GMP位移位不能在负数运行 [英] GMP Bit shift doesn't work on negative numbers

问题描述

我发现在php.net此功能。这似乎在正数的工作，但未能对消极的：

I found this function at php.net. It seems to work on positive numbers, but fails on negative ones:

function gmp_shiftr($x,$n) { // shift right
  return(gmp_div($x,gmp_pow(2,$n)));
} 

echo -1 >> 8; //returns -1, presumably correctly
echo "<br />";
echo gmp_strval(gmp_shiftr(-1,8)); //returns 0, presumably incorrectly

我怎么能修复了底片工作中的作用？

How could I fix up the function to work with negatives?

两个想法我有：

也许我可以做线沿线的东西。

Maybe I could do something along the lines of

if (whatever) { $a >> $b} else{ gmp_shiftr($a, $b) }?

或者，也许我可以根据自己的价值减去负的成绩东西..？

Or, maybe I could subtract something from the negative results depending on their value..?

我只想得到>>将给予价值，也得到它> 32位数字，当我使用GMP。

I just want to get the value that >> would give, but also get it for >32bit numbers when I use GMP.

推荐答案

综观的为师例程的GMP文件，有一个函数

Looking at the GMP documentation for the division routines, there's a function

void mpz_tdiv_q_2exp (mpz_t q, mpz_t n, unsigned long int b)

这看起来可能是你想要什么：算术右移的治疗
N ，如果它被重新presented在二进制补码，和（我认为）转变为 B 地方
在右边。不幸的是，API的那个水平似乎并没有被PHP GMP暴露出来。

that seems like it might be what you want: an arithmetic right shift that treats n as if it were represented in twos-complement, and (I think) shifts it b places to the right. Unfortunately, that level of the API doesn't seem to be exposed by PHP GMP.

我发现了一个位摆弄黑客做符号扩展时比特数
中联重presentation是未知的：

I found a bit twiddling hack for doing sign extension when the number of bits in the representation is unknown:

unsigned b; // number of bits representing the number in x
int x;      // sign extend this b-bit number to r
int r;      // resulting sign-extended number
int const m = 1U << (b - 1); // mask can be pre-computed if b is fixed

x = x & ((1U << b) - 1);  // (Skip this if bits in x above position b are already zero.)
r = (x ^ m) - m;

由于按位与和XOR的是的由PHP支持GMP，你也许能够使
这项工作......

Since bitwise AND and XOR are supported by PHP GMP, you might be able to make this work...

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