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/*
* Copyright (c) 2008, 2011, 2012, 2013 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.
*/
#ifndef TAG_H
#define TAG_H 1
#include <stdbool.h>
#include <stdint.h>
#include <limits.h>
#include "util.h"
/*
* Tagging support.
*
* A 'tag' represents an arbitrary category. Currently, tags are used to
* represent categories of flows and in particular the value of the 64-bit
* "metadata" field in the flow. The universe of possible categories is very
* large (2**64). The number of categories in use at a given time can also be
* large. This means that keeping track of category membership via
* conventional means (lists, bitmaps, etc.) is likely to be expensive.
*
* Tags are actually implemented via a "superimposed coding", as discussed in
* Knuth TAOCP v.3 section 6.5 "Retrieval on Secondary Keys". A tag is an
* unsigned integer in which exactly 2 bits are set to 1 and the rest set to 0.
* For 32-bit integers (as currently used) there are 32 * 31 / 2 = 496 unique
* tags; for 64-bit integers there are 64 * 63 / 2 = 2,016.
*
* Because there is a small finite number of unique tags, tags must collide
* after some number of them have been created. In practice we generally
* create tags by choosing bits randomly or based on a hash function.
*
* The key property of tags is that we can combine them without increasing the
* amount of data required using bitwise-OR, since the result has the 1-bits
* from both tags set. The necessary tradeoff is that the result is even more
* ambiguous: if combining two tags yields a value with 4 bits set to 1, then
* the result value will test as having 4 * 3 / 2 = 6 unique tags, not just the
* two tags that we combined.
*
* The upshot is this: a value that is the bitwise-OR combination of a number
* of tags will always include the tags that were combined, but it may contain
* any number of additional tags as well. This is acceptable for our use,
* since we want to be sure that we check every classifier table that contains
* a rule with a given metadata value, but it is OK if we check a few extra
* tables as well.
*
* If we combine too many tags, then the result will have every bit set, so
* that it will test as including every tag. This can happen, but we hope that
* this is not the common case.
*/
/* Represents a tag, or the combination of 0 or more tags. */
typedef uint32_t tag_type;
#define N_TAG_BITS (CHAR_BIT * sizeof(tag_type))
BUILD_ASSERT_DECL(IS_POW2(N_TAG_BITS));
/* A 'tag_type' value that intersects every tag. */
#define TAG_ALL UINT32_MAX
/* An arbitrary tag. */
#define TAG_ARBITRARY UINT32_C(3)
tag_type tag_create_deterministic(uint32_t seed);
static inline bool tag_intersects(tag_type, tag_type);
/* Returns true if 'a' and 'b' have at least one tag in common,
* false if their set of tags is disjoint. */
static inline bool
tag_intersects(tag_type a, tag_type b)
{
tag_type x = a & b;
return (x & (x - 1)) != 0;
}
/* Adding tags is easy, but subtracting is hard because you can't tell whether
* a bit was set only by the tag you're removing or by multiple tags. The
* tag_tracker data structure counts the number of tags that set each bit,
* which allows for efficient subtraction. */
struct tag_tracker {
unsigned int counts[N_TAG_BITS];
};
void tag_tracker_init(struct tag_tracker *);
void tag_tracker_add(struct tag_tracker *, tag_type *, tag_type);
void tag_tracker_subtract(struct tag_tracker *, tag_type *, tag_type);
#endif /* tag.h */
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