[core] use rapidjson's dtoa implementation to stringify floating point numbers

include/mbgl/util/string.hpp

@@ -5,6 +5,8 @@
 #include <cassert>
 #include <exception>
 
+#include <mbgl/util/dtoa.hpp>
+
 namespace mbgl {
 namespace util {
 
@@ -21,6 +23,18 @@ inline std::string toString(uint8_t num) {
     return std::to_string(unsigned(num));
 }
 
+inline std::string toString(float num) {
+    return dtoa(num);
+}
+
+inline std::string toString(double num) {
+    return dtoa(num);
+}
+
+inline std::string toString(long double num) {
+    return dtoa(num);
+}
+
 inline std::string toString(std::exception_ptr error) {
     assert(error);
 

src/mbgl/util/dtoa.cpp

@@ -0,0 +1,97 @@
+#include "dtoa.hpp"
+
+#include <rapidjson/internal/dtoa.h>
+
+namespace mbgl {
+namespace util {
+
+namespace {
+
+// From https://github.com/miloyip/rapidjson/blob/master/include/rapidjson/internal/dtoa.h
+
+inline 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);
+    }
+}
+
+} // namespace util
+} // namespace mbgl

src/mbgl/util/dtoa.hpp

@@ -0,0 +1,22 @@
+#ifndef MBGL_UTIL_DTOA
+#define MBGL_UTIL_DTOA
+
+#include <string>
+
+namespace mbgl {
+namespace util {
+
+char* dtoa(double value, char* buffer);
+
+inline 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());
+    return data;
+}
+
+} // end namespace util
+} // end namespace mbgl
+
+#endif

test/test.gypi

@@ -25,6 +25,7 @@
         'util/mapbox.cpp',
         'util/merge_lines.cpp',
         'util/run_loop.cpp',
+        'util/number_conversions.cpp',
         'util/text_conversions.cpp',
         'util/thread.cpp',
         'util/thread_local.cpp',

test/util/number_conversions.cpp

@@ -0,0 +1,55 @@
+#include <iostream>
+#include <mbgl/test/util.hpp>
+
+#include <mbgl/util/string.hpp>
+
+using namespace mbgl;
+
+TEST(NumberConversions, number_to_string) {
+    EXPECT_EQ("0", util::toString(0));
+    EXPECT_EQ("10", util::toString(10));
+    EXPECT_EQ("-10", util::toString(-10));
+    EXPECT_EQ("10", util::toString(10.0));
+    EXPECT_EQ("-10", util::toString(-10.0));
+
+    EXPECT_EQ("1", util::toString(1));
+    EXPECT_EQ("-1", util::toString(-1));
+    EXPECT_EQ("32768", util::toString(32768));
+    EXPECT_EQ("-32768", util::toString(-32768));
+
+    EXPECT_EQ("1", util::toString(1.000000));
+    EXPECT_EQ("-1", util::toString(-1.000000));
+    EXPECT_EQ("32768", util::toString(32768.000000));
+    EXPECT_EQ("-32768", util::toString(-32768.000000));
+
+    EXPECT_EQ("1.01", util::toString(1.01));
+    EXPECT_EQ("-1.01", util::toString(-1.01));
+    EXPECT_EQ("32768.01", util::toString(32768.01));
+    EXPECT_EQ("-32768.01", util::toString(-32768.01));
+
+    EXPECT_EQ("1.01", util::toString(1.0100));
+    EXPECT_EQ("-1.01", util::toString(-1.0100));
+    EXPECT_EQ("32768.01", util::toString(32768.0100));
+    EXPECT_EQ("-32768.01", util::toString(-32768.0100));
+
+    EXPECT_EQ("1.123456", util::toString(1.123456));
+    EXPECT_EQ("-1.123456", util::toString(-1.123456));
+    EXPECT_EQ("32768.123456", util::toString(32768.123456));
+    EXPECT_EQ("-32768.123456", util::toString(-32768.123456));
+
+    EXPECT_EQ("1.123456789", util::toString(1.123456789));
+    EXPECT_EQ("-1.123456789", util::toString(-1.123456789));
+    EXPECT_EQ("32768.123456789", util::toString(32768.123456789));
+    EXPECT_EQ("-32768.123456789", util::toString(-32768.123456789));
+
+    EXPECT_EQ("3.141592653589793", util::toString(3.1415926535897932385128089594061862044327426701784));
+    EXPECT_EQ("-3.141592653589793", util::toString(-3.1415926535897932385128089594061862044327426701784));
+
+    EXPECT_EQ("123456789", util::toString(123456789));
+
+    EXPECT_EQ("0.0000532", util::toString((long double)5.32e-5));
+    EXPECT_EQ("5.32e-40", util::toString((long double)5.32e-40));
+
+    EXPECT_EQ("10", util::toString(unsigned(10)));
+    EXPECT_EQ("10", util::toString(unsigned(10.0)));
+}