartment = new Compartment(endowments, modules, {
+    resolveHook: resolve,
+    importHook: makeImportHook(descriptor.root)
+  });
+
+  loaded[name] = compartment;
+  return compartment;
+};