asar coverage - build #323

src/asar/
File: src/asar/frozen/bits/pmh.h
Date: 2025-10-31 21:01:18
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1 /*
2 * Frozen
3 * Copyright 2016 QuarksLab
4 *
5 * Licensed to the Apache Software Foundation (ASF) under one
6 * or more contributor license agreements. See the NOTICE file
7 * distributed with this work for additional information
8 * regarding copyright ownership. The ASF licenses this file
9 * to you under the Apache License, Version 2.0 (the
10 * "License"); you may not use this file except in compliance
11 * with the License. You may obtain a copy of the License at
12 *
13 * http://www.apache.org/licenses/LICENSE-2.0
14 *
15 * Unless required by applicable law or agreed to in writing,
16 * software distributed under the License is distributed on an
17 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
18 * KIND, either express or implied. See the License for the
19 * specific language governing permissions and limitations
20 * under the License.
21 */
22
23 // inspired from http://stevehanov.ca/blog/index.php?id=119
24 #ifndef FROZEN_LETITGO_PMH_H
25 #define FROZEN_LETITGO_PMH_H
26
27 #include "frozen/bits/algorithms.h"
28 #include "frozen/bits/basic_types.h"
29
30 #include <array>
31 #include <cstddef>
32 #include <cstdint>
33 #include <limits>
34
35 namespace frozen {
36
37 namespace bits {
38
39 // Function object for sorting buckets in decreasing order of size
40 struct bucket_size_compare {
41 template <typename B>
42 bool constexpr operator()(B const &b0,
43 B const &b1) const {
44 return b0.size() > b1.size();
45 }
46 };
47
48 // Step One in pmh routine is to take all items and hash them into buckets,
49 // with some collisions. Then process those buckets further to build a perfect
50 // hash function.
51 // pmh_buckets represents the initial placement into buckets.
52
53 template <std::size_t M>
54 struct pmh_buckets {
55 // Step 0: Bucket max is 2 * sqrt M
56 // TODO: Come up with justification for this, should it not be O(log M)?
57 static constexpr auto bucket_max = 2 * (1u << (log(M) / 2));
58
59 using bucket_t = cvector<std::size_t, bucket_max>;
60 carray<bucket_t, M> buckets;
61 std::uint64_t seed;
62
63 // Represents a reference to a bucket. This is used because the buckets
64 // have to be sorted, but buckets are big, making it slower than sorting refs
65 struct bucket_ref {
66 unsigned hash;
67 const bucket_t * ptr;
68
69 // Forward some interface of bucket
70 using value_type = typename bucket_t::value_type;
71 using const_iterator = typename bucket_t::const_iterator;
72
73 constexpr auto size() const { return ptr->size(); }
74 constexpr const auto & operator[](std::size_t idx) const { return (*ptr)[idx]; }
75 constexpr auto begin() const { return ptr->begin(); }
76 constexpr auto end() const { return ptr->end(); }
77 };
78
79 // Make a bucket_ref for each bucket
80 template <std::size_t... Is>
81 carray<bucket_ref, M> constexpr make_bucket_refs(std::index_sequence<Is...>) const {
82 return {{ bucket_ref{Is, &buckets[Is]}... }};
83 }
84
85 // Makes a bucket_ref for each bucket and sorts them by size
86 carray<bucket_ref, M> constexpr get_sorted_buckets() const {
87 carray<bucket_ref, M> result{this->make_bucket_refs(std::make_index_sequence<M>())};
88 bits::quicksort(result.begin(), result.end() - 1, bucket_size_compare{});
89 return result;
90 }
91 };
92
93 template <std::size_t M, class Item, std::size_t N, class Hash, class Key, class PRG>
94 pmh_buckets<M> constexpr make_pmh_buckets(const carray<Item, N> & items,
95 Hash const & hash,
96 Key const & key,
97 PRG & prg) {
98 using result_t = pmh_buckets<M>;
99 // Continue until all items are placed without exceeding bucket_max
100 while (1) {
101 result_t result{};
102 result.seed = prg();
103 bool rejected = false;
104 for (std::size_t i = 0; i < items.size(); ++i) {
105 auto & bucket = result.buckets[hash(key(items[i]), static_cast<std::size_t>(result.seed)) % M];
106 if (bucket.size() >= result_t::bucket_max) {
107 rejected = true;
108 break;
109 }
110 bucket.push_back(i);
111 }
112 if (!rejected) { return result; }
113 }
114 }
115
116 // Check if an item appears in a cvector
117 template<class T, std::size_t N>
118 constexpr bool all_different_from(cvector<T, N> & data, T & a) {
119 for (std::size_t i = 0; i < data.size(); ++i)
120 if (data[i] == a)
121 return false;
122
123 return true;
124 }
125
126 // Represents either an index to a data item array, or a seed to be used with
127 // a hasher. Seed must have high bit of 1, value has high bit of zero.
128 struct seed_or_index {
129 using value_type = std::uint64_t;
130
131 private:
132 static constexpr value_type MINUS_ONE = std::numeric_limits<value_type>::max();
133 static constexpr value_type HIGH_BIT = ~(MINUS_ONE >> 1);
134
135 value_type value_ = 0;
136
137 public:
138 88359 constexpr value_type value() const { return value_; }
139 88359 constexpr bool is_seed() const { return value_ & HIGH_BIT; }
140
141 constexpr seed_or_index(bool is_seed, value_type value)
142 : value_(is_seed ? (value | HIGH_BIT) : (value & ~HIGH_BIT)) {}
143
144 constexpr seed_or_index() = default;
145 constexpr seed_or_index(const seed_or_index &) = default;
146 constexpr seed_or_index & operator =(const seed_or_index &) = default;
147 };
148
149 // Represents the perfect hash function created by pmh algorithm
150 template <std::size_t M, class Hasher>
151 struct pmh_tables : private Hasher {
152 std::uint64_t first_seed_;
153 carray<seed_or_index, M> first_table_;
154 carray<std::size_t, M> second_table_;
155
156 constexpr pmh_tables(
157 std::uint64_t first_seed,
158 carray<seed_or_index, M> first_table,
159 carray<std::size_t, M> second_table,
160 Hasher hash) noexcept
161 : Hasher(hash)
162 , first_seed_(first_seed)
163 , first_table_(first_table)
164 , second_table_(second_table)
165 {}
166
167 129431 constexpr Hasher const& hash_function() const noexcept {
168 129431 return static_cast<Hasher const&>(*this);
169 }
170
171 template <typename KeyType>
172 constexpr std::size_t lookup(const KeyType & key) const {
173 return lookup(key, hash_function());
174 }
175
176 // Looks up a given key, to find its expected index in carray<Item, N>
177 // Always returns a valid index, must use KeyEqual test after to confirm.
178 template <typename KeyType, typename HasherType>
179 88359 constexpr std::size_t lookup(const KeyType & key, const HasherType& hasher) const {
180
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 88359 auto const d = first_table_[hasher(key, static_cast<std::size_t>(first_seed_)) % M];
181
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 88359 if (!d.is_seed()) { return static_cast<std::size_t>(d.value()); } // this is narrowing std::uint64 -> std::size_t but should be fine
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 9836 else { return second_table_[hasher(key, static_cast<std::size_t>(d.value())) % M]; }
183 }
184 };
185
186 // Make pmh tables for given items, hash function, prg, etc.
187 template <std::size_t M, class Item, std::size_t N, class Hash, class Key, class KeyEqual, class PRG>
188 pmh_tables<M, Hash> constexpr make_pmh_tables(const carray<Item, N> &
189 items,
190 Hash const &hash,
191 KeyEqual const &equal,
192 Key const &key,
193 PRG prg) {
194 // Step 1: Place all of the keys into buckets
195 auto step_one = make_pmh_buckets<M>(items, hash, key, prg);
196
197 // Step 1.5: Detect redundant keys.
198 for(auto const& bucket : step_one.buckets)
199 for(std::size_t i = 1; i < bucket.size(); ++i)
200 constexpr_assert(!equal(key(items[0]), key(items[i])), "structure keys should be unique");
201
202 // Step 2: Sort the buckets to process the ones with the most items first.
203 auto buckets = step_one.get_sorted_buckets();
204
205 // Special value for unused slots. This is purposefully the index
206 // one-past-the-end of 'items' to function as a sentinel value. Both to avoid
207 // the need to apply the KeyEqual predicate and to be easily convertible to
208 // end().
209 // Unused entries in both hash tables (G and H) have to contain this value.
210 const auto UNUSED = items.size();
211
212 // G becomes the first hash table in the resulting pmh function
213 carray<seed_or_index, M> G({false, UNUSED});
214
215 // H becomes the second hash table in the resulting pmh function
216 carray<std::size_t, M> H(UNUSED);
217
218 // Step 3: Map the items in buckets into hash tables.
219 for (const auto & bucket : buckets) {
220 auto const bsize = bucket.size();
221
222 if (bsize == 1) {
223 // Store index to the (single) item in G
224 // assert(bucket.hash == hash(key(items[bucket[0]]), step_one.seed) % M);
225 G[bucket.hash] = {false, static_cast<std::uint64_t>(bucket[0])};
226 } else if (bsize > 1) {
227
228 // Repeatedly try different H of d until we find a hash function
229 // that places all items in the bucket into free slots
230 seed_or_index d{true, prg()};
231 cvector<std::size_t, decltype(step_one)::bucket_max> bucket_slots;
232
233 while (bucket_slots.size() < bsize) {
234 auto slot = hash(key(items[bucket[bucket_slots.size()]]), static_cast<std::size_t>(d.value())) % M;
235
236 if (H[slot] != UNUSED || !all_different_from(bucket_slots, slot)) {
237 bucket_slots.clear();
238 d = {true, prg()};
239 continue;
240 }
241
242 bucket_slots.push_back(slot);
243 }
244
245 // Put successful seed in G, and put indices to items in their slots
246 // assert(bucket.hash == hash(key(items[bucket[0]]), step_one.seed) % M);
247 G[bucket.hash] = d;
248 for (std::size_t i = 0; i < bsize; ++i)
249 H[bucket_slots[i]] = bucket[i];
250 }
251 }
252
253 return {step_one.seed, G, H, hash};
254 }
255
256 } // namespace bits
257
258 } // namespace frozen
259
260 #endif
261