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tag: Be more precise about choosing tags to add, in tag_set_add().

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It is not necessary to add a "tag" if all of the bits in it are already
present in some member of the set.  This commit adds that optimization.
This commit is contained in:
Ben Pfaff
2010-08-06 11:42:07 -07:00
parent c393047875
commit d12f7113b0
1 changed files with 26 additions and 1 deletions
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27
lib/tag.c
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@@ -61,11 +61,36 @@ tag_set_init(struct tag_set *set)
memset(set, 0, sizeof *set);
}
static bool
tag_is_worth_adding(const struct tag_set *set, tag_type tag)
{
if (!tag) {
/* Nothing to add. */
return false;
} else if ((set->total & tag) != tag) {
/* 'set' doesn't have all the bits in 'tag', so we need to add it. */
return true;
} else {
/* We can drop it if some member of 'set' already includes all of the
* 1-bits in 'tag'. (tag_set_intersects() does a different test:
* whether some member of 'set' has at least two 1-bit in common with
* 'tag'.) */
int i;
for (i = 0; i < TAG_SET_SIZE; i++) {
if ((set->tags[i] & tag) == tag) {
return false;
}
}
return true;
}
}
/* Adds 'tag' to 'set'. */
void
tag_set_add(struct tag_set *set, tag_type tag)
{
if (tag && (!tag_is_valid(tag) || !tag_set_intersects(set, tag))) {
if (tag_is_worth_adding(set, tag)) {
/* XXX We could do better by finding the set member to which we would
* add the fewest number of 1-bits. This would reduce the amount of
* ambiguity, since e.g. three 1-bits match 3 * 2 / 2 = 3 unique tags