Added conversion operations that can be used inline in JSON.

e.g.: { myfield: cos(rad(180)) } is equivalent to writing { myfield: -1.0 }

Bug: 29338398
Change-Id: I6fc4ef1fd10bda3ba78cba464414dd071a2f50ca
Tested: on Linux.

docs/source/Schemas.md

@@ -333,6 +333,10 @@ JSON:
 -   A field that has the value `null` (e.g. `field: null`) is intended to
     have the default value for that field (thus has the same effect as if
     that field wasn't specified at all).
+-   It has some built in conversion functions, so you can write for example
+    `rad(180)` where ever you'd normally write `3.14159`.
+    Currently supports the following functions: `rad`, `deg`, `cos`, `sin`,
+    `tan`, `acos`, `asin`, `atan`.
 
 When parsing JSON, it recognizes the following escape codes in strings:
 

src/idl_parser.cpp

@@ -17,6 +17,14 @@
 #include <algorithm>
 #include <list>
 
+#ifdef _WIN32
+#if !defined(_USE_MATH_DEFINES)
+#define _USE_MATH_DEFINES  // For M_PI.
+#endif                     // !defined(_USE_MATH_DEFINES)
+#endif                     // _WIN32
+
+#include <math.h>
+
 #include "flatbuffers/idl.h"
 #include "flatbuffers/util.h"
 
@@ -1004,8 +1012,30 @@ CheckedError Parser::ParseHash(Value &e, FieldDef* field) {
 }
 
 CheckedError Parser::ParseSingleValue(Value &e) {
-  // First check if this could be a string/identifier enum value:
-  if (e.type.base_type != BASE_TYPE_STRING &&
+  // First see if this could be a conversion function:
+  if (token_ == kTokenIdentifier && *cursor_ == '(') {
+    auto functionname = attribute_;
+    NEXT();
+    EXPECT('(');
+    ECHECK(ParseSingleValue(e));
+    EXPECT(')');
+    #define FLATBUFFERS_FN_DOUBLE(name, op) \
+      if (functionname == name) { \
+        auto x = strtod(e.constant.c_str(), nullptr); \
+        e.constant = NumToString(op); \
+      }
+    FLATBUFFERS_FN_DOUBLE("deg", x / M_PI * 180);
+    FLATBUFFERS_FN_DOUBLE("rad", x * M_PI / 180);
+    FLATBUFFERS_FN_DOUBLE("sin", sin(x));
+    FLATBUFFERS_FN_DOUBLE("cos", cos(x));
+    FLATBUFFERS_FN_DOUBLE("tan", tan(x));
+    FLATBUFFERS_FN_DOUBLE("asin", asin(x));
+    FLATBUFFERS_FN_DOUBLE("acos", acos(x));
+    FLATBUFFERS_FN_DOUBLE("atan", atan(x));
+    // TODO(wvo): add more useful conversion functions here.
+    #undef FLATBUFFERS_FN_DOUBLE
+  // Then check if this could be a string/identifier enum value:
+  } else if (e.type.base_type != BASE_TYPE_STRING &&
       e.type.base_type != BASE_TYPE_NONE &&
       (token_ == kTokenIdentifier || token_ == kTokenStringConstant)) {
     if (IsIdentifierStart(attribute_[0])) {  // Enum value.

tests/test.cpp

@@ -814,20 +814,29 @@ void ErrorTest() {
   TestError("table X { Y:byte; } root_type X; { Y:1, Y:2 }", "more than once");
 }
 
-// Additional parser testing not covered elsewhere.
-void ScientificTest() {
+float TestValue(const char *json) {
   flatbuffers::Parser parser;
 
   // Simple schema.
   TEST_EQ(parser.Parse("table X { Y:float; } root_type X;"), true);
 
-  // Test scientific notation numbers.
-  TEST_EQ(parser.Parse("{ Y:0.0314159e+2 }"), true);
+  TEST_EQ(parser.Parse(json), true);
   auto root = flatbuffers::GetRoot<float>(parser.builder_.GetBufferPointer());
   // root will point to the table, which is a 32bit vtable offset followed
   // by a float:
-  TEST_EQ(sizeof(flatbuffers::soffset_t) == 4 &&  // Test assumes 32bit offsets
-          fabs(root[1] - 3.14159) < 0.001, true);
+  TEST_EQ(sizeof(flatbuffers::soffset_t), 4);  // Test assumes 32bit offsets
+  return root[1];
+}
+
+bool FloatCompare(float a, float b) { return fabs(a - b) < 0.001; }
+
+// Additional parser testing not covered elsewhere.
+void ValueTest() {
+  // Test scientific notation numbers.
+  TEST_EQ(FloatCompare(TestValue("{ Y:0.0314159e+2 }"), 3.14159), true);
+
+  // Test conversion functions.
+  TEST_EQ(FloatCompare(TestValue("{ Y:cos(rad(180)) }"), -1), true);
 }
 
 void EnumStringsTest() {
@@ -963,7 +972,7 @@ int main(int /*argc*/, const char * /*argv*/[]) {
   FuzzTest2();
 
   ErrorTest();
-  ScientificTest();
+  ValueTest();
   EnumStringsTest();
   IntegerOutOfRangeTest();
   UnicodeTest();