Implement a is_empty_schema utility function (#1833)

Fixes: #1832
Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

src/core/jsonschema/include/sourcemeta/core/jsonschema.h

@@ -104,6 +104,22 @@ auto schema_keyword_priority(std::string_view keyword,
 SOURCEMETA_CORE_JSONSCHEMA_EXPORT
 auto is_schema(const JSON &schema) -> bool;
 
+/// @ingroup jsonschema
+///
+/// This function returns true if the given JSON instance is a schema
+/// semantically equivalent to the empty schema. For example:
+///
+/// ```cpp
+/// #include <sourcemeta/core/json.h>
+/// #include <sourcemeta/core/jsonschema.h>
+/// #include <cassert>
+///
+/// const sourcemeta::core::JSON document{true};
+/// assert(sourcemeta::core::is_empty_schema(document));
+/// ```
+SOURCEMETA_CORE_JSONSCHEMA_EXPORT
+auto is_empty_schema(const JSON &schema) -> bool;
+
 /// @ingroup jsonschema
 ///
 /// This function returns the URI identifier of the given schema, if any. For

src/core/jsonschema/jsonschema.cc

@@ -13,6 +13,14 @@ auto sourcemeta::core::is_schema(const sourcemeta::core::JSON &schema) -> bool {
   return schema.is_object() || schema.is_boolean();
 }
 
+// TODO: Make this function detect schemas only using identifier/comment
+// keywords, etc
+auto sourcemeta::core::is_empty_schema(const sourcemeta::core::JSON &schema)
+    -> bool {
+  return (schema.is_boolean() && schema.to_boolean()) ||
+         (schema.is_object() && schema.empty());
+}
+
 namespace {
 
 auto id_keyword_guess(const sourcemeta::core::JSON &schema)

test/jsonschema/CMakeLists.txt

@@ -12,6 +12,7 @@ sourcemeta_googletest(NAMESPACE sourcemeta PROJECT core NAME jsonschema
     jsonschema_identify_draft0_test.cc
     jsonschema_identify_test.cc
     jsonschema_is_schema_test.cc
+    jsonschema_is_empty_schema_test.cc
     jsonschema_frame_2020_12_test.cc
     jsonschema_frame_2019_09_test.cc
     jsonschema_frame_draft7_test.cc

test/jsonschema/jsonschema_is_empty_schema_test.cc

@@ -0,0 +1,57 @@
+#include <gtest/gtest.h>
+
+#include <sourcemeta/core/json.h>
+#include <sourcemeta/core/jsonschema.h>
+
+TEST(JSONSchema, is_empty_schema_true) {
+  const sourcemeta::core::JSON document{true};
+  EXPECT_TRUE(sourcemeta::core::is_empty_schema(document));
+}
+
+TEST(JSONSchema, is_empty_schema_false) {
+  const sourcemeta::core::JSON document{false};
+  EXPECT_FALSE(sourcemeta::core::is_empty_schema(document));
+}
+
+TEST(JSONSchema, is_empty_schema_int) {
+  const sourcemeta::core::JSON document{5};
+  EXPECT_FALSE(sourcemeta::core::is_empty_schema(document));
+}
+
+TEST(JSONSchema, is_empty_schema_real) {
+  const sourcemeta::core::JSON document{5.3};
+  EXPECT_FALSE(sourcemeta::core::is_empty_schema(document));
+}
+
+TEST(JSONSchema, is_empty_schema_null) {
+  const sourcemeta::core::JSON document{nullptr};
+  EXPECT_FALSE(sourcemeta::core::is_empty_schema(document));
+}
+
+TEST(JSONSchema, is_empty_schema_string) {
+  const sourcemeta::core::JSON document{"foo"};
+  EXPECT_FALSE(sourcemeta::core::is_empty_schema(document));
+}
+
+TEST(JSONSchema, is_empty_schema_array) {
+  const sourcemeta::core::JSON document =
+      sourcemeta::core::parse_json("[ 1, 2, 3 ]");
+  EXPECT_FALSE(sourcemeta::core::is_empty_schema(document));
+}
+
+TEST(JSONSchema, is_empty_schema_empty_object) {
+  const sourcemeta::core::JSON document = sourcemeta::core::parse_json("{}");
+  EXPECT_TRUE(sourcemeta::core::is_empty_schema(document));
+}
+
+TEST(JSONSchema, is_empty_schema_non_empty_object) {
+  const sourcemeta::core::JSON document =
+      sourcemeta::core::parse_json("{ \"type\": true }");
+  EXPECT_FALSE(sourcemeta::core::is_empty_schema(document));
+}
+
+TEST(JSONSchema, is_empty_schema_unknown_keyword) {
+  const sourcemeta::core::JSON document =
+      sourcemeta::core::parse_json("{ \"x-foo\": true }");
+  EXPECT_FALSE(sourcemeta::core::is_empty_schema(document));
+}