C 中的波动算子

我见过 ELF 哈希算法中使用的波浪运算符，我很好奇它是做什么的。(密码是 来自《永远的困惑》)

unsigned elf_hash ( void *key, int len )
{
unsigned char *p = key;
unsigned h = 0, g;
int i;


for ( i = 0; i < len; i++ ) {
h = ( h << 4 ) + p[i];
g = h & 0xf0000000L;


if ( g != 0 )
h ^= g >> 24;


h &= ~g;
}


return h;
}
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~ is the bitwise NOT operator. It inverts the bits of the operand.

For example, if you have:

char b = 0xF0;  /* Bits are 11110000 */
char c = ~b;    /* Bits are 00001111 */
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This is the bitwise NOT operator. It flips all the bits in a number: 100110 -> 011001

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最佳答案

The ~ operator is bitwise NOT, it inverts the bits in a binary number:

NOT 011100
= 100011
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It is the bitwise NOT operator. It inverts all bits in an integer value.

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The tilde character is used as an operator to invert all bits of an integer (bitwise NOT).

For example: ~0x0044 = 0xFFBB.

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Tilde operator (~) also called bitwise NOT operator, performs one's complement of any binary number as argument. If the operand to NOT is decimal number then it convert it as binary and perform's one's complement operation.

To calculate one's complement simply invert all the digit [0-->1] and [1-->0] Ex : 0101 = 5; ~(0101) = 1010. Use of tilde operator : 1. It is used in masking operation , Masking means setting and resetting the values inside any register . for ex : 

char mask ;
mask = 1 << 5 ;

It will set mask to a binary value of 10000 and this mask can be used to check the bit value present inside other variable . 

int a = 4;
int k = a&mask ; if the 5th bit is 1 , then k=1 otherwise k=0.

This is called Masking of bits. 2.To find binary equivalent of any number using masking properties.

#include<stdio.h>
void equi_bits(unsigned char);
int main()
{
unsigned char num = 10 ;
printf("\nDecimal %d is same as binary ", num);
equi_bits(num);
return 0;
}
void equi_bits(unsigned char n)
{
int i ;
unsigned char j , k ,mask ;
for( i = 7 ; i >= 0 ; i--)
{
j=i;
mask = 1 << j;
k = n&mask ; // Masking
k==0?printf("0"):printf("1");
}
}

Output : Decimal 10 is same as 00001010

My observation :For the maximum range of any data type , one's complement provide the negative value decreased by 1 to any corresponding value. ex:
~1 --------> -2
~2---------> -3
and so on... I will show you this observation using little code snippet

#include<stdio.h>
int main()
{
int a , b;
a=10;
b=~a; // b-----> -11
printf("%d\n",a+~b+1);// equivalent to a-b
return 0;
}
Output: 0

Note : This is valid only for the range of data type. means for int data type this rule will be applicable only for the value of range[-2,147,483,648 to 2,147,483,647].
Thankyou .....May this help you


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