getbits.c

来自「自己做的常用库和实现的数据结构。public domain.」· C语言 代码 · 共 43 行

#include <stdio.h>

static unsigned getbits(unsigned x, unsigned a, unsigned b); 
static unsigned get_value(unsigned n);

int main(void)
{
	unsigned x;

	x = getbits(931u, 5u, 8u);
	printf("%u ", x);

	return 0;
}

static unsigned getbits(unsigned x, unsigned a, unsigned b) 
{
	unsigned i, j;

	i = get_value(a);
	i <<= (sizeof(unsigned) * 8) - a;
	i = ~i;
	j = get_value(b);
	j = ~j;
	x &= i;
	x &= j;

	return x;
}

static unsigned get_value(unsigned n)
{
	unsigned i, s1=1, s2=0;

	for(i=1; i<n; i++) {
		s1 *= 2;
		s2 += s1;
	}	

	return s2 + 1;
}