Merge pull request #38 from wtsi-hgi/feature/small-float-serialization

Feature/small float serialization

src/json.hpp

@@ -1882,7 +1882,8 @@ class basic_json
     recursively. Note that
 
     - strings and object keys are escaped using escape_string()
-    - numbers are converted to a string before output using std::to_string()
+    - integer numbers are converted to a string before output using std::to_string()
+    - floating-point numbers are converted to a string using "%g" format
 
     @param prettyPrint    whether the output shall be pretty-printed
     @param indentStep     the indent level
@@ -1990,7 +1991,12 @@ class basic_json
 
             case (value_t::number_float):
             {
-                return std::to_string(m_value.number_float);
+                // 15 digits of precision allows round-trip IEEE 754 string->double->string
+                unsigned int sz = (unsigned int)std::snprintf(nullptr, 0, "%.15g", m_value.number_float);
+                std::vector<char> buf(sz + 1);
+                std::snprintf(&buf[0], buf.size(), "%.15g", m_value.number_float);
+                string_t formatted = buf.data();
+                return formatted;
             }
 
             default:

test/unit.cpp

@@ -1160,6 +1160,12 @@ TEST_CASE("object inspection")
             CHECK(s.find("42.23") != std::string::npos);
         }
 
+        SECTION("dump and small floating-point numbers")
+        {
+            auto s = json(1.23456e-78).dump();
+            CHECK(s.find("1.23456e-78") != std::string::npos);
+        }
+
         SECTION("dump and non-ASCII characters")
         {
             CHECK(json("ä").dump() == "\"ä\"");