Merge pull request Tencent#1922 from jack-perisich/grisu2_precision

Fix small errors in dtoa output for certain doubles

Author: miloyip

include/rapidjson/internal/dtoa.h

@@ -58,7 +58,11 @@ inline int CountDecimalDigit32(uint32_t n) {
 }
 
 inline void DigitGen(const DiyFp& W, const DiyFp& Mp, uint64_t delta, char* buffer, int* len, int* K) {
-    static const uint32_t kPow10[] = { 1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000 };
+    static const uint64_t kPow10[] = { 1U, 10U, 100U, 1000U, 10000U, 100000U, 1000000U, 10000000U, 100000000U,
+                                       1000000000U, 10000000000U, 100000000000U, 1000000000000U,
+                                       10000000000000U, 100000000000000U, 1000000000000000U,
+                                       10000000000000000U, 100000000000000000U, 1000000000000000000U,
+                                       10000000000000000000U };
     const DiyFp one(uint64_t(1) << -Mp.e, Mp.e);
     const DiyFp wp_w = Mp - W;
     uint32_t p1 = static_cast<uint32_t>(Mp.f >> -one.e);
@@ -86,7 +90,7 @@ inline void DigitGen(const DiyFp& W, const DiyFp& Mp, uint64_t delta, char* buff
         uint64_t tmp = (static_cast<uint64_t>(p1) << -one.e) + p2;
         if (tmp <= delta) {
             *K += kappa;
-            GrisuRound(buffer, *len, delta, tmp, static_cast<uint64_t>(kPow10[kappa]) << -one.e, wp_w.f);
+            GrisuRound(buffer, *len, delta, tmp, kPow10[kappa] << -one.e, wp_w.f);
             return;
         }
     }
@@ -103,7 +107,7 @@ inline void DigitGen(const DiyFp& W, const DiyFp& Mp, uint64_t delta, char* buff
         if (p2 < delta) {
             *K += kappa;
             int index = -kappa;
-            GrisuRound(buffer, *len, delta, p2, one.f, wp_w.f * (index < 9 ? kPow10[index] : 0));
+            GrisuRound(buffer, *len, delta, p2, one.f, wp_w.f * (index < 20 ? kPow10[index] : 0));
             return;
         }
     }

test/unittest/dtoatest.cpp

@@ -38,6 +38,7 @@ TEST(dtoa, normal) {
     TEST_DTOA(0.123456789012, "0.123456789012");
     TEST_DTOA(1234567.8, "1234567.8");
     TEST_DTOA(-79.39773355813419, "-79.39773355813419");
+    TEST_DTOA(-36.973846435546875, "-36.973846435546875");
     TEST_DTOA(0.000001, "0.000001");
     TEST_DTOA(0.0000001, "1e-7");
     TEST_DTOA(1e30, "1e30");