[core] deduplicate dtoa() from RapidJSON and add tests

cmake/test-files.txt

@@ -144,6 +144,7 @@ test/util/peer.test.cpp
 test/util/position.test.cpp
 test/util/projection.test.cpp
 test/util/run_loop.test.cpp
+test/util/string.test.cpp
 test/util/text_conversions.test.cpp
 test/util/thread.test.cpp
 test/util/thread_local.test.cpp

src/mbgl/util/dtoa.cpp

@@ -13,99 +13,16 @@ namespace util {
 
 #if !defined(_WINDOWS)
 
-namespace {
-
-// From https://github.com/miloyip/rapidjson/blob/master/include/rapidjson/internal/dtoa.h
-
-char* Prettify(char* buffer, int length, int k) {
-    constexpr int maxDecimalPlaces = 324;
-    const int kk = length + k;  // 10^(kk-1) <= v < 10^kk
-
-    if (0 <= k && kk <= 21) {
-        // 1234e7 -> 12340000000
-        for (int i = length; i < kk; i++)
-            buffer[i] = '0';
-        return &buffer[kk];
-    }
-    else if (0 < kk && kk <= 21) {
-        // 1234e-2 -> 12.34
-        std::memmove(&buffer[kk + 1], &buffer[kk], static_cast<size_t>(length - kk));
-        buffer[kk] = '.';
-        if (0 > k + maxDecimalPlaces) {
-            // When maxDecimalPlaces = 2, 1.2345 -> 1.23, 1.102 -> 1.1
-            // Remove extra trailing zeros (at least one) after truncation.
-            for (int i = kk + maxDecimalPlaces; i > kk + 1; i--)
-                if (buffer[i] != '0')
-                    return &buffer[i + 1];
-            return &buffer[kk + 2]; // Reserve one zero
-        }
-        else
-            return &buffer[length + 1];
-    }
-    else if (-6 < kk && kk <= 0) {
-        // 1234e-6 -> 0.001234
-        const int offset = 2 - kk;
-        std::memmove(&buffer[offset], &buffer[0], static_cast<size_t>(length));
-        buffer[0] = '0';
-        buffer[1] = '.';
-        for (int i = 2; i < offset; i++)
-            buffer[i] = '0';
-        if (length - kk > maxDecimalPlaces) {
-            // When maxDecimalPlaces = 2, 0.123 -> 0.12, 0.102 -> 0.1
-            // Remove extra trailing zeros (at least one) after truncation.
-            for (int i = maxDecimalPlaces + 1; i > 2; i--)
-                if (buffer[i] != '0')
-                    return &buffer[i + 1];
-            return &buffer[3]; // Reserve one zero
-        }
-        else
-            return &buffer[length + offset];
-    }
-    else if (kk < -maxDecimalPlaces) {
-        // Truncate to zero
-        buffer[0] = '0';
-        return &buffer[1];
-    }
-    else if (length == 1) {
-        // 1e30
-        buffer[1] = 'e';
-        return rapidjson::internal::WriteExponent(kk - 1, &buffer[2]);
-    }
-    else {
-        // 1234e30 -> 1.234e33
-        std::memmove(&buffer[2], &buffer[1], static_cast<size_t>(length - 1));
-        buffer[1] = '.';
-        buffer[length + 1] = 'e';
-        return rapidjson::internal::WriteExponent(kk - 1, &buffer[0 + length + 2]);
-    }
-}
-
-} // namespace
-
-char* dtoa(double value, char* buffer) {
-    rapidjson::internal::Double d(value);
-    if (d.IsZero()) {
-        if (d.Sign())
-            *buffer++ = '-';     // -0.0, Issue #289
-        buffer[0] = '0';
-        return &buffer[1];
-    }
-    else {
-        if (value < 0) {
-            *buffer++ = '-';
-            value = -value;
-        }
-        int length, K;
-        rapidjson::internal::Grisu2(value, buffer, &length, &K);
-        return Prettify(buffer, length, K);
-    }
-}
-
 std::string dtoa(double value) {
     std::string data;
     data.resize(25);
-    auto end = dtoa(value, const_cast<char*>(data.data()));
-    data.resize(end - data.data());
+    auto end = rapidjson::internal::dtoa(value, const_cast<char*>(data.data()));
+    auto length = end - data.data();
+    if (length >= 3 && end[-1] == '0' && end[-2] == '.') {
+        // Remove trailing ".0" for integers
+        length -= 2;
+    }
+    data.resize(length);
     return data;
 }
 

test/util/string.test.cpp

@@ -0,0 +1,13 @@
+#include <mbgl/test/util.hpp>
+
+#include <mbgl/util/string.hpp>
+
+using namespace mbgl;
+
+TEST(ToString, FloatingPoint) {
+    EXPECT_EQ("0", util::toString(0.0));
+    EXPECT_EQ("1.33", util::toString(1.33));
+    EXPECT_EQ("-1", util::toString(-1.0));
+    EXPECT_EQ("12340000000", util::toString(12340000000.0));
+    EXPECT_EQ("12340000000", util::toString(12340000000.0));
+}