2 * Copyright (c) 2008, 2009 Nicira Networks.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at:
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
21 #include "type-props.h"
24 #define N_TAG_BITS (CHAR_BIT * sizeof(tag_type))
25 BUILD_ASSERT_DECL(IS_POW2(N_TAG_BITS));
27 #define LOG2_N_TAG_BITS (N_TAG_BITS == 32 ? 5 : N_TAG_BITS == 64 ? 6 : 0)
28 BUILD_ASSERT_DECL(LOG2_N_TAG_BITS > 0);
30 /* Returns a randomly selected tag. */
32 tag_create_random(void)
36 uint16_t r = random_uint16();
37 x = r & (N_TAG_BITS - 1);
38 y = r >> (16 - LOG2_N_TAG_BITS);
40 return (1u << x) | (1u << y);
43 /* Returns a tag deterministically generated from 'seed'.
45 * 'seed' should have data in all of its bits; if it has data only in its
46 * low-order bits then the resulting tags will be poorly distributed. Use a
47 * hash function such as hash_bytes() to generate 'seed' if necessary. */
49 tag_create_deterministic(uint32_t seed)
51 int x = seed & (N_TAG_BITS - 1);
52 int y = (seed >> LOG2_N_TAG_BITS) % 31;
54 return (1u << x) | (1u << y);
57 /* Initializes 'set' as an empty tag set. */
59 tag_set_init(struct tag_set *set)
61 memset(set, 0, sizeof *set);
64 /* Adds 'tag' to 'set'. */
66 tag_set_add(struct tag_set *set, tag_type tag)
68 if (tag && (!tag_is_valid(tag) || !tag_set_intersects(set, tag))) {
69 /* XXX We could do better by finding the set member to which we would
70 * add the fewest number of 1-bits. This would reduce the amount of
71 * ambiguity, since e.g. three 1-bits match 3 * 2 / 2 = 3 unique tags
72 * whereas four 1-bits match 4 * 3 / 2 = 6 unique tags. */
73 tag_type *t = &set->tags[set->n++ % TAG_SET_SIZE];
75 if (*t == TYPE_MAXIMUM(tag_type)) {