Bit Conversions

IEEE 754 specifies two different encodings for decimal floating point types: Binary Integer Significand Field (BID), and Densely Packed Decimal Significand Field (DPD). The former is designed for software implementations and the latter for hardware implementations. Internally this library is implemented in the BID format for the IEEE-754 compliant types. Should the user want to capture the bit format in BID or convert to DPD we offer a family of conversion functions: to_bid, from_bid, to_dpd, and from_dpd that allow conversion to or from the bit strings regardless of encoding.

#include <boost/decimal/bid_conversions.hpp>
#include <boost/decimal/dpd_conversions.hpp>
#include <boost/decimal/uint128_t.hpp>

namespace boost {
namespace decimal {

struct uint128_t
{
    std::uint64_t high;
    std::uint64_t low;
};

// ----- BID Conversions -----

constexpr std::uint32_t to_bid_d32(decimal32_t val) noexcept;

constexpr decimal32_t from_bid_d32(std::uint32_t bits) noexcept;

constexpr std::uint32_t to_bid_d32f(decimal_fast32_t val) noexcept;

constexpr decimal_fast32_t from_bid_d32f(std::uint32_t bits) noexcept;

constexpr std::uint64_t to_bid_d64(decimal64_t val) noexcept;

constexpr decimal64_t from_bid_d64(std::uint64_t bits) noexcept;

constexpr std::uint64_t to_bid_d64f(decimal_fast64_t val) noexcept;

constexpr decimal_fast64_t from_bid_d64f(std::uint64_t bits) noexcept;

constexpr uint128_t to_bid_d128(decimal128_t val) noexcept;

constexpr decimal128_t from_bid_d128(uint128_t bits) noexcept;

// Automatic detection if your platform has built-in unsigned __int128 or not to enable/disable the overload
#ifdef BOOST_DECIMAL_HAS_INT128

constexpr decimal128_t from_bid_d128(unsigned __int128 bits) noexcept;

#endif // BOOST_DECIMAL_HAS_INT128

constexpr uint128_t to_bid_d128f(decimal_fast128_t val) noexcept;

constexpr decimal128_t from_bid_d128f(uint128_t bits) noexcept;

#ifdef BOOST_DECIMAL_HAS_INT128

constexpr decimal128_t from_bid_d128f(unsigned __int128 bits) noexcept;

#endif // BOOST_DECIMAL_HAS_INT128

template <typename T>
constexpr auto to_bid(T val) noexcept;

template <typename T = decimal32_t>
constexpr T from_bid(std::uint32_t bits) noexcept;

template <typename T = decimal64_t>
constexpr T from_bid(std::uint64_t bits) noexcept;

template <typename T = decimal128_t>
constexpr T from_bid(uint128_t bits) noexcept;

// ----- DPD Conversions -----

constexpr std::uint32_t to_dpd_d32(decimal32_t val) noexcept;

constexpr std::uint32_t to_dpd_d32f(decimal_fast32_t val) noexcept;

constexpr std::uint64_t to_dpd_d64(decimal64_t val) noexcept;

constexpr std::uint64_t to_dpd_d64f(decimal_fast64_t val) noexcept;

constexpr uint128_t to_dpd_d128(decimal128_t val) noexcept;

constexpr uint128_t to_dpd_d128f(decimal_fast128_t val) noexcept;

template <typename T>
constexpr auto to_dpd(T val) noexcept;

template <typename T = decimal32_t>
constexpr T from_dpd(std::uint32_t bits) noexcept;

template <typename T = decimal64_t>
constexpr T from_dpd(std::uint64_t bits) noexcept;

template <typename T = decimal128_t>
constexpr T from_dpd(uint128_t bits) noexcept;

#ifdef BOOST_DECIMAL_HAS_INT128

template <typename T = decimal128_t>
constexpr T from_dpd(unsigned __int128 bits) noexcept;

#endif

} // namespace decimal
} // namespace boost

Bit Conversions Example

The following example is copied and pasted from the examples/ folder of the library and is called bit_conversions.cpp.

#include <boost/decimal/decimal32_t.hpp>
#include <boost/decimal/dpd_conversion.hpp>
#include <boost/decimal/bid_conversion.hpp>
#include <iostream>

int main()
{
    using boost::decimal::decimal32_t;
    using boost::decimal::from_bid;
    using boost::decimal::from_dpd;
    using boost::decimal::to_bid;
    using boost::decimal::to_dpd;

    // First we construct a decimal value and then convert it into both BID and DPD encoded bits
    const decimal32_t decimal_value {5};
    const std::uint32_t BID_bits {to_bid(decimal_value)};
    const std::uint32_t DPD_bits {to_dpd(decimal_value)};

    // Display the difference between the hex values of both bit encodings
    std::cout << std::hex
              << "BID format: " << BID_bits << '\n'
              << "DPD format: " << DPD_bits << std::endl;

    // Recover the original value by encoding two new decimals using the from_bid and from_dpd functions
    const decimal32_t bid_decimal {from_bid<decimal32_t>(BID_bits)};
    const decimal32_t dpd_decimal {from_dpd<decimal32_t>(DPD_bits)};

    if (bid_decimal == dpd_decimal)
    {
        // These should both have recovered the original value of 5
        return 0;
    }
    else
    {
        // Something has gone wrong recovering the decimal values
        return 1;
    }
}

Output:

BID format: 32800005
DPD format: 22500005

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