integer_to_string.cc

#include <iostream>
/* Fast integer to string conversion.
Source: https://github.com/miloyip/itoa-benchmark
Local modifications:
1. Return end of buffer instead of null terminating
2. Collapse to single file
3. Namespace
4. Remove test hook
5. Non-x86 support from the branch_lut code
6. Rename functions
7. Require __SSE2__ on i386

Copyright (C) 2014 Milo Yip

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.

Which is based on: http://0x80.pl/snippets/asm/sse-utoa.c

  SSE: conversion integers to decimal representation

  Author: Wojciech Muła
  e-mail: wojciech_mula@poczta.onet.pl
  www:    http://0x80.pl/

  License: BSD

  initial release 2011-10-21
  $Id$
*/

#include "util/integer_to_string.hh"
#include <cassert>
#include <stdint.h>

namespace util {

namespace {
const char gDigitsLut[200] = {
  '0','0','0','1','0','2','0','3','0','4','0','5','0','6','0','7','0','8','0','9',
  '1','0','1','1','1','2','1','3','1','4','1','5','1','6','1','7','1','8','1','9',
  '2','0','2','1','2','2','2','3','2','4','2','5','2','6','2','7','2','8','2','9',
  '3','0','3','1','3','2','3','3','3','4','3','5','3','6','3','7','3','8','3','9',
  '4','0','4','1','4','2','4','3','4','4','4','5','4','6','4','7','4','8','4','9',
  '5','0','5','1','5','2','5','3','5','4','5','5','5','6','5','7','5','8','5','9',
  '6','0','6','1','6','2','6','3','6','4','6','5','6','6','6','7','6','8','6','9',
  '7','0','7','1','7','2','7','3','7','4','7','5','7','6','7','7','7','8','7','9',
  '8','0','8','1','8','2','8','3','8','4','8','5','8','6','8','7','8','8','8','9',
  '9','0','9','1','9','2','9','3','9','4','9','5','9','6','9','7','9','8','9','9'
};
} // namespace

// SSE2 implementation according to http://0x80.pl/articles/sse-itoa.html
// Modifications: (1) fix incorrect digits (2) accept all ranges (3) write to user provided buffer.

#if defined(__amd64) || defined(_M_X64) || (defined(__SSE2__) && (defined(_M_IX86) || defined(i386)))

#include <emmintrin.h>

#ifdef _MSC_VER
#include "intrin.h"
#endif

#ifdef _MSC_VER
#define ALIGN_PRE __declspec(align(16))
#define ALIGN_SUF
#else
#define ALIGN_PRE
#define ALIGN_SUF  __attribute__ ((aligned(16)))
#endif

namespace {

static const uint32_t kDiv10000 = 0xd1b71759;
ALIGN_PRE static const uint32_t kDiv10000Vector[4] ALIGN_SUF = { kDiv10000, kDiv10000, kDiv10000, kDiv10000 };
ALIGN_PRE static const uint32_t k10000Vector[4] ALIGN_SUF = { 10000, 10000, 10000, 10000 };
ALIGN_PRE static const uint16_t kDivPowersVector[8] ALIGN_SUF = { 8389, 5243, 13108, 32768, 8389, 5243, 13108, 32768 }; // 10^3, 10^2, 10^1, 10^0
ALIGN_PRE static const uint16_t kShiftPowersVector[8] ALIGN_SUF = {
    1 << (16 - (23 + 2 - 16)),
    1 << (16 - (19 + 2 - 16)),
    1 << (16 - 1 - 2),
    1 << (15),
    1 << (16 - (23 + 2 - 16)),
    1 << (16 - (19 + 2 - 16)),
    1 << (16 - 1 - 2),
    1 << (15)
};
ALIGN_PRE static const uint16_t k10Vector[8] ALIGN_SUF = { 10, 10, 10, 10, 10, 10, 10, 10 };
ALIGN_PRE static const char kAsciiZero[16] ALIGN_SUF = { '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '0' };

inline __m128i Convert8DigitsSSE2(uint32_t value) {
    assert(value <= 99999999);

    // abcd, efgh = abcdefgh divmod 10000
    const __m128i abcdefgh = _mm_cvtsi32_si128(value);
    const __m128i abcd = _mm_srli_epi64(_mm_mul_epu32(abcdefgh, reinterpret_cast<const __m128i*>(kDiv10000Vector)[0]), 45);
    const __m128i efgh = _mm_sub_epi32(abcdefgh, _mm_mul_epu32(abcd, reinterpret_cast<const __m128i*>(k10000Vector)[0]));

    // v1 = [ abcd, efgh, 0, 0, 0, 0, 0, 0 ]
    const __m128i v1 = _mm_unpacklo_epi16(abcd, efgh);

    // v1a = v1 * 4 = [ abcd * 4, efgh * 4, 0, 0, 0, 0, 0, 0 ]
    const __m128i v1a = _mm_slli_epi64(v1, 2);

    // v2 = [ abcd * 4, abcd * 4, abcd * 4, abcd * 4, efgh * 4, efgh * 4, efgh * 4, efgh * 4 ]
    const __m128i v2a = _mm_unpacklo_epi16(v1a, v1a);
    const __m128i v2 = _mm_unpacklo_epi32(v2a, v2a);

    // v4 = v2 div 10^3, 10^2, 10^1, 10^0 = [ a, ab, abc, abcd, e, ef, efg, efgh ]
    const __m128i v3 = _mm_mulhi_epu16(v2, reinterpret_cast<const __m128i*>(kDivPowersVector)[0]);
    const __m128i v4 = _mm_mulhi_epu16(v3, reinterpret_cast<const __m128i*>(kShiftPowersVector)[0]);

    // v5 = v4 * 10 = [ a0, ab0, abc0, abcd0, e0, ef0, efg0, efgh0 ]
    const __m128i v5 = _mm_mullo_epi16(v4, reinterpret_cast<const __m128i*>(k10Vector)[0]);

    // v6 = v5 << 16 = [ 0, a0, ab0, abc0, 0, e0, ef0, efg0 ]
    const __m128i v6 = _mm_slli_epi64(v5, 16);

    // v7 = v4 - v6 = { a, b, c, d, e, f, g, h }
    const __m128i v7 = _mm_sub_epi16(v4, v6);

    return v7;
}

inline __m128i ShiftDigits_SSE2(__m128i a, unsigned digit) {
    assert(digit <= 8);
    switch (digit) {
        case 0: return a;
        case 1: return _mm_srli_si128(a, 1);
        case 2: return _mm_srli_si128(a, 2);
        case 3: return _mm_srli_si128(a, 3);
        case 4: return _mm_srli_si128(a, 4);
        case 5: return _mm_srli_si128(a, 5);
        case 6: return _mm_srli_si128(a, 6);
        case 7: return _mm_srli_si128(a, 7);
        case 8: return _mm_srli_si128(a, 8);
    }
    return a; // should not execute here.
}

} // namespace

// Original name: u32toa_sse2
char *ToString(uint32_t value, char* buffer) {
    if (value < 10000) {
        const uint32_t d1 = (value / 100) << 1;
        const uint32_t d2 = (value % 100) << 1;

        if (value >= 1000)
            *buffer++ = gDigitsLut[d1];
        if (value >= 100)
            *buffer++ = gDigitsLut[d1 + 1];
        if (value >= 10)
            *buffer++ = gDigitsLut[d2];
        *buffer++ = gDigitsLut[d2 + 1];
        //*buffer++ = '\0';
        return buffer;
    }
    else if (value < 100000000) {
        // Experiment shows that this case SSE2 is slower
#if 0
        const __m128i a = Convert8DigitsSSE2(value);

        // Convert to bytes, add '0'
        const __m128i va = _mm_add_epi8(_mm_packus_epi16(a, _mm_setzero_si128()), reinterpret_cast<const __m128i*>(kAsciiZero)[0]);

        // Count number of digit
        const unsigned mask = _mm_movemask_epi8(_mm_cmpeq_epi8(va, reinterpret_cast<const __m128i*>(kAsciiZero)[0]));
        unsigned long digit;
#ifdef _MSC_VER
        _BitScanForward(&digit, ~mask | 0x8000);
#else
        digit = __builtin_ctz(~mask | 0x8000);
#endif

        // Shift digits to the beginning
        __m128i result = ShiftDigits_SSE2(va, digit);
        //__m128i result = _mm_srl_epi64(va, _mm_cvtsi32_si128(digit * 8));
        _mm_storel_epi64(reinterpret_cast<__m128i*>(buffer), result);
        buffer[8 - digit] = '\0';
#else
        // value = bbbbcccc
        const uint32_t b = value / 10000;
        const uint32_t c = value % 10000;

        const uint32_t d1 = (b / 100) << 1;
        const uint32_t d2 = (b % 100) << 1;

        const uint32_t d3 = (c / 100) << 1;
        const uint32_t d4 = (c % 100) << 1;

        if (value >= 10000000)
            *buffer++ = gDigitsLut[d1];
        if (value >= 1000000)
            *buffer++ = gDigitsLut[d1 + 1];
        if (value >= 100000)
            *buffer++ = gDigitsLut[d2];
        *buffer++ = gDigitsLut[d2 + 1];

        *buffer++ = gDigitsLut[d3];
        *buffer++ = gDigitsLut[d3 + 1];
        *buffer++ = gDigitsLut[d4];
        *buffer++ = gDigitsLut[d4 + 1];
//        *buffer++ = '\0';
        return buffer;
#endif
    }
    else {
        // value = aabbbbbbbb in decimal

        const uint32_t a = value / 100000000; // 1 to 42
        value %= 100000000;

        if (a >= 10) {
            const unsigned i = a << 1;
            *buffer++ = gDigitsLut[i];
            *buffer++ = gDigitsLut[i + 1];
        }
        else
            *buffer++ = '0' + static_cast<char>(a);

        const __m128i b = Convert8DigitsSSE2(value);
        const __m128i ba = _mm_add_epi8(_mm_packus_epi16(_mm_setzero_si128(), b), reinterpret_cast<const __m128i*>(kAsciiZero)[0]);
        const __m128i result = _mm_srli_si128(ba, 8);
        _mm_storel_epi64(reinterpret_cast<__m128i*>(buffer), result);
//        buffer[8] = '\0';
        return buffer + 8;
    }
}

// Original name: u64toa_sse2
char *ToString(uint64_t value, char* buffer) {
    if (value < 100000000) {
        uint32_t v = static_cast<uint32_t>(value);
        if (v < 10000) {
            const uint32_t d1 = (v / 100) << 1;
            const uint32_t d2 = (v % 100) << 1;

            if (v >= 1000)
                *buffer++ = gDigitsLut[d1];
            if (v >= 100)
                *buffer++ = gDigitsLut[d1 + 1];
            if (v >= 10)
                *buffer++ = gDigitsLut[d2];
            *buffer++ = gDigitsLut[d2 + 1];
            //*buffer++ = '\0';
            return buffer;
        }
        else {
            // Experiment shows that this case SSE2 is slower
#if 0
            const __m128i a = Convert8DigitsSSE2(v);

            // Convert to bytes, add '0'
            const __m128i va = _mm_add_epi8(_mm_packus_epi16(a, _mm_setzero_si128()), reinterpret_cast<const __m128i*>(kAsciiZero)[0]);

            // Count number of digit
            const unsigned mask = _mm_movemask_epi8(_mm_cmpeq_epi8(va, reinterpret_cast<const __m128i*>(kAsciiZero)[0]));
            unsigned long digit;
#ifdef _MSC_VER
            _BitScanForward(&digit, ~mask | 0x8000);
#else
            digit = __builtin_ctz(~mask | 0x8000);
#endif

            // Shift digits to the beginning
            __m128i result = ShiftDigits_SSE2(va, digit);
            _mm_storel_epi64(reinterpret_cast<__m128i*>(buffer), result);
            buffer[8 - digit] = '\0';
#else
            // value = bbbbcccc
            const uint32_t b = v / 10000;
            const uint32_t c = v % 10000;

            const uint32_t d1 = (b / 100) << 1;
            const uint32_t d2 = (b % 100) << 1;

            const uint32_t d3 = (c / 100) << 1;
            const uint32_t d4 = (c % 100) << 1;

            if (value >= 10000000)
                *buffer++ = gDigitsLut[d1];
            if (value >= 1000000)
                *buffer++ = gDigitsLut[d1 + 1];
            if (value >= 100000)
                *buffer++ = gDigitsLut[d2];
            *buffer++ = gDigitsLut[d2 + 1];

            *buffer++ = gDigitsLut[d3];
            *buffer++ = gDigitsLut[d3 + 1];
            *buffer++ = gDigitsLut[d4];
            *buffer++ = gDigitsLut[d4 + 1];
            //*buffer++ = '\0';
            return buffer;
#endif
        }
    }
    else if (value < 10000000000000000) {
        const uint32_t v0 = static_cast<uint32_t>(value / 100000000);
        const uint32_t v1 = static_cast<uint32_t>(value % 100000000);

        const __m128i a0 = Convert8DigitsSSE2(v0);
        const __m128i a1 = Convert8DigitsSSE2(v1);

        // Convert to bytes, add '0'
        const __m128i va = _mm_add_epi8(_mm_packus_epi16(a0, a1), reinterpret_cast<const __m128i*>(kAsciiZero)[0]);

        // Count number of digit
        const unsigned mask = _mm_movemask_epi8(_mm_cmpeq_epi8(va, reinterpret_cast<const __m128i*>(kAsciiZero)[0]));
#ifdef _MSC_VER
        unsigned long digit;
        _BitScanForward(&digit, ~mask | 0x8000);
#else
        unsigned digit = __builtin_ctz(~mask | 0x8000);
#endif

        // Shift digits to the beginning
        __m128i result = ShiftDigits_SSE2(va, digit);
        _mm_storeu_si128(reinterpret_cast<__m128i*>(buffer), result);
//        buffer[16 - digit] = '\0';
        return &buffer[16 - digit];
    }
    else {
        const uint32_t a = static_cast<uint32_t>(value / 10000000000000000); // 1 to 1844
        value %= 10000000000000000;

        if (a < 10)
            *buffer++ = '0' + static_cast<char>(a);
        else if (a < 100) {
            const uint32_t i = a << 1;
            *buffer++ = gDigitsLut[i];
            *buffer++ = gDigitsLut[i + 1];
        }
        else if (a < 1000) {
            *buffer++ = '0' + static_cast<char>(a / 100);

            const uint32_t i = (a % 100) << 1;
            *buffer++ = gDigitsLut[i];
            *buffer++ = gDigitsLut[i + 1];
        }
        else {
            const uint32_t i = (a / 100) << 1;
            const uint32_t j = (a % 100) << 1;
            *buffer++ = gDigitsLut[i];
            *buffer++ = gDigitsLut[i + 1];
            *buffer++ = gDigitsLut[j];
            *buffer++ = gDigitsLut[j + 1];
        }

        const uint32_t v0 = static_cast<uint32_t>(value / 100000000);
        const uint32_t v1 = static_cast<uint32_t>(value % 100000000);

        const __m128i a0 = Convert8DigitsSSE2(v0);
        const __m128i a1 = Convert8DigitsSSE2(v1);

        // Convert to bytes, add '0'
        const __m128i va = _mm_add_epi8(_mm_packus_epi16(a0, a1), reinterpret_cast<const __m128i*>(kAsciiZero)[0]);
        _mm_storeu_si128(reinterpret_cast<__m128i*>(buffer), va);
//        buffer[16] = '\0';
        return &buffer[16];
    }
}

#else // Generic Non-x86 case

// Orignal name: u32toa_branchlut
char *ToString(uint32_t value, char* buffer) {
    if (value < 10000) {
        const uint32_t d1 = (value / 100) << 1;
        const uint32_t d2 = (value % 100) << 1;

        if (value >= 1000)
            *buffer++ = gDigitsLut[d1];
        if (value >= 100)
            *buffer++ = gDigitsLut[d1 + 1];
        if (value >= 10)
            *buffer++ = gDigitsLut[d2];
        *buffer++ = gDigitsLut[d2 + 1];
    }
    else if (value < 100000000) {
        // value = bbbbcccc
        const uint32_t b = value / 10000;
        const uint32_t c = value % 10000;

        const uint32_t d1 = (b / 100) << 1;
        const uint32_t d2 = (b % 100) << 1;

        const uint32_t d3 = (c / 100) << 1;
        const uint32_t d4 = (c % 100) << 1;

        if (value >= 10000000)
            *buffer++ = gDigitsLut[d1];
        if (value >= 1000000)
            *buffer++ = gDigitsLut[d1 + 1];
        if (value >= 100000)
            *buffer++ = gDigitsLut[d2];
        *buffer++ = gDigitsLut[d2 + 1];

        *buffer++ = gDigitsLut[d3];
        *buffer++ = gDigitsLut[d3 + 1];
        *buffer++ = gDigitsLut[d4];
        *buffer++ = gDigitsLut[d4 + 1];
    }
    else {
        // value = aabbbbcccc in decimal

        const uint32_t a = value / 100000000; // 1 to 42
        value %= 100000000;

        if (a >= 10) {
            const unsigned i = a << 1;
            *buffer++ = gDigitsLut[i];
            *buffer++ = gDigitsLut[i + 1];
        }
        else
            *buffer++ = '0' + static_cast<char>(a);

        const uint32_t b = value / 10000; // 0 to 9999
        const uint32_t c = value % 10000; // 0 to 9999

        const uint32_t d1 = (b / 100) << 1;
        const uint32_t d2 = (b % 100) << 1;

        const uint32_t d3 = (c / 100) << 1;
        const uint32_t d4 = (c % 100) << 1;

        *buffer++ = gDigitsLut[d1];
        *buffer++ = gDigitsLut[d1 + 1];
        *buffer++ = gDigitsLut[d2];
        *buffer++ = gDigitsLut[d2 + 1];
        *buffer++ = gDigitsLut[d3];
        *buffer++ = gDigitsLut[d3 + 1];
        *buffer++ = gDigitsLut[d4];
        *buffer++ = gDigitsLut[d4 + 1];
    }
    return buffer; //*buffer++ = '\0';
}

// Original name: u64toa_branchlut
char *ToString(uint64_t value, char* buffer) {
    if (value < 100000000) {
        uint32_t v = static_cast<uint32_t>(value);
        if (v < 10000) {
            const uint32_t d1 = (v / 100) << 1;
            const uint32_t d2 = (v % 100) << 1;

            if (v >= 1000)
                *buffer++ = gDigitsLut[d1];
            if (v >= 100)
                *buffer++ = gDigitsLut[d1 + 1];
            if (v >= 10)
                *buffer++ = gDigitsLut[d2];
            *buffer++ = gDigitsLut[d2 + 1];
        }
        else {
            // value = bbbbcccc
            const uint32_t b = v / 10000;
            const uint32_t c = v % 10000;

            const uint32_t d1 = (b / 100) << 1;
            const uint32_t d2 = (b % 100) << 1;

            const uint32_t d3 = (c / 100) << 1;
            const uint32_t d4 = (c % 100) << 1;

            if (value >= 10000000)
                *buffer++ = gDigitsLut[d1];
            if (value >= 1000000)
                *buffer++ = gDigitsLut[d1 + 1];
            if (value >= 100000)
                *buffer++ = gDigitsLut[d2];
            *buffer++ = gDigitsLut[d2 + 1];

            *buffer++ = gDigitsLut[d3];
            *buffer++ = gDigitsLut[d3 + 1];
            *buffer++ = gDigitsLut[d4];
            *buffer++ = gDigitsLut[d4 + 1];
        }
    }
    else if (value < 10000000000000000) {
        const uint32_t v0 = static_cast<uint32_t>(value / 100000000);
        const uint32_t v1 = static_cast<uint32_t>(value % 100000000);

        const uint32_t b0 = v0 / 10000;
        const uint32_t c0 = v0 % 10000;

        const uint32_t d1 = (b0 / 100) << 1;
        const uint32_t d2 = (b0 % 100) << 1;

        const uint32_t d3 = (c0 / 100) << 1;
        const uint32_t d4 = (c0 % 100) << 1;

        const uint32_t b1 = v1 / 10000;
        const uint32_t c1 = v1 % 10000;

        const uint32_t d5 = (b1 / 100) << 1;
        const uint32_t d6 = (b1 % 100) << 1;

        const uint32_t d7 = (c1 / 100) << 1;
        const uint32_t d8 = (c1 % 100) << 1;

        if (value >= 1000000000000000)
            *buffer++ = gDigitsLut[d1];
        if (value >= 100000000000000)
            *buffer++ = gDigitsLut[d1 + 1];
        if (value >= 10000000000000)
            *buffer++ = gDigitsLut[d2];
        if (value >= 1000000000000)
            *buffer++ = gDigitsLut[d2 + 1];
        if (value >= 100000000000)
            *buffer++ = gDigitsLut[d3];
        if (value >= 10000000000)
            *buffer++ = gDigitsLut[d3 + 1];
        if (value >= 1000000000)
            *buffer++ = gDigitsLut[d4];
        if (value >= 100000000)
            *buffer++ = gDigitsLut[d4 + 1];

        *buffer++ = gDigitsLut[d5];
        *buffer++ = gDigitsLut[d5 + 1];
        *buffer++ = gDigitsLut[d6];
        *buffer++ = gDigitsLut[d6 + 1];
        *buffer++ = gDigitsLut[d7];
        *buffer++ = gDigitsLut[d7 + 1];
        *buffer++ = gDigitsLut[d8];
        *buffer++ = gDigitsLut[d8 + 1];
    }
    else {
        const uint32_t a = static_cast<uint32_t>(value / 10000000000000000); // 1 to 1844
        value %= 10000000000000000;

        if (a < 10)
            *buffer++ = '0' + static_cast<char>(a);
        else if (a < 100) {
            const uint32_t i = a << 1;
            *buffer++ = gDigitsLut[i];
            *buffer++ = gDigitsLut[i + 1];
        }
        else if (a < 1000) {
            *buffer++ = '0' + static_cast<char>(a / 100);

            const uint32_t i = (a % 100) << 1;
            *buffer++ = gDigitsLut[i];
            *buffer++ = gDigitsLut[i + 1];
        }
        else {
            const uint32_t i = (a / 100) << 1;
            const uint32_t j = (a % 100) << 1;
            *buffer++ = gDigitsLut[i];
            *buffer++ = gDigitsLut[i + 1];
            *buffer++ = gDigitsLut[j];
            *buffer++ = gDigitsLut[j + 1];
        }

        const uint32_t v0 = static_cast<uint32_t>(value / 100000000);
        const uint32_t v1 = static_cast<uint32_t>(value % 100000000);

        const uint32_t b0 = v0 / 10000;
        const uint32_t c0 = v0 % 10000;

        const uint32_t d1 = (b0 / 100) << 1;
        const uint32_t d2 = (b0 % 100) << 1;

        const uint32_t d3 = (c0 / 100) << 1;
        const uint32_t d4 = (c0 % 100) << 1;

        const uint32_t b1 = v1 / 10000;
        const uint32_t c1 = v1 % 10000;

        const uint32_t d5 = (b1 / 100) << 1;
        const uint32_t d6 = (b1 % 100) << 1;

        const uint32_t d7 = (c1 / 100) << 1;
        const uint32_t d8 = (c1 % 100) << 1;

        *buffer++ = gDigitsLut[d1];
        *buffer++ = gDigitsLut[d1 + 1];
        *buffer++ = gDigitsLut[d2];
        *buffer++ = gDigitsLut[d2 + 1];
        *buffer++ = gDigitsLut[d3];
        *buffer++ = gDigitsLut[d3 + 1];
        *buffer++ = gDigitsLut[d4];
        *buffer++ = gDigitsLut[d4 + 1];
        *buffer++ = gDigitsLut[d5];
        *buffer++ = gDigitsLut[d5 + 1];
        *buffer++ = gDigitsLut[d6];
        *buffer++ = gDigitsLut[d6 + 1];
        *buffer++ = gDigitsLut[d7];
        *buffer++ = gDigitsLut[d7 + 1];
        *buffer++ = gDigitsLut[d8];
        *buffer++ = gDigitsLut[d8 + 1];
    }
    return buffer;
}

#endif // End of architecture if statement.

// Signed wrappers.  The negation is done on the unsigned version because
// doing so has defined behavior for INT_MIN.
char *ToString(int32_t value, char *to) {
  uint32_t un = static_cast<uint32_t>(value);
  if (value < 0) {
    *to++ = '-';
    un = -un;
  }
  return ToString(un, to);
}

char *ToString(int64_t value, char *to) {
  uint64_t un = static_cast<uint64_t>(value);
  if (value < 0) {
    *to++ = '-';
    un = -un;
  }
  return ToString(un, to);
}

// No optimization for this case yet.
char *ToString(int16_t value, char *to) {
  return ToString((int32_t)value, to);
}
char *ToString(uint16_t value, char *to) {
  return ToString((uint32_t)value, to);
}

// void * to string.  This hasn't been optimized at all really.
namespace {
const char kHexDigits[] = "0123456789abcdef";
} // namespace

char *ToString(const void *v, char *to) {
  *to++ = '0';
  *to++ = 'x';

  // Fun fact: gcc/clang boost::lexical_cast on Linux do just "0" while clang on OS X does "0x0"
  // I happen to prefer 0x0.
  if (!v) {
    *to++ = '0';
    return to;
  }

  uintptr_t value = reinterpret_cast<uintptr_t>(v);
  uint8_t shift = sizeof(void*) * 8 - 4;
  for (; !(value >> shift); shift -= 4) {}
  for (; ; shift -= 4) {
    *to++ = kHexDigits[(value >> shift) & 0xf];
    if (!shift) break;
  }
  return to;
}

} // namespace util