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Avoid overflow by only using 9 digits per uint32_t

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Matt Borland
2023-01-25 13:00:54 -08:00
parent 6a9eeacafe
commit 4f46f6a31d
1 changed files with 44 additions and 10 deletions
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54
include/boost/charconv/to_chars.hpp
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@@ -144,23 +144,57 @@ BOOST_CXX14_CONSTEXPR to_chars_result to_chars_integer_impl(char* first, char* l
else if (std::numeric_limits<Integer>::digits <= std::numeric_limits<std::uint64_t>::digits ||
static_cast<std::uint64_t>(value) <= (std::numeric_limits<std::uint64_t>::max)())
{
const auto converted_value = static_cast<std::uint64_t>(value);
auto converted_value = static_cast<std::uint64_t>(value);
const auto converted_value_digits = num_digits(converted_value);
const auto x = static_cast<std::uint32_t>(converted_value / UINT64_C(10000000000));
const auto y = static_cast<std::uint32_t>(converted_value % UINT64_C(10000000000));
const int first_value_chars = num_digits(x);
decompose32(x, buffer);
if (is_negative)
{
*first++ = '-';
}
std::memcpy(first, buffer + (sizeof(buffer) - first_value_chars), first_value_chars);
// Only store 9 digits in each to avoid overflow
if (num_digits(converted_value) <= 18)
{
const auto x = static_cast<std::uint32_t>(converted_value / UINT64_C(1000000000));
const auto y = static_cast<std::uint32_t>(converted_value % UINT64_C(1000000000));
const int first_value_chars = num_digits(x);
decompose32(y, buffer);
std::memcpy(first + first_value_chars, buffer, sizeof(buffer));
decompose32(x, buffer);
std::memcpy(first, buffer + (sizeof(buffer) - first_value_chars), first_value_chars);
decompose32(y, buffer);
std::memcpy(first + first_value_chars, buffer + 1, sizeof(buffer) - 1);
}
else
{
const auto x = static_cast<std::uint32_t>(converted_value / UINT64_C(100000000000));
converted_value -= x * UINT64_C(100000000000);
const auto y = static_cast<std::uint32_t>(converted_value / UINT64_C(100));
const auto z = static_cast<std::uint32_t>(converted_value % UINT64_C(100));
if (converted_value_digits == 19)
{
decompose32(x, buffer);
std::memcpy(first, buffer + 2, sizeof(buffer) - 2);
decompose32(y, buffer);
std::memcpy(first + 8, buffer + 1, sizeof(buffer) - 1);
decompose32(z, buffer);
std::memcpy(first + 17, buffer + 8, 2);
}
else // 20
{
decompose32(x, buffer);
std::memcpy(first, buffer + 1, sizeof(buffer) - 1);
decompose32(y, buffer);
std::memcpy(first + 9, buffer + 1, sizeof(buffer) - 1);
decompose32(z, buffer);
std::memcpy(first + 18, buffer + 8, 2);
}
}
}
#if 0
// unsigned __128 requires 4 shifts