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/*
* Copyright (c) 2008, 2009, 2010, 2011 Nicira, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at:
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include <config.h>
#include "tag.h"
#include <limits.h>
#include "random.h"
#include "type-props.h"
#include "util.h"
#define N_TAG_BITS (CHAR_BIT * sizeof(tag_type))
BUILD_ASSERT_DECL(IS_POW2(N_TAG_BITS));
#define LOG2_N_TAG_BITS (N_TAG_BITS == 32 ? 5 : N_TAG_BITS == 64 ? 6 : 0)
BUILD_ASSERT_DECL(LOG2_N_TAG_BITS > 0);
/* Returns a randomly selected tag. */
tag_type
tag_create_random(void)
{
int x, y;
do {
uint16_t r = random_uint16();
x = r & (N_TAG_BITS - 1);
y = r >> (16 - LOG2_N_TAG_BITS);
} while (x == y);
return (1u << x) | (1u << y);
}
/* Returns a tag deterministically generated from 'seed'.
*
* 'seed' should have data in all of its bits; if it has data only in its
* low-order bits then the resulting tags will be poorly distributed. Use a
* hash function such as hash_bytes() to generate 'seed' if necessary. */
tag_type
tag_create_deterministic(uint32_t seed)
{
int x = seed & (N_TAG_BITS - 1);
int y = (seed >> LOG2_N_TAG_BITS) % (N_TAG_BITS - 1);
y += y >= x;
return (1u << x) | (1u << y);
}
/* Initializes 'set' as an empty tag set. */
void
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_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
* whereas four 1-bits match 4 * 3 / 2 = 6 unique tags. */
tag_type *t = &set->tags[set->n++ % TAG_SET_SIZE];
*t |= tag;
if (*t == TYPE_MAXIMUM(tag_type)) {
set->tags[0] = *t;
}
set->total |= tag;
}
}
/* Adds all the tags in 'other' to 'set'. */
void
tag_set_union(struct tag_set *set, const struct tag_set *other)
{
size_t i;
for (i = 0; i < TAG_SET_SIZE; i++) {
tag_set_add(set, other->tags[i]);
}
}
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