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183126a1dc
It's easy to add two tags together, but it's hard to subtract them. The new "tag_tracker" data structure provides a solution. Signed-off-by: Ben Pfaff <blp@nicira.com>
65 lines
2.0 KiB
C
65 lines
2.0 KiB
C
/*
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* Copyright (c) 2008, 2009, 2010, 2011, 2013 Nicira, Inc.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at:
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include <config.h>
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#include "tag.h"
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#define LOG2_N_TAG_BITS (N_TAG_BITS == 32 ? 5 : N_TAG_BITS == 64 ? 6 : 0)
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BUILD_ASSERT_DECL(LOG2_N_TAG_BITS > 0);
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/* Returns a tag deterministically generated from 'seed'.
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*
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* 'seed' should have data in all of its bits; if it has data only in its
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* low-order bits then the resulting tags will be poorly distributed. Use a
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* hash function such as hash_bytes() to generate 'seed' if necessary. */
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tag_type
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tag_create_deterministic(uint32_t seed)
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{
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int x = seed & (N_TAG_BITS - 1);
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int y = (seed >> LOG2_N_TAG_BITS) % (N_TAG_BITS - 1);
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y += y >= x;
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return (1u << x) | (1u << y);
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}
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/* Initializes 'tracker'. */
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void
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tag_tracker_init(struct tag_tracker *tracker)
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{
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memset(tracker, 0, sizeof *tracker);
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}
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/* Adds 'add' to '*tags' and records the bits added in 'tracker'. */
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void
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tag_tracker_add(struct tag_tracker *tracker, tag_type *tags, tag_type add)
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{
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*tags |= add;
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for (; add; add = zero_rightmost_1bit(add)) {
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tracker->counts[rightmost_1bit_idx(add)]++;
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}
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}
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/* Removes 'sub' from 'tracker' and unsets any bits in '*tags' that no
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* remaining tag includes. */
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void
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tag_tracker_subtract(struct tag_tracker *tracker, tag_type *tags, tag_type sub)
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{
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for (; sub; sub = zero_rightmost_1bit(sub)) {
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if (!--tracker->counts[rightmost_1bit_idx(sub)]) {
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*tags &= ~rightmost_1bit(sub);
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}
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}
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}
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