Add IssuableReferenceUtility, `foo/bar#123`

See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1797

app/assets/javascripts/lib/utils/issuable_reference_utils.js

+const ISSUABLE_REFERENCE_RE = /^((?:[^\s/]+(?:\/(?!#))?)*)#(\d+)$/i;
+
+function getReferencePieces(partialReference, namespacePath, projectPath) {
+  const [
+    ,
+    fullNamespace = '',
+    resultantIssue,
+  ] = partialReference.match(ISSUABLE_REFERENCE_RE);
+  const namespacePieces = fullNamespace.split('/');
+  const resultantNamespace = namespacePieces.length > 1 ? namespacePieces.slice(0, -1).join('/') : namespacePath;
+  const resultantProject = namespacePieces.slice(-1)[0] || projectPath;
+
+  return {
+    namespace: resultantNamespace,
+    project: resultantProject,
+    issue: resultantIssue,
+  };
+}
+
+function assembleNecessaryIssuableReference(
+  partialReference,
+  currentNamespacePath,
+  currentProjectPath,
+) {
+  const {
+    namespace,
+    project,
+    issue,
+  } = getReferencePieces(partialReference, currentNamespacePath, currentProjectPath);
+
+  let necessaryReference = `#${issue}`;
+  if (currentProjectPath !== project) {
+    necessaryReference = project + necessaryReference;
+  }
+  if (currentNamespacePath !== namespace) {
+    necessaryReference = `${namespace}/${necessaryReference}`;
+  }
+
+  return necessaryReference;
+}
+
+function assembleFullIssuableReference(partialReference, currentNamespacePath, currentProjectPath) {
+  const {
+    namespace,
+    project,
+    issue,
+  } = getReferencePieces(partialReference, currentNamespacePath, currentProjectPath);
+  return `${namespace}/${project}#${issue}`;
+}
+
+export {
+  ISSUABLE_REFERENCE_RE,
+  getReferencePieces,
+  assembleNecessaryIssuableReference,
+  assembleFullIssuableReference,
+};

spec/javascripts/lib/utils/issuable_reference_utils_spec.js

+import {
+  getReferencePieces,
+  assembleNecessaryIssuableReference,
+  assembleFullIssuableReference,
+} from '~/lib/utils/issuable_reference_utils';
+
+describe('issuable_reference_utils', () => {
+  describe('getReferencePieces', () => {
+    it('should work with only issue number reference', () => {
+      expect(getReferencePieces('#111', 'foo', 'bar')).toEqual({
+        namespace: 'foo',
+        project: 'bar',
+        issue: '111',
+      });
+    });
+    it('should work with project and issue number reference', () => {
+      expect(getReferencePieces('qux#111', 'foo', 'bar')).toEqual({
+        namespace: 'foo',
+        project: 'qux',
+        issue: '111',
+      });
+    });
+    it('should work with full reference', () => {
+      expect(getReferencePieces('foo/garply#111', 'foo', 'bar')).toEqual({
+        namespace: 'foo',
+        project: 'garply',
+        issue: '111',
+      });
+    });
+    it('should work with sub-groups', () => {
+      expect(getReferencePieces('some/with/sub/groups/other#111', 'foo', 'bar')).toEqual({
+        namespace: 'some/with/sub/groups',
+        project: 'other',
+        issue: '111',
+      });
+    });
+    it('does not mangle other group references', () => {
+      expect(getReferencePieces('some/other#111', 'foo', 'bar')).toEqual({
+        namespace: 'some',
+        project: 'other',
+        issue: '111',
+      });
+    });
+    it('does not mangle other group even with partial match', () => {
+      expect(getReferencePieces('bar/baz/fido#111', 'foo/bar/baz', 'garply')).toEqual({
+        namespace: 'bar/baz',
+        project: 'fido',
+        issue: '111',
+      });
+    });
+  });
+
+  describe('assembleNecessaryIssuableReference', () => {
+    it('should work with only issue number reference', () => {
+      expect(assembleNecessaryIssuableReference('#111', 'foo', 'bar')).toEqual('#111');
+    });
+    it('should work with project and issue number reference', () => {
+      expect(assembleNecessaryIssuableReference('qux#111', 'foo', 'bar')).toEqual('qux#111');
+    });
+    it('should work with full reference to current project', () => {
+      expect(assembleNecessaryIssuableReference('foo/garply#111', 'foo', 'bar')).toEqual('garply#111');
+    });
+    it('should work with sub-groups', () => {
+      expect(assembleNecessaryIssuableReference('some/with/sub/groups/other#111', 'foo', 'bar')).toEqual('some/with/sub/groups/other#111');
+    });
+    it('does not mangle other group references', () => {
+      expect(assembleNecessaryIssuableReference('some/other#111', 'foo', 'bar')).toEqual('some/other#111');
+    });
+    it('does not mangle other group even with partial match', () => {
+      expect(assembleNecessaryIssuableReference('bar/baz/fido#111', 'foo/bar/baz', 'garply')).toEqual('bar/baz/fido#111');
+    });
+  });
+
+  describe('assembleFullIssuableReference', () => {
+    it('should work with only issue number reference', () => {
+      expect(assembleFullIssuableReference('#111', 'foo', 'bar')).toEqual('foo/bar#111');
+    });
+    it('should work with project and issue number reference', () => {
+      expect(assembleFullIssuableReference('qux#111', 'foo', 'bar')).toEqual('foo/qux#111');
+    });
+    it('should work with full reference', () => {
+      expect(assembleFullIssuableReference('foo/garply#111', 'foo', 'bar')).toEqual('foo/garply#111');
+    });
+    it('should work with sub-groups', () => {
+      expect(assembleFullIssuableReference('some/with/sub/groups/other#111', 'foo', 'bar')).toEqual('some/with/sub/groups/other#111');
+    });
+    it('does not mangle other group references', () => {
+      expect(assembleFullIssuableReference('some/other#111', 'foo', 'bar')).toEqual('some/other#111');
+    });
+    it('does not mangle other group even with partial match', () => {
+      expect(assembleFullIssuableReference('bar/baz/fido#111', 'foo/bar/baz', 'garply')).toEqual('bar/baz/fido#111');
+    });
+  });
+});