Add conversion functions for id (#38)

Test Plan:
Run `yarn test` and note that tests pass.

wchargin-branch: id-conversion

src/backend/graph.js

@@ -24,3 +24,32 @@ export type Graph = {
   nodes: {[stringID: string]: GraphNode<mixed>},
   edges: {[stringID: string]: GraphEdge<mixed>},
 };
+
+export function idToString(id: ID) {
+  if (id.pluginName.includes("$")) {
+    const escaped = JSON.stringify(id.pluginName);
+    throw new Error(`id.pluginName must not include "\$": ${escaped}`);
+  }
+  if (id.repositoryName.includes("$")) {
+    const escaped = JSON.stringify(id.repositoryName);
+    throw new Error(`id.repositoryName must not include "\$": ${escaped}`);
+  }
+  if (id.name.includes("$")) {
+    const escaped = JSON.stringify(id.name);
+    throw new Error(`id.name must not include "\$": ${escaped}`);
+  }
+  return `${id.pluginName}\$${id.repositoryName}\$${id.name}`;
+}
+
+export function stringToID(string: string) {
+  const parts = string.split("$");
+  if (parts.length !== 3) {
+    const escaped = JSON.stringify(string);
+    throw new Error(`Input should have exactly two \$s: ${escaped}`);
+  }
+  return {
+    pluginName: parts[0],
+    repositoryName: parts[1],
+    name: parts[2],
+  };
+}

src/backend/graph.test.js

@@ -0,0 +1,76 @@
+import {idToString, stringToID} from "./graph";
+
+describe("graph", () => {
+  describe("idToString", () => {
+    const makeSimpleID = () => ({
+      pluginName: "widgets",
+      repositoryName: "megacorp/megawidget",
+      name: "issue#123",
+    });
+    it("stringifies a simple ID", () => {
+      const input = makeSimpleID();
+      const expected = "widgets$megacorp/megawidget$issue#123";
+      expect(idToString(input)).toEqual(expected);
+    });
+    function expectRejection(attribute, value) {
+      const input = {...makeSimpleID(), [attribute]: value};
+      expect(() => idToString(input)).toThrow(RegExp(attribute));
+      // (escaping regexp in JavaScript is a nightmare; ignore it)
+    }
+    it("rejects an ID with $-signs in plugin name", () => {
+      expectRejection("pluginName", "widg$ets");
+    });
+    it("rejects an ID with $-signs in repository name", () => {
+      expectRejection("repositoryName", "megacorp$megawidget");
+    });
+    it("rejects an ID with $-signs in name", () => {
+      expectRejection("name", "issue$123");
+    });
+  });
+
+  describe("stringToID", () => {
+    it("parses a simple ID-string", () => {
+      const input = "widgets$megacorp/megawidget$issue#123";
+      const expected = {
+        pluginName: "widgets",
+        repositoryName: "megacorp/megawidget",
+        name: "issue#123",
+      };
+      expect(stringToID(input)).toEqual(expected);
+    });
+    [0, 1, 3, 4].forEach((n) => {
+      it(`rejects an ID-string with ${n} occurrences of "\$"`, () => {
+        const dollars = Array(n + 1).join("$");
+        const input = `mega${dollars}corp`;
+        expect(() => stringToID(input)).toThrow(/exactly two \$s/);
+      });
+    });
+  });
+
+  describe("stringToID and idToString interop", () => {
+    const examples = () => [
+      {
+        object: {
+          pluginName: "widgets",
+          repositoryName: "megacorp/megawidget",
+          name: "issue#123",
+        },
+        string: "widgets$megacorp/megawidget$issue#123",
+      },
+    ];
+    examples().forEach((example, index) => {
+      describe(`for example at 0-index ${index}`, () => {
+        it("has stringToID a left identity for idToString", () => {
+          expect(stringToID(idToString(example.object))).toEqual(
+            example.object
+          );
+        });
+        it("has stringToID a right identity for idToString", () => {
+          expect(idToString(stringToID(example.string))).toEqual(
+            example.string
+          );
+        });
+      });
+    });
+  });
+});