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

1 files changed, 97 insertions, 0 deletions

diff --git a/src/mbgl/util/dtoa.cpp b/src/mbgl/util/dtoa.cpp
new file mode 100644
index 0000000000..7ccee345f6
--- /dev/null
+++ b/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