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libdm: add dm_bit_get_last()/dm_bit_get_prev()

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It is sometimes convenient to iterate over the set bits in a dm
bitset in reverse order (from the highest set bit toward zero), or
to quickly find the last set bit.

Add dm_bit_get_last() and dm_bit_get_prev(), mirroring the existing
dm_bit_get_first() and dm_bit_get_next().

dm_bit_get_prev() uses __builtin_clz when available to efficiently
test the bitset in reverse.
This commit is contained in:
Bryn M. Reeves 2016-12-11 12:44:45 +00:00
parent 107bc13db3
commit 5d1d65e735
3 changed files with 40 additions and 0 deletions
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2
libdm/.exported_symbols.DM_1_02_138 Normal file
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@ -0,0 +1,2 @@
dm_bit_get_last
dm_bit_get_prev

36
libdm/datastruct/bitset.c
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@ -76,6 +76,13 @@ static int _test_word(uint32_t test, int bit)
return (tb ? ffs(tb) + bit - 1 : -1);
}
static int _test_word_rev(uint32_t test, int bit)
{
uint32_t tb = test << (DM_BITS_PER_INT - 1 - bit);
return (tb ? bit - clz(tb) : -1);
}
int dm_bit_get_next(dm_bitset_t bs, int last_bit)
{
int bit, word;
@ -101,11 +108,40 @@ int dm_bit_get_next(dm_bitset_t bs, int last_bit)
return -1;
}
int dm_bit_get_prev(dm_bitset_t bs, int last_bit)
{
int bit, word;
uint32_t test;
last_bit--; /* otherwise we'll return the same bit again */
/*
* bs[0] holds number of bits
*/
while (last_bit >= 0) {
word = last_bit >> INT_SHIFT;
test = bs[word + 1];
bit = last_bit & (DM_BITS_PER_INT - 1);
if ((bit = _test_word_rev(test, bit)) >= 0)
return (word * DM_BITS_PER_INT) + bit;
last_bit = (last_bit & ~(DM_BITS_PER_INT - 1)) - 1;
}
return -1;
}
int dm_bit_get_first(dm_bitset_t bs)
{
return dm_bit_get_next(bs, -1);
}
int dm_bit_get_last(dm_bitset_t bs)
{
return dm_bit_get_prev(bs, bs[0] + 1);
}
/*
* Based on the Linux kernel __bitmap_parselist from lib/bitmap.c
*/

2
libdm/libdevmapper.h
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@ -2069,6 +2069,8 @@ void dm_bit_and(dm_bitset_t out, dm_bitset_t in1, dm_bitset_t in2);
void dm_bit_union(dm_bitset_t out, dm_bitset_t in1, dm_bitset_t in2);
int dm_bit_get_first(dm_bitset_t bs);
int dm_bit_get_next(dm_bitset_t bs, int last_bit);
int dm_bit_get_last(dm_bitset_t bs);
int dm_bit_get_prev(dm_bitset_t bs, int last_bit);
#define DM_BITS_PER_INT (sizeof(int) * CHAR_BIT)