C++ 中文周刊 2025-01-12 第175期
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标准委员会动态/ide/编译器信息放在这里
编译器信息最新动态推荐关注hellogcc公众号 本周更新 2025-01-08 第288期
文章
Counting the digits of 64-bit integers
其实比较常规，就是查表
int int_log2(uint64_t x) { return 63 - __builtin_clzll(x | 1); } // c++20可以用bit_width
int digit_count(uint64_t x) {
static uint64_t table[] = {9,
99,
999,
9999,
99999,
999999,
9999999,
99999999,
999999999,
9999999999,
99999999999,
999999999999,
9999999999999,
99999999999999,
999999999999999ULL,
9999999999999999ULL,
99999999999999999ULL,
999999999999999999ULL,
9999999999999999999ULL};
int y = (19 * int_log2(x) >> 6);
y += x > table[y];
return y + 1;
}
或者更极端一点
int alternative_digit_count(uint64_t x) {
static uint64_t table[64][2] = {
{ 0x01, 0xfffffffffffffff6ULL },
{ 0x01, 0xfffffffffffffff6ULL },
{ 0x01, 0xfffffffffffffff6ULL },
{ 0x01, 0xfffffffffffffff6ULL },
{ 0x02, 0xffffffffffffff9cULL },
{ 0x02, 0xffffffffffffff9cULL },
{ 0x02, 0xffffffffffffff9cULL },
{ 0x03, 0xfffffffffffffc18ULL },
{ 0x03, 0xfffffffffffffc18ULL },
{ 0x03, 0xfffffffffffffc18ULL },
{ 0x04, 0xffffffffffffd8f0ULL },
{ 0x04, 0xffffffffffffd8f0ULL },
{ 0x04, 0xffffffffffffd8f0ULL },
{ 0x04, 0xffffffffffffd8f0ULL },
{ 0x05, 0xfffffffffffe7960ULL },
{ 0x05, 0xfffffffffffe7960ULL },
{ 0x05, 0xfffffffffffe7960ULL },
{ 0x06, 0xfffffffffff0bdc0ULL },
{ 0x06, 0xfffffffffff0bdc0ULL },
{ 0x06, 0xfffffffffff0bdc0ULL },
{ 0x07, 0xffffffffff676980ULL },
{ 0x07, 0xffffffffff676980ULL },
{ 0x07, 0xffffffffff676980ULL },
{ 0x07, 0xffffffffff676980ULL },
{ 0x08, 0xfffffffffa0a1f00ULL },
{ 0x08, 0xfffffffffa0a1f00ULL },
{ 0x08, 0xfffffffffa0a1f00ULL },
{ 0x09, 0xffffffffc4653600ULL },
{ 0x09, 0xffffffffc4653600ULL },
{ 0x09, 0xffffffffc4653600ULL },
{ 0x0a, 0xfffffffdabf41c00ULL },
{ 0x0a, 0xfffffffdabf41c00ULL },
{ 0x0a, 0xfffffffdabf41c00ULL },
{ 0x0a, 0xfffffffdabf41c00ULL },
{ 0x0b, 0xffffffe8b7891800ULL },
{ 0x0b, 0xffffffe8b7891800ULL },
{ 0x0b, 0xffffffe8b7891800ULL },
{ 0x0c, 0xffffff172b5af000ULL },
{ 0x0c, 0xffffff172b5af000ULL },
{ 0x0c, 0xffffff172b5af000ULL },
{ 0x0d, 0xfffff6e7b18d6000ULL },
{ 0x0d, 0xfffff6e7b18d6000ULL },
{ 0x0d, 0xfffff6e7b18d6000ULL },
{ 0x0d, 0xfffff6e7b18d6000ULL },
{ 0x0e, 0xffffa50cef85c000ULL },
{ 0x0e, 0xffffa50cef85c000ULL },
{ 0x0e, 0xffffa50cef85c000ULL },
{ 0x0f, 0xfffc72815b398000ULL },
{ 0x0f, 0xfffc72815b398000ULL },
{ 0x0f, 0xfffc72815b398000ULL },
{ 0x10, 0xffdc790d903f0000ULL },
{ 0x10, 0xffdc790d903f0000ULL },
{ 0x10, 0xffdc790d903f0000ULL },
{ 0x10, 0xffdc790d903f0000ULL },
{ 0x11, 0xfe9cba87a2760000ULL },
{ 0x11, 0xfe9cba87a2760000ULL },
{ 0x11, 0xfe9cba87a2760000ULL },
{ 0x12, 0xf21f494c589c0000ULL },
{ 0x12, 0xf21f494c589c0000ULL },
{ 0x12, 0xf21f494c589c0000ULL },
{ 0x13, 0x7538dcfb76180000ULL },
{ 0x13, 0x7538dcfb76180000ULL },
{ 0x13, 0x7538dcfb76180000ULL },
{ 0x13, 0x7538dcfb76180000ULL },
};
int log = int_log2(x);
uint64_t low = table[log][1];
uint64_t high = table[log][0];
return (x + low < x ) + high;
}
评论区大哥给了一个更快的
inline uint8_t digitCounts64[]{ 19, 19, 19, 19, 18, 18, 18, 17, 17, 17, 16, 16, 16, 16, 15, 15, 15, 14, 14, 14, 13, 13, 13, 13, 12, 12, 12, 11, 11, 11, 10, 10, 10,
10, 9, 9, 9, 8, 8, 8, 7, 7, 7, 7, 6, 6, 6, 5, 5, 5, 4, 4, 4, 4, 3, 3, 3, 2, 2, 2, 1, 1, 1, 1, 1 };
inline uint64_t digitCountThresholds64[]{ 0ull, 9ull, 99ull, 999ull, 9999ull, 99999ull, 999999ull, 9999999ull, 99999999ull, 999999999ull, 9999999999ull,
99999999999ull, 999999999999ull, 9999999999999ull, 99999999999999ull, 999999999999999ull, 9999999999999999ull, 99999999999999999ull, 999999999999999999ull,
9999999999999999999ull };
inline uint64_t fastDigitCount(const uint64_t inputValue) {
const uint64_t originalDigitCount{ digitCounts64[__builtin_clzll(inputValue)] };
return originalDigitCount + (inputValue > digitCountThresholds64[originalDigitCount]);
}
另外这个大哥写了个json库很快。和glaze有一拼 https://github.com/RealTimeChris/Jsonifier/
C++26: a placeholder with no name
总算不用使用 decltype(std::ignore) _;了 c++26支持了
[Inside STL: Waiting for a std::atomic<std::shared_ptr> to change, part 1](https://devblogs.microsoft.com/oldnewthing/20250108-00/?p=110732) [Part2](https://devblogs.microsoft.com/oldnewthing/20250109-00/?p=110738)
省流std::atomic<std::shared_ptr>::notify_one linux平台由于futex的问题，没有很好的实现
windows有waitonaddress
constexpr std::string的一个好处
可以写编译期测试 static_assert
constexpr std::string
decode(std::span<const unsigned char> payload)
{
static constexpr unsigned char MASK{0x7F};
std::string result{};
int shift{};
unsigned char mask{MASK};
for(unsigned char scratch{}; auto c : payload) {
const unsigned char realChar =
((c & mask) << shift) | scratch;
scratch = (c & (~mask)) >> (7 - shift);
++shift;
mask >>= 1;
result += static_cast<char>(realChar);
if(7 == shift) {
result += static_cast<char>(scratch);
shift = 0;
mask = MASK;
scratch = 0;
}
}
return result;
}
constexpr auto make_array(auto... values)
{
return std::array<unsigned char, sizeof...(values)>{
static_cast<unsigned char>(values)...};
}
void Use()
{
constexpr auto payload{make_array(0xd3,
0x74,
0x1b,
0xce,
0x2e,
0x83,
0xa6,
0xcd,
0x29,
0x88,
0x5e,
0xc6,
0xd3,
0x5d)};
constexpr auto payload2{make_array(0xc8,
0x32,
0x9b,
0xfd,
0x66,
0x81,
0x86,
0xab,
0x55,
0x08,
0x44,
0x45,
0xa7,
0xe7,
0xa0,
0xf4,
0x1c,
0x34,
0x46,
0x97,
0xc7,
0xeb,
0xb4,
0xfb,
0x0c,
0x0a,
0x83,
0xe6,
0x74,
0xb2,
0x4e,
0x37,
0xa7,
0xcb,
0xd3,
0xee,
0x33,
0x28,
0x4c,
0x07,
0x8d,
0xdf,
0x6d,
0x78,
0x9a,
0x5d,
0x06,
0xd1,
0xd3,
0xed,
0x72,
0x08,
0xe4,
0x4c,
0x8f,
0xcb,
0x2c,
0x50,
0x7a,
0xee,
0x3e,
0xd1,
0x41,
0x69,
0xfa,
0x0f)};
auto smsText = decode(payload);
std::println("{}", smsText);
auto smsText2 = decode(payload2);
std::println("{}", smsText2);
static_assert("Simple SMS text." == decode(payload));
static_assert(
"Hello, C++! This is checking a std::string at "
"compile time! Nice, isn't it?" ==
decode(payload2));
}
“Trivially relocatable” versus “trivially destructible after move”
抽象
开源项目介绍
互动环节
本期内容不多。估计以后周更基本上都这样，太长了看不完有点
大家有啥建议欢迎交流
