DoublePrecision: add a unit test for the precision handling

test/unittest/writertest.cpp

 #include "unittest.h"
+#include "rapidjson/document.h"
 #include "rapidjson/reader.h"
 #include "rapidjson/writer.h"
 #include "rapidjson/stringbuffer.h"
@@ -56,6 +57,61 @@ TEST(Writer, String) {
 	TEST_ROUNDTRIP("[\"\\\"\\\\/\\b\\f\\n\\r\\t\"]");
 }
 
+TEST(Writer,DoublePrecision) {
+	const char json[] = "[1.2345,1.2345678,0.123456789012,1234567.8]";
+
+	StringBuffer buffer;
+	Writer<StringBuffer> writer(buffer);
+
+	const int kDefaultDoublePrecision = 6;
+	// handling the double precision
+	EXPECT_EQ(writer.GetDoublePrecision(), kDefaultDoublePrecision);
+	writer.SetDoublePrecision(17);
+	EXPECT_EQ(writer.GetDoublePrecision(), 17);
+	writer.SetDoublePrecision(-1); // negative equivalent to reset
+	EXPECT_EQ(writer.GetDoublePrecision(), kDefaultDoublePrecision);
+	writer.SetDoublePrecision(1);
+	writer.SetDoublePrecision();   // reset again
+	EXPECT_EQ(writer.GetDoublePrecision(), kDefaultDoublePrecision);
+
+	{ // write with explicitly increased precision
+		StringStream s(json);
+		Reader reader;
+		reader.Parse<0>(s, writer.SetDoublePrecision(12));
+		EXPECT_EQ(writer.GetDoublePrecision(), 12);
+		EXPECT_STREQ(json, buffer.GetString());
+		buffer.Clear();
+	}
+	{ // explicit individual double precisions
+		writer.SetDoublePrecision(2)
+			.StartArray()
+			.Double(1.2345,5)
+			.Double(1.2345678,9)
+			.Double(0.123456789012,12)
+			.Double(1234567.8,8)
+			.EndArray();
+
+		EXPECT_EQ(writer.GetDoublePrecision(), 2);
+		EXPECT_STREQ(json, buffer.GetString());
+		buffer.Clear();
+	}
+	{ // write with default precision (output with precision loss)
+		Document d;
+		d.Parse<0>(json);
+		d.Accept(writer.SetDoublePrecision());
+
+		// parsed again to avoid platform-dependent floating point outputs
+		// (e.g. width of exponents)
+		d.Parse<0>(buffer.GetString());
+		EXPECT_EQ(writer.GetDoublePrecision(), kDefaultDoublePrecision);
+		EXPECT_DOUBLE_EQ(d[0u].GetDouble(), 1.2345);
+		EXPECT_DOUBLE_EQ(d[1u].GetDouble(), 1.23457);
+		EXPECT_DOUBLE_EQ(d[2u].GetDouble(), 0.123457);
+		EXPECT_DOUBLE_EQ(d[3u].GetDouble(), 1234570);
+		buffer.Clear();
+	}
+}
+
 TEST(Writer, Transcode) {
 	// UTF8 -> UTF16 -> UTF8
 	StringStream s("{ \"hello\" : \"world\", \"t\" : true , \"f\" : false, \"n\": null, \"i\":123, \"pi\": 3.1416, \"a\":[1, 2, 3], \"dollar\":\"\x24\", \"cents\":\"\xC2\xA2\", \"euro\":\"\xE2\x82\xAC\", \"gclef\":\"\xF0\x9D\x84\x9E\" } ");