From 6b2494e2f67dc447cebca9ed447c5e7dc6776d80 Mon Sep 17 00:00:00 2001
From: iscai-msft <isabellavcai@gmail.com>
Date: Fri, 1 Mar 2024 14:11:15 -0500
Subject: [PATCH 1/2] add getCrossLanguagePackageId

---
 .../typespec-client-generator-core/src/lib.ts |  6 ++++
 .../src/public-utils.ts                       | 26 +++++++++++++++-
 .../test/decorators.test.ts                   | 30 +++++++++++++++++--
 3 files changed, 59 insertions(+), 3 deletions(-)

diff --git a/packages/typespec-client-generator-core/src/lib.ts b/packages/typespec-client-generator-core/src/lib.ts
index 40145e062d..8ada1f4a8c 100644
--- a/packages/typespec-client-generator-core/src/lib.ts
+++ b/packages/typespec-client-generator-core/src/lib.ts
@@ -105,6 +105,12 @@ export const $lib = createTypeSpecLibrary({
         default: paramMessage`Unsupported kind ${"kind"}`,
       },
     },
+    "multiple-services": {
+      severity: "warning",
+      messages: {
+        default: paramMessage`Multiple services found in definition. Only one service is supported, so we will choose the first one ${"service"}`,
+      },
+    },
   },
 });
 
diff --git a/packages/typespec-client-generator-core/src/public-utils.ts b/packages/typespec-client-generator-core/src/public-utils.ts
index 29f72b390d..6d90267815 100644
--- a/packages/typespec-client-generator-core/src/public-utils.ts
+++ b/packages/typespec-client-generator-core/src/public-utils.ts
@@ -1,4 +1,6 @@
 import {
+  createDiagnosticCollector,
+  Diagnostic,
   getDeprecationDetails,
   getDoc,
   getEffectiveModelType,
@@ -36,7 +38,7 @@ import {
   listOperationsInOperationGroup,
 } from "./decorators.js";
 import { parseEmitterName, TCGCContext } from "./internal-utils.js";
-import { reportDiagnostic } from "./lib.js";
+import { createDiagnostic, reportDiagnostic } from "./lib.js";
 
 /**
  * Return the default api version for a versioned service. Will return undefined if one does not exist
@@ -284,6 +286,28 @@ export function getCrossLanguageDefinitionId(type: {
   return retval;
 }
 
+/**
+ * Helper function return the cross langauge package id for a package
+ */
+export function getCrossLanguagePackageId(context: TCGCContext): [string, readonly Diagnostic[]] {
+  const diagnostics = createDiagnosticCollector();
+  const services = listServices(context.program);
+  if (services.length === 0) return diagnostics.wrap("");
+  const serviceNamespace = getNamespaceFullName(services[0].type);
+  if (services.length > 1) {
+    diagnostics.add(
+      createDiagnostic({
+        code: "multiple-services",
+        target: services[0].type,
+        format: {
+          service: serviceNamespace,
+        },
+      })
+    );
+  }
+  return diagnostics.wrap(serviceNamespace);
+}
+
 /**
  * Create a name for anonymous model
  * @param context
diff --git a/packages/typespec-client-generator-core/test/decorators.test.ts b/packages/typespec-client-generator-core/test/decorators.test.ts
index 5f708d6911..238a7baf89 100644
--- a/packages/typespec-client-generator-core/test/decorators.test.ts
+++ b/packages/typespec-client-generator-core/test/decorators.test.ts
@@ -1,4 +1,12 @@
-import { Enum, Interface, Model, Namespace, Operation, UsageFlags } from "@typespec/compiler";
+import {
+  Enum,
+  Interface,
+  Model,
+  Namespace,
+  Operation,
+  UsageFlags,
+  ignoreDiagnostics,
+} from "@typespec/compiler";
 import { expectDiagnostics } from "@typespec/compiler/testing";
 import { deepStrictEqual, ok, strictEqual } from "assert";
 import { beforeEach, describe, it } from "vitest";
@@ -14,7 +22,7 @@ import {
   shouldGenerateProtocol,
 } from "../src/decorators.js";
 import { SdkOperationGroup } from "../src/interfaces.js";
-import { getCrossLanguageDefinitionId } from "../src/public-utils.js";
+import { getCrossLanguageDefinitionId, getCrossLanguagePackageId } from "../src/public-utils.js";
 import { getAllModels } from "../src/types.js";
 import { SdkTestRunner, createSdkContextTestHelper, createSdkTestRunner } from "./test-host.js";
 
@@ -307,6 +315,24 @@ describe("typespec-client-generator-core: decorators", () => {
       strictEqual(getCrossLanguageDefinitionId(one), "MyClient.SubNamespace.Widgets.one");
     });
 
+    it("crossLanguagePackageId", async () => {
+      await runner.compile(`
+        @client({name: "MyPackageClient"})
+        @service({})
+        namespace My.Package.Namespace;
+
+        namespace SubNamespace {
+          interface Widgets {
+            @test op one(): void;
+          }
+        }
+      `);
+      strictEqual(
+        ignoreDiagnostics(getCrossLanguagePackageId(runner.context)),
+        "My.Package.Namespace"
+      );
+    });
+
     it("@operationGroup with scope", async () => {
       const testCode = `
         @service({

From b5339c26639875a809d5b6b91c36b9293a4cc4e3 Mon Sep 17 00:00:00 2001
From: iscai-msft <isabellavcai@gmail.com>
Date: Fri, 1 Mar 2024 14:13:46 -0500
Subject: [PATCH 2/2] add changeset

---
 .../changes/getCrossLanguagePackageId-2024-2-1-14-13-36.md | 7 +++++++
 1 file changed, 7 insertions(+)
 create mode 100644 .chronus/changes/getCrossLanguagePackageId-2024-2-1-14-13-36.md

diff --git a/.chronus/changes/getCrossLanguagePackageId-2024-2-1-14-13-36.md b/.chronus/changes/getCrossLanguagePackageId-2024-2-1-14-13-36.md
new file mode 100644
index 0000000000..4c846768fb
--- /dev/null
+++ b/.chronus/changes/getCrossLanguagePackageId-2024-2-1-14-13-36.md
@@ -0,0 +1,7 @@
+---
+changeKind: feature
+packages:
+  - "@azure-tools/typespec-client-generator-core"
+---
+
+add helper function getCrossLanguagePackageId. getCrossLanguagePackageId returns a package id that is consistent across languages, allowing emitters to identify that they are generating from the same service tsp
\ No newline at end of file