User-defined double output precision

include/rapidjson/prettywriter.h

@@ -26,6 +26,9 @@ class PrettyWriter : public Writer<OutputStream, SourceEncoding, TargetEncoding,
 	PrettyWriter(OutputStream& os, Allocator* allocator = 0, size_t levelDepth = Base::kDefaultLevelDepth) : 
 		Base(os, allocator, levelDepth), indentChar_(' '), indentCharCount_(4) {}
 
+	//! Overridden for fluent API, see \ref Writer::SetDoublePrecision()
+	PrettyWriter& SetDoublePrecision(int p) { Base::SetDoublePrecision(p); return *this; }
+
 	//! Set custom indentation.
 	/*! \param indentChar		Character for indentation. Must be whitespace character (' ', '\\t', '\\n', '\\r').
 		\param indentCharCount	Number of indent characters for each indentation level.
@@ -48,6 +51,11 @@ class PrettyWriter : public Writer<OutputStream, SourceEncoding, TargetEncoding,
 	PrettyWriter& Int64(int64_t i64)	{ PrettyPrefix(kNumberType); Base::WriteInt64(i64);		return *this; }
 	PrettyWriter& Uint64(uint64_t u64)	{ PrettyPrefix(kNumberType); Base::WriteUint64(u64);	return *this; }
 	PrettyWriter& Double(double d)		{ PrettyPrefix(kNumberType); Base::WriteDouble(d);		return *this; }
+	//! Overridden for fluent API, see \ref Writer::Double()
+	PrettyWriter& Double(double d, int precision) {
+		int oldPrecision = Base::GetDoublePrecision();
+		return SetDoublePrecision(precision).Double(d).SetDoublePrecision(oldPrecision);
+	}
 
 	PrettyWriter& String(const Ch* str, SizeType length, bool copy = false) {
 		(void)copy;

include/rapidjson/writer.h

@@ -35,7 +35,23 @@ class Writer {
 	typedef typename SourceEncoding::Ch Ch;
 
 	Writer(OutputStream& os, Allocator* allocator = 0, size_t levelDepth = kDefaultLevelDepth) : 
-		os_(os), level_stack_(allocator, levelDepth * sizeof(Level)) {}
+		os_(os), level_stack_(allocator, levelDepth * sizeof(Level)),
+		doublePrecision_(kDefaultDoublePrecision) {}
+
+	//! Set the number of significant digits for \c double values
+	/*! When writing a \c double value to the \c OutputStream, the number
+		of significant digits is limited to 6 by default.
+		\param p maximum number of significant digits (default: 6)
+		\return The Writer itself for fluent API.
+	*/
+	Writer& SetDoublePrecision(int p = kDefaultDoublePrecision) {
+		if (p < 0) p = kDefaultDoublePrecision; // negative precision is ignored
+		doublePrecision_ = p;
+		return *this;
+	}
+
+	//! \see SetDoublePrecision()
+	int GetDoublePrecision() const { return doublePrecision_; }
 
 	//@name Implementation of Handler
 	//@{
@@ -45,8 +61,34 @@ class Writer {
 	Writer& Uint(unsigned u)		{ Prefix(kNumberType); WriteUint(u);		return *this; }
 	Writer& Int64(int64_t i64)		{ Prefix(kNumberType); WriteInt64(i64);		return *this; }
 	Writer& Uint64(uint64_t u64)	{ Prefix(kNumberType); WriteUint64(u64);	return *this; }
+
+	//! Writes the given \c double value to the stream
+	/*!
+		The number of significant digits (the precision) to be written
+		can be set by \ref SetDoublePrecision() for the Writer:
+		\code
+		Writer<...> writer(...);
+		writer.SetDoublePrecision(12).Double(M_PI);
+		\endcode
+		\param d The value to be written.
+		\return The Writer itself for fluent API.
+	*/
 	Writer& Double(double d)		{ Prefix(kNumberType); WriteDouble(d);		return *this; }
 
+	//! Writes the given \c double value to the stream (explicit precision)
+	/*!
+		The currently set double precision is ignored in favor of the explicitly
+		given precision for this value.
+		\see Double(), SetDoublePrecision(), GetDoublePrecision()
+		\param d The value to be written
+		\param precision The number of significant digits for this value
+		\return The Writer itself for fluent API.
+	*/
+	Writer& Double(double d, int precision) {
+		int oldPrecision = GetDoublePrecision();
+		return SetDoublePrecision(precision).Double(d).SetDoublePrecision(oldPrecision);
+	}
+
 	Writer& String(const Ch* str, SizeType length, bool copy = false) {
 		(void)copy;
 		Prefix(kStringType);
@@ -161,18 +203,21 @@ class Writer {
 		} while (p != buffer);
 	}
 
-	//! \todo Optimization with custom double-to-string converter.
-	void WriteDouble(double d) {
-		char buffer[100];
 #ifdef _MSC_VER
-		int ret = sprintf_s(buffer, sizeof(buffer), "%g", d);
+#define RAPIDJSON_SNPRINTF sprintf_s
 #else
-		int ret = snprintf(buffer, sizeof(buffer), "%g", d);
+#define RAPIDJSON_SNPRINTF snprintf
 #endif
+
+	//! \todo Optimization with custom double-to-string converter.
+	void WriteDouble(double d) {
+		char buffer[100];
+		int ret = RAPIDJSON_SNPRINTF(buffer, sizeof(buffer), "%.*g", doublePrecision_, d);
 		RAPIDJSON_ASSERT(ret >= 1);
 		for (int i = 0; i < ret; i++)
 			os_.Put(buffer[i]);
 	}
+#undef RAPIDJSON_SNPRINTF
 
 	void WriteString(const Ch* str, SizeType length)  {
 		static const char hexDigits[16] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' };
@@ -234,6 +279,9 @@ class Writer {
 
 	OutputStream& os_;
 	internal::Stack<Allocator> level_stack_;
+	int doublePrecision_;
+
+	static const int kDefaultDoublePrecision = 6;
 
 private:
 	// Prohibit assignment for VC C4512 warning

test/unittest/writertest.cpp

@@ -1,4 +1,5 @@
 #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\" } ");