SERVER-41271 allow logging doubles with arbitrary precision

3 files changed, 23 insertions, 0 deletions

diff --git a/src/mongo/util/str.cpp b/src/mongo/util/str.cpp
index 7209fd90e2e..0f04497f95f 100644
--- a/src/mongo/util/str.cpp
+++ b/src/mongo/util/str.cpp
@@ -236,4 +236,11 @@ boost::optional<size_t> parseUnsignedBase10Integer(StringData fieldName) {
     return boost::none;
 }
 
+std::string convertDoubleToString(double d, int prec) {
+    char buffer[StringBuilder::MONGO_DBL_SIZE];
+    int z = snprintf(buffer, sizeof(buffer), "%.*g", prec, d);
+    invariant(z >= 0);
+    return std::string(buffer);
+}
+
 }  // namespace mongo::str
diff --git a/src/mongo/util/str.h b/src/mongo/util/str.h
index bddfff282b2..b2ee26dbbdb 100644
--- a/src/mongo/util/str.h
+++ b/src/mongo/util/str.h
@@ -344,4 +344,10 @@ std::string escape(StringData s, bool escape_slash = false);
  */
 boost::optional<size_t> parseUnsignedBase10Integer(StringData integer);
 
+/**
+ * Converts a double to a string with specified precision. If unspecified, default to 17, which is
+ * the maximum decimal precision possible from a standard double.
+ */
+std::string convertDoubleToString(double d, int prec = 17);
+
 }  // namespace mongo::str
diff --git a/src/mongo/util/str_test.cpp b/src/mongo/util/str_test.cpp
index 68d06b7535c..66ee0f77196 100644
--- a/src/mongo/util/str_test.cpp
+++ b/src/mongo/util/str_test.cpp
@@ -270,4 +270,14 @@ TEST(StringUtilsTest, WhitespaceWithinNumberFailsToParse) {
     boost::optional<size_t> result = parseUnsignedBase10Integer(" 10");
     ASSERT(!result);
 }
+
+TEST(StringUtilsTest, ConvertDoubleToStringWithProperPrecision) {
+    ASSERT_EQUALS(std::string("1.9876543219876543"), convertDoubleToString(1.98765432198765432));
+    ASSERT_EQUALS(std::string("1.987654321"), convertDoubleToString(1.987654321, 10));
+    ASSERT_EQUALS(std::string("1.988"), convertDoubleToString(1.987654321, 4));
+    ASSERT_EQUALS(std::string("6e-07"), convertDoubleToString(6E-7, 10));
+    ASSERT_EQUALS(std::string("6e-07"), convertDoubleToString(6E-7, 6));
+    ASSERT_EQUALS(std::string("0.1000000006"), convertDoubleToString(0.1 + 6E-10, 10));
+    ASSERT_EQUALS(std::string("0.1"), convertDoubleToString(0.1 + 6E-8, 6));
+}
 }  // namespace mongo::str