Switch template metaprogramming helper for promises library (#26908)

* Switch template metaprogramming helper for promises library

* py3

BUILD

@@ -878,6 +878,15 @@ grpc_cc_library(
     deps = ["gpr_platform"],
 )
 
+grpc_cc_library(
+    name = "switch",
+    language = "c++",
+    public_hdrs = [
+        "src/core/lib/promise/detail/switch.h",
+    ],
+    deps = ["gpr_platform"],
+)
+
 grpc_cc_library(
     name = "ref_counted",
     language = "c++",

tools/codegen/core/gen_switch.py

@@ -0,0 +1,75 @@
+#!/usr/bin/env python3
+
+# Copyright 2021 gRPC authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import sys
+
+
+# utility: print a big comment block into a set of files
+def put_banner(files, banner):
+    for f in files:
+        print('/*', file=f)
+        for line in banner:
+            print(' * %s' % line, file=f)
+        print(' */', file=f)
+        print('', file=f)
+
+
+with open('src/core/lib/promise/detail/switch.h', 'w') as H:
+    # copy-paste copyright notice from this file
+    with open(sys.argv[0]) as my_source:
+        copyright = []
+        for line in my_source:
+            if line[0] != '#':
+                break
+        for line in my_source:
+            if line[0] == '#':
+                copyright.append(line)
+                break
+        for line in my_source:
+            if line[0] != '#':
+                break
+            copyright.append(line)
+        put_banner([H], [line[2:].rstrip() for line in copyright])
+
+    put_banner([H], ["Automatically generated by %s" % sys.argv[0]])
+
+    print("#ifndef GRPC_CORE_LIB_PROMISE_DETAIL_SWITCH_H", file=H)
+    print("#define GRPC_CORE_LIB_PROMISE_DETAIL_SWITCH_H", file=H)
+    print('', file=H)
+    print('#include <grpc/impl/codegen/port_platform.h>', file=H)
+    print('', file=H)
+    print("#include <stdlib.h>", file=H)
+    print('', file=H)
+    print("namespace grpc_core {", file=H)
+
+    for n in range(1, 33):
+        print('', file=H)
+        print("template <typename R, %s> R Switch(char idx, %s) {" % (
+            ", ".join("typename F%d" % i for i in range(0, n)),
+            ", ".join("F%d f%d" % (i, i) for i in range(0, n)),
+        ),
+              file=H)
+        print("  switch (idx) {", file=H)
+        for i in range(0, n):
+            print("   case %d: return f%d();" % (i, i), file=H)
+        print("  }", file=H)
+        print("  abort();", file=H)
+        print("}", file=H)
+
+    print('', file=H)
+    print("}", file=H)
+    print('', file=H)
+    print("#endif // GRPC_CORE_LIB_PROMISE_DETAIL_SWITCH_H", file=H)