reduce the size of the nlohmann json library download

WORKSPACE

@@ -12,21 +12,21 @@ http_archive(
     ],
 )
 
+# This should also be kept up to date with the version used by Tensorflow.
+http_file(
+    name = "com_github_nlohmann_json_single_header",
+    urls = [
+        "https://github.com/nlohmann/json/releases/download/v3.4.0/json.hpp",
+    ],
+)
+
 http_archive(
     name = "org_tensorflow",
     sha256 = "902a6d90bb69549fe241377210aa459773459820da1333b67dcfdef37836f25f",
     strip_prefix = "tensorflow-1.13.1",
     urls = ["https://github.com/tensorflow/tensorflow/archive/v1.13.1.zip"],
 )
 
-http_archive(
-    name = "com_github_nlohmann_json",
-    build_file = "//cc:json.BUILD",
-    sha256 = "2ef2fe6f1a615ad97beb39f91ef5e319d776f6ba0af91570003276e6ffb1c47c",
-    strip_prefix = "json-3.2.0",
-    urls = ["https://github.com/nlohmann/json/archive/v3.2.0.zip"],
-)
-
 load("@org_tensorflow//tensorflow:workspace.bzl", "tf_workspace")
 
 tf_workspace()

cc/BUILD

@@ -129,6 +129,7 @@ minigo_cc_library(
     deps = [
         ":base",
         ":gtp_client",
+        ":json",
         ":logging",
         ":mcts",
         ":sgf",
@@ -137,7 +138,6 @@ minigo_cc_library(
         "//cc/dual_net",
         "//cc/dual_net:batching_dual_net",
         "//cc/file",
-        "@com_github_nlohmann_json//:json",
         "@com_google_absl//absl/container:flat_hash_map",
         "@com_google_absl//absl/strings",
         "@com_google_absl//absl/strings:str_format",
@@ -146,6 +146,14 @@ minigo_cc_library(
     ],
 )
 
+cc_library(
+    name = "json",
+    hdrs = [
+        "json.h",
+        "@com_github_nlohmann_json_single_header//file",
+    ],
+)
+
 minigo_cc_library(
     name = "mcts",
     srcs = [

cc/json.h

@@ -0,0 +1,20 @@
+// Copyright 2019 Google LLC
+//
+// 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.
+
+#ifndef MINIGO_CC_JSON_H_
+#define MINIGO_CC_JSON_H_
+
+#include "external/com_github_nlohmann_json_single_header/file/json.hpp"
+
+#endif  // MINIGO_CC_JSON_H_

cc/minigui_gtp_client.cc

@@ -27,9 +27,9 @@
 #include "absl/time/clock.h"
 #include "cc/constants.h"
 #include "cc/file/utils.h"
+#include "cc/json.h"
 #include "cc/logging.h"
 #include "cc/sgf.h"
-#include "nlohmann/json.hpp"
 
 namespace minigo {