[英]Issue clearing bits in 128 bit integer array
My goal with this program is that I want to be able to set 1000 bits to 0 or 1 separately, for this I'm using an array of 128 bit integers.我对这个程序的目标是我希望能够将 1000 位分别设置为 0 或 1,为此我使用了一个 128 位整数数组。
Now the issue is when I simply clear_bit(3), bit 35 is also being cleared (and vice versa).现在的问题是当我只是 clear_bit(3) 时,第 35 位也被清除(反之亦然)。 3 & 35 are always cleared together, same for 4 & 36, 5 & 37 etc. So there's clearly a pattern.
3 和 35 总是一起清零,4 和 36、5 和 37 等也一样。所以显然有一个模式。 I simply want 3 to be cleared while the rest stays 1.
我只想清除 3,而其余的保持 1。
Any ideas why this is happening?任何想法为什么会发生这种情况? Thanks!
谢谢!
#include <stdio.h>
__uint128_t array [(1000/128) + 1];
// Set bit to 0.
void clear_bit(int k)
{
array[k/128] &= ~(1 << (k%128));
}
// Find the value of bit.
int test_bit(int k)
{
return((array[k/128] & (1 << (k%128) )) != 0);
}
// Set bit to 1.
void set_bit(int k)
{
array[k/128] |= 1 << (k%128); // Set the bit at the k-th position in A[i]
}
int main (void)
{
// Set all bits to 1.
for(int i = 0; i < 40; i++) {
set_bit(i);
}
// I want to clear bit 3, but it also clears 35 for an unknown reason.
clear_bit(3);
for(int i = 0; i < 40; i++) {
printf("%d is now:%d\n", i, test_bit(i));
}
return (0);
}
OP's code is not using 128-bit math with 1 << (k%128)
. OP 的代码没有使用
1 << (k%128)
128 位数学。 @user2357112 @用户2357112
Insure the integer math is done with at least 128-bit math.确保整数数学至少使用 128 位数学完成。
array[k/128] &= ~(1 << (k%128));
array[k/128] &= ~((__uint128_t)1 << (k%128));
Alternative, use unsigned
in a portable fashion should unsigned
have a bit width of be 16,32 64, 36, etc. No need to rely on __uint128_t
.或者,以可移植方式使用
unsigned
应该unsigned
的位宽为 16,32 __uint128_t
等。无需依赖__uint128_t
。
#include <limits.h>
#define UNS_WIDTH (sizeof(unsigned)*CHAR_BIT)
unsigned array[(1000 + UNS_WIDTH - 1)/UNS_WIDTH];
void clear_bit(int k) {
array[k/UNS_WIDTH] &= ~(1u << (k%UNS_WIDTH));
}
Best to insure the 1
is unsigned
with 1u
.最好确保
1
与1u
unsigned
。
I would suggest using the native size of platform registers.我建议使用平台寄存器的本机大小。 just compare the generated code (in this example 64 bit platform) - https://godbolt.org/g/Y316vU
只需比较生成的代码(在此示例中为 64 位平台)- https://godbolt.org/g/Y316vU
__uint128_t array [(1000/128) + 1];
uint64_t array1[(1000/64) + 1];
void assignBit64(unsigned bit, unsigned value)
{
array1[bit >> 6] &= ~((uint64_t)1 << (bit & 63));
array1[bit >> 6] |= ((uint64_t)!!value) << (bit & 63);
}
void assignBit128(unsigned bit, unsigned value)
{
array[bit >> 7] &= ~((__uint128_t)1 << (bit & 127));
array[bit >> 7] |= ((__uint128_t)(!!value)) << (bit & 127);
}
void resetBit64(unsigned bit)
{
array1[bit >> 6] &= ~((uint64_t)1 << (bit & 63));
}
void resetBit128(unsigned bit)
{
array[bit >> 7] &= ~((__uint128_t)1 << (bit & 127));
}
void setBit64(unsigned bit)
{
array1[bit >> 6] |= ((uint64_t)1 << (bit & 63));
}
void setBit128(unsigned bit)
{
array[bit >> 7] |= ((__uint128_t)1 << (bit & 127));
}
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