Return 0 if binary exponent is too small

include/rapidjson/internal/diyfp.h

 // Tencent is pleased to support the open source community by making RapidJSON available.
-// 
+//
 // Copyright (C) 2015 THL A29 Limited, a Tencent company, and Milo Yip. All rights reserved.
 //
 // Licensed under the MIT License (the "License"); you may not use this file except
@@ -7,9 +7,9 @@
 //
 // http://opensource.org/licenses/MIT
 //
-// Unless required by applicable law or agreed to in writing, software distributed 
-// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR 
-// CONDITIONS OF ANY KIND, either express or implied. See the License for the 
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
 // specific language governing permissions and limitations under the License.
 
 // This is a C++ header-only implementation of Grisu2 algorithm from the publication:
@@ -56,7 +56,7 @@ struct DiyFp {
         if (biased_e != 0) {
             f = significand + kDpHiddenBit;
             e = biased_e - kDpExponentBias;
-        } 
+        }
         else {
             f = significand;
             e = kDpMinExponent + 1;
@@ -142,9 +142,12 @@ struct DiyFp {
             uint64_t u64;
         }u;
         RAPIDJSON_ASSERT(f <= kDpHiddenBit + kDpSignificandMask);
-        RAPIDJSON_ASSERT(e >= kDpDenormalExponent);
+        if (e < kDpDenormalExponent) {
+            // Underflow.
+            return 0.0;
+        }
         RAPIDJSON_ASSERT(e < kDpMaxExponent);
-        const uint64_t be = (e == kDpDenormalExponent && (f & kDpHiddenBit) == 0) ? 0 : 
+        const uint64_t be = (e == kDpDenormalExponent && (f & kDpHiddenBit) == 0) ? 0 :
             static_cast<uint64_t>(e + kDpExponentBias);
         u.u64 = (f & kDpSignificandMask) | (be << kDpSignificandSize);
         return u.d;
@@ -226,7 +229,7 @@ inline DiyFp GetCachedPowerByIndex(size_t index) {
     RAPIDJSON_ASSERT(index < 87);
     return DiyFp(kCachedPowers_F[index], kCachedPowers_E[index]);
 }
-    
+
 inline DiyFp GetCachedPower(int e, int* K) {
 
     //int k = static_cast<int>(ceil((-61 - e) * 0.30102999566398114)) + 374;

test/unittest/readertest.cpp

@@ -392,6 +392,15 @@ static void TestParseDouble() {
         "83723677529752585477247372368372368547354737253685475529752",
         6223372036854775808.0);
 
+
+    TEST_DOUBLE(fullPrecision, "1e-325", 0.0);
+    TEST_DOUBLE(fullPrecision, "1e-324", 0.0);
+    TEST_DOUBLE(fullPrecision, "2e-324", 0.0);
+    TEST_DOUBLE(fullPrecision, "2.4703282292062327e-324", 0.0);
+    TEST_DOUBLE(fullPrecision, "2.4703282292062328e-324", 2.4703282292062328e-324);
+    TEST_DOUBLE(fullPrecision, "2.48e-324", 2.48e-324);
+    TEST_DOUBLE(fullPrecision, "2.5e-324", 2.5e-324);
+
 #undef TEST_DOUBLE
 }
 
@@ -1346,20 +1355,20 @@ TEST(Reader, IterativePullParsing_General) {
             handler.LOG_DOUBLE,
             handler.LOG_ENDARRAY | 7
         };
-        
+
         StringStream is("[1, {\"k\": [1, 2]}, null, false, true, \"string\", 1.2]");
         Reader reader;
-        
+
         reader.IterativeParseInit();
         while (!reader.IterativeParseComplete()) {
             size_t oldLogCount = handler.LogCount;
             EXPECT_TRUE(oldLogCount < sizeof(e) / sizeof(int)) << "overrun";
-            
+
             EXPECT_TRUE(reader.IterativeParseNext<kParseDefaultFlags>(is, handler)) << "parse fail";
             EXPECT_EQ(handler.LogCount, oldLogCount + 1) << "handler should be invoked exactly once each time";
             EXPECT_EQ(e[oldLogCount], handler.Logs[oldLogCount]) << "wrong event returned";
         }
-        
+
         EXPECT_FALSE(reader.HasParseError());
         EXPECT_EQ(sizeof(e) / sizeof(int), handler.LogCount) << "handler invoked wrong number of times";