// b: uint32_t array of size n => 32*n bits
// The bit index, i, is in the range 0 <= i < 32 * n
// The bit in b at bit index 0 is always 0!
unsigned idx_of_first_zero_bit_before_or_at (uint32_t *b, unsigned n, unsigned i) {
// Returns a bit index, k, such that k <= i and k is the largest bit index
// for which bit k in b is 0.
}
// As above, value == 0 or 1
void set_bit (uint32_t *b, unsigned n, unsigned i, unsigned value) {
// Sets bit at bit index i to value.
// It could be something like (untested):
if (value)
b[i >> 5] |= (1 << (i&31));
else
b[i >> 5] &= (~(1 << (i&31)));
}
我正在寻找最有效的,但仍然可移植的(在不同的目标,但仅使用了克++编译器)的方式来实现这些功能(尤其是第一一)。位(大,小端或其他)的存储顺序无关紧要。两个操作/ C++
天真实现(未经测试):
uint32_t get_bit (uint32_t *b, unsigned n, unsigned i) {
return b[i >> 5] & (1 << (i&31));
}
unsigned idx_of_first_zero_bit_before_or_at (uint32_t *b, unsigned n, unsigned i) {
while (get_bit (b, n, i))
i--;
return i;
}
跳过所有-1元素:
unsigned idx_of_first_zero_bit_before_or_at (uint32_t *b, unsigned n, unsigned i) {
for (unsigned k = i >> 5; ~(b[k]) == 0; i = (--k << 5) + 31);
while (get_bit (b, n, i))
i--;
return i;
}
您未经测试的'get_bit'似乎检查除了有问题的位以外的所有内容(在相关的32位值中)。只是舍弃反转。 :-)为了优化,考虑跳过全是1的32位值,通过反转和检查0来轻松检查。Cheers&hth。, – 2010-11-06 16:47:39
@Alf:谢谢,试图添加您的解决方案 - 可能不尽可能好... – Thomas 2010-11-06 16:57:03
GCC有一些扩展,例如__builtin_clz,所以如果你只需要使用GCC,你可以使用这些扩展。 http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html – 2010-11-06 17:11:40