Uri template fix (microsoft#7616)

Author: JoshLove-msft

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/RestClientProvider.cs

@@ -386,13 +386,47 @@ private IReadOnlyList<MethodBodyStatement> AppendPathParameters(ScopedApi uri, I
         {
             Dictionary<string, InputParameter> inputParamMap = new(operation.Parameters.ToDictionary(p => p.NameInRequest));
             List<MethodBodyStatement> statements = new(operation.Parameters.Count);
-            string? endpoint = ClientProvider.EndpointParameterName;
-            int uriOffset = endpoint is null || !operation.Uri.StartsWith(endpoint, StringComparison.Ordinal) ? 0 : endpoint.Length;
+            int uriOffset = GetUriOffset(operation.Uri);
             AddUriSegments(operation.Uri, uriOffset, uri, statements, inputParamMap, paramMap, operation);
             AddUriSegments(operation.Path, 0, uri, statements, inputParamMap, paramMap, operation);
             return statements;
         }
 
+        private int GetUriOffset(string uriTemplate)
+        {
+            string? endpointParameter = ClientProvider.EndpointParameterName;
+            if (endpointParameter == null)
+            {
+                return 0;
+            }
+
+            ReadOnlySpan<char> templateSpan = uriTemplate.AsSpan();
+            ReadOnlySpan<char> parameterSpan = endpointParameter.AsSpan();
+
+            if (templateSpan.StartsWith(parameterSpan, StringComparison.Ordinal))
+            {
+                return endpointParameter.Length;
+            }
+
+            const string httpPrefix = "http://";
+            const string httpsPrefix = "https://";
+
+            // Use span based comparison to avoid allocations
+            if (templateSpan.StartsWith(httpsPrefix.AsSpan(), StringComparison.Ordinal) &&
+                templateSpan[httpsPrefix.Length..].StartsWith(parameterSpan, StringComparison.Ordinal))
+            {
+                return httpsPrefix.Length + endpointParameter.Length;
+            }
+
+            if (templateSpan.StartsWith(httpPrefix.AsSpan(), StringComparison.Ordinal) &&
+                templateSpan[httpPrefix.Length..].StartsWith(parameterSpan, StringComparison.Ordinal))
+            {
+                return httpPrefix.Length + endpointParameter.Length;
+            }
+
+            return 0;
+        }
+
         private void AddUriSegments(
             string segments,
             int offset,

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ClientProviders/ClientProviderTests.cs

@@ -999,6 +999,27 @@ public void EndpointFieldAssignedFromStringParameter(string serverTemplate, stri
             StringAssert.Contains($"_endpoint = new global::System.Uri($\"{serverTemplate}\");", constructor?.BodyStatements?.ToDisplayString());
         }
 
+        [TestCase("{endpoint}", "endpoint")]
+        [TestCase("http://{hostName}", "hostName")]
+        [TestCase("https://{hostName}", "hostName")]
+        public void EndpointAppliedInCreateMethodRequest(string serverTemplate, string parameterName)
+        {
+            MockHelpers.LoadMockGenerator();
+            var client = InputFactory.Client(
+                TestClientName,
+                methods: [InputFactory.BasicServiceMethod("Foo", InputFactory.Operation("bar", uri: $"{serverTemplate}/foo"))],
+                parameters: [InputFactory.Parameter(
+                    parameterName,
+                    InputPrimitiveType.String,
+                    isRequired: true,
+                    kind: InputParameterKind.Client,
+                    serverUrlTemplate: serverTemplate,
+                    isEndpoint: true)]);
+            var clientProvider = new ClientProvider(client);
+            var createMethod = clientProvider.RestClient.Methods.FirstOrDefault();
+            StringAssert.Contains($"uri.Reset(_endpoint);", createMethod?.BodyStatements?.ToDisplayString());
+        }
+
         private static InputClient GetEnumQueryParamClient()
         {
             return InputFactory.Client(