ClickHouse/dbms/src/IO/WriteIntText.h

#pragma once

#include <limits>
#include <type_traits>

#include <common/likely.h>

#include <Core/Types.h>
#include <IO/WriteBuffer.h>

/// 20 digits or 19 digits and a sign
#define WRITE_HELPERS_MAX_INT_WIDTH 20U


namespace DB
{

namespace detail
{
    /** See:
      * https://github.com/localvoid/cxx-benchmark-itoa
      * http://www.slideshare.net/andreialexandrescu1/three-optimization-tips-for-c-15708507
      * http://vimeo.com/55639112
      */


    /// The usual way, if there is not enough space in the buffer for any number to fit there.
    template <typename T>
    void writeUIntTextFallback(T x, WriteBuffer & buf)
    {
        if (x == 0)
        {
            buf.nextIfAtEnd();
            *buf.position() = '0';
            ++buf.position();

            return;
        }

        char tmp[WRITE_HELPERS_MAX_INT_WIDTH];

        char * pos;
        for (pos = tmp + WRITE_HELPERS_MAX_INT_WIDTH - 1; x != 0; --pos)
        {
            *pos = '0' + x % 10;
            x /= 10;
        }

        ++pos;

        buf.write(pos, tmp + WRITE_HELPERS_MAX_INT_WIDTH - pos);
    }


    /** Count the number of decimal digits in the number.
      * Works well for nonuniform distribution of numbers, which usually happens.
      * If most of the numbers are long, then a "branchless" code with a `bsr` instruction and a smart conversion would work better.
      */
    template <typename T>
    UInt32 digits10(T x)
    {
        if (x < 10ULL)
            return 1;
        if (x < 100ULL)
            return 2;
        if (x < 1000ULL)
            return 3;

        if (x < 1000000000000ULL)
        {
            if (x < 100000000ULL)
            {
                if (x < 1000000ULL)
                {
                    if (x < 10000ULL)
                        return 4;
                    else
                        return 5 + (x >= 100000ULL);
                }

                return 7 + (x >= 10000000ULL);
            }

            if (x < 10000000000ULL)
                return 9 + (x >= 1000000000ULL);

            return 11 + (x >= 100000000000ULL);
        }

        return 12 + digits10(x / 1000000000000ULL);
    }


    /** Converts two digits per iteration.
      * Works well for the nonuniform distribution of numbers, which usually happens.
      * If most of the numbers are long, and if you do care about the cache, then the variant
      *  with a large table and four digits per iteration.
      */
    template <typename T>
    UInt32 writeUIntText(T x, char * dst)
    {
        static const char digits[201] =
            "00010203040506070809"
            "10111213141516171819"
            "20212223242526272829"
            "30313233343536373839"
            "40414243444546474849"
            "50515253545556575859"
            "60616263646566676869"
            "70717273747576777879"
            "80818283848586878889"
            "90919293949596979899";

        const UInt32 length = digits10(x);
        UInt32 next = length - 1;

        while (x >= 100)
        {
            const UInt32 i = (x % 100) * 2;
            x /= 100;
            dst[next] = digits[i + 1];
            dst[next - 1] = digits[i];
            next -= 2;
        }

        if (x < 10)
        {
            dst[next] = '0' + x;
        }
        else
        {
            const UInt32 i = x * 2;
            dst[next] = digits[i + 1];
            dst[next - 1] = digits[i];
        }

        return length;
    }


    /** If there is enough space in the buffer - calls an optimized version, otherwise - the normal version.
      */
    template <typename T>
    void writeUIntText(T x, WriteBuffer & buf)
    {
        if (likely(buf.position() + WRITE_HELPERS_MAX_INT_WIDTH < buf.buffer().end()))
            buf.position() += writeUIntText(x, buf.position());
        else
            writeUIntTextFallback(x, buf);
    }


    /** Wrapper for signed numbers.
      */
    template <typename T>
    void writeSIntText(T x, WriteBuffer & buf)
    {
        /// A special case for the smallest negative number
        if (unlikely(x == std::numeric_limits<T>::min()))
        {
            if (sizeof(x) == 1)
                buf.write("-128", 4);
            else if (sizeof(x) == 2)
                buf.write("-32768", 6);
            else if (sizeof(x) == 4)
                buf.write("-2147483648", 11);
            else
                buf.write("-9223372036854775808", 20);
            return;
        }

        if (x < 0)
        {
            x = -x;
            buf.nextIfAtEnd();
            *buf.position() = '-';
            ++buf.position();
        }

        writeUIntText(static_cast<std::make_unsigned_t<T>>(x), buf);
    }

}


template <typename T>
std::enable_if_t<std::is_signed_v<T>, void> writeIntText(T x, WriteBuffer & buf)
{
    detail::writeSIntText(x, buf);
}

template <typename T>
std::enable_if_t<std::is_unsigned_v<T>, void> writeIntText(T x, WriteBuffer & buf)
{
    detail::writeUIntText(x, buf);
}

}