Make unflatten() linear in the length of the keys
unflatten() decides in a first pass which prefixes of the keys denote arrays
(those that have a reference token 0 among their children), and looks up
prefixes again in the second pass. A prefix was identified by hashing it, and
by comparing it byte by byte with the entry in the set, from its beginning.
A key with n tokens therefore cost n^2/2 bytes: 100,000 tokens "/0" took
2.4 s (3.12.0: 5 ms), and so did "/0" followed by "/1" tokens, because every
new level looks its prefix up. Keys in the order of an unsorted object type
(ordered_json) defeated the shortcut for keys that share a beginning with the
previous one: four keys were enough to be quadratic.
Prefixes of less than 64 bytes are kept in a hash table and compared as
before, which costs a bounded number of bytes. Longer prefixes are kept in a
tree whose nodes are looked up by their parent and the text of one token. The
node of a prefix is found by walking the tokens of the key from the root, so
all prefixes of a key together cost one pass over the key, and no prefix is
compared as a whole. The nodes along the key walked last are kept, so keys
that share a beginning do not walk it again. The array/object decision and
the independence from the order of the keys are unchanged.
The hash function is seeded with a value derived from the address of a local
variable, which address space layout randomization changes between
processes, so keys cannot be crafted to collide in advance: with the fixed
constants, 20,000 keys of 16 + 2 bytes with the same hash took 0.5 s (random
keys, and 3.12.0: 7 ms). Where addresses are not randomized, or when the seed
is known, k short prefixes can still collide and cost k^2 / 2 comparisons of
fewer than 64 bytes. Results do not depend on the seed.
Measurements (Apple clang -O2, arm64), unflatten() of one key with n tokens:
tokens "/0" n=25k n=50k n=100k n=200k
before 175 ms 617 ms 2437 ms 9729 ms
after 4 ms 6 ms 12 ms 27 ms
3.12.0 1 ms 3 ms 5 ms 10 ms
"/0" then "/1" n=25k n=50k n=100k n=200k
before 153 ms 603 ms 2436 ms 9678 ms
after 1 ms 3 ms 5 ms 10 ms
3.12.0 1 ms 3 ms 5 ms 11 ms
ordered_json, 4 keys /0*m/a0, /10, /0*m/a1, /11
m=25k m=50k m=100k m=200k
before 397 ms 1402 ms 5191 ms 20723 ms
after 4 ms 7 ms 15 ms 33 ms
3.12.0 2 ms 3 ms 8 ms 17 ms
Ordinary documents (flatten() then unflatten(), instructions retired): within
0.3 % of before for canada, citm_catalog, twitter, jeopardy, mesh,
update-center and random from the benchmark corpus.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
3 files changed
+995
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@@ -852,139 +852,6 @@ class json_pointer
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}
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private:
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/*!
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@brief the pointer prefixes of a flattened object that denote arrays
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@ref unflatten creates arrays exactly below the prefixes that have a
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reference token 0 among their children, and objects everywhere else.
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Deciding this up front keeps the result independent of the order in which
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the flattened object is iterated, which is unspecified for some object
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types.
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A prefix is identified by the text of the key it is the beginning of:
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escaping is unambiguous, so equal prefixes of valid keys are equal texts.
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The set refers to the keys, which must outlive it.
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*/
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class array_prefix_set
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{
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public:
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void insert(const char* data, std::size_t size)
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{
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// flattened objects are usually sorted, so most keys that have a
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// 0 yield the prefix of the previous one
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if (last_data != nullptr && last_size == size && std::memcmp(last_data, data, size) == 0)
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{
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return;
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}
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last_data = data;
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last_size = size;
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if (table.empty())
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{
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table.resize(16);
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}
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const std::size_t hash = hash_of(data, size);
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const std::size_t mask = table.size() - 1;
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std::size_t i = hash & mask;
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while (table[i].data != nullptr)
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{
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if (table[i].hash == hash && table[i].size == size && std::memcmp(table[i].data, data, size) == 0)
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{
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return;
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}
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i = (i + 1) & mask;
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}
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table[i] = slot(hash, data, size);
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// keep the load factor below 1/2
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if (++used * 2 > table.size())
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{
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grow();
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}
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}
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bool empty() const noexcept
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{
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return used == 0;
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}
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bool contains(const char* data, std::size_t size) const noexcept
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{
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if (used == 0)
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{
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return false;
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}
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const std::size_t hash = hash_of(data, size);
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const std::size_t mask = table.size() - 1;
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for (std::size_t i = hash & mask; table[i].data != nullptr; i = (i + 1) & mask)
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{
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if (table[i].hash == hash && table[i].size == size && std::memcmp(table[i].data, data, size) == 0)
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{
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return true;
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}
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}
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return false;
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}
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private:
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struct slot
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{
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slot() noexcept = default;
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slot(std::size_t h, const char* d, std::size_t s) noexcept
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: hash(h), data(d), size(s)
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{}
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std::size_t hash = 0;
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const char* data = nullptr; // nullptr: the slot is empty
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std::size_t size = 0;
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};
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static std::size_t hash_of(const char* data, std::size_t size) noexcept
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{
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std::uint64_t h = 0x9E3779B97F4A7C15ULL ^ size;
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std::size_t i = 0;
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for (; i + 8 <= size; i += 8)
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{
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std::uint64_t w = 0;
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std::memcpy(&w, data + i, 8);
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h = (h ^ w) * 0x100000001B3ULL;
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h ^= h >> 32u;
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}
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std::uint64_t w = 0;
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if (i < size)
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{
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std::memcpy(&w, data + i, size - i);
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}
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h = (h ^ w) * 0x9E3779B97F4A7C15ULL;
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return detail::conditional_static_cast<std::size_t>(h ^ (h >> 29u));
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}
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void grow()
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{
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std::vector<slot> old(table.size() * 2);
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old.swap(table);
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const std::size_t mask = table.size() - 1;
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for (const auto& s : old)
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{
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if (s.data != nullptr)
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{
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std::size_t i = s.hash & mask;
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while (table[i].data != nullptr)
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{
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i = (i + 1) & mask;
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}
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table[i] = s;
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}
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}
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}
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std::vector<slot> table{};
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std::size_t used = 0;
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const char* last_data = nullptr;
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std::size_t last_size = 0;
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};
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/// @brief the length of the longest common prefix of the first @a limit bytes of @a a and @a b
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static std::size_t common_prefix_length(const char* a, const char* b, const std::size_t limit) noexcept
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{
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@@ -1007,6 +874,354 @@ class json_pointer
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return i;
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}
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/*!
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@brief the prefixes of a flattened object that are marked as arrays, for
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prefixes that are not short
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A prefix is identified by the tokens it consists of: a node of a tree
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stands for the prefix of its parent followed by a token, and is looked up
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by its parent and the text of the token. The node of a prefix is found by
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walking the tokens of the key from the root, so identifying all prefixes
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of a key together costs one pass over the key, however long the prefixes
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are, and no prefix is ever compared byte by byte as a whole. The nodes
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along the key that was walked last are kept, and the walk of the next key
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starts below the beginning it has in common with that key. The tree refers
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to the keys, which must outlive it.
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*/
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class prefix_tree
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{
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public:
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/// @brief set the seed of the hash function, before anything is added
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void reseed(const std::uint64_t seed_value) noexcept
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{
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seed = seed_value;
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}
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/// @brief mark the first @a prefix_size bytes of the key of @a key_size bytes at @a data
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void mark(const char* data, std::size_t key_size, std::size_t prefix_size)
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{
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nodes[walk(data, key_size, prefix_size)].marked = true;
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any_marked = true;
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}
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bool is_marked(const char* data, std::size_t key_size, std::size_t prefix_size)
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{
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return any_marked && nodes[walk(data, key_size, prefix_size)].marked;
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}
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bool empty() const noexcept
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{
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return !any_marked;
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}
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/// @brief the hash of the first @a size bytes at @a data, for the seed @a seed_value
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static std::size_t hash_bytes(const char* data, std::size_t size, const std::uint64_t seed_value) noexcept
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{
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std::uint64_t h = seed_value ^ size;
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std::size_t i = 0;
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for (; i + 8 <= size; i += 8)
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{
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std::uint64_t w = 0;
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std::memcpy(&w, data + i, 8);
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h = (h ^ w) * 0x100000001B3ULL;
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h ^= h >> 32u;
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}
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std::uint64_t w = 0;
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if (i < size)
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{
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std::memcpy(&w, data + i, size - i);
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}
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h = (h ^ w) * 0x9E3779B97F4A7C15ULL;
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return detail::conditional_static_cast<std::size_t>(h ^ (h >> 29u));
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}
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private:
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struct node
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{
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std::size_t parent = 0;
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std::size_t hash = 0;
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const char* token = nullptr;
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std::size_t token_size = 0;
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bool marked = false;
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};
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/// @brief a slot of the hash table; the index of a node, 0 if the slot is empty
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struct bucket
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{
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std::size_t index = 0;
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};
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/// @brief the node after a token of the key that was walked last, and where the token ends
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struct step
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{
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std::size_t node = 0;
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std::size_t end = 0;
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};
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/// @brief the index of the node of the prefix that ends at the slash (or the end) at @a prefix_size
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std::size_t walk(const char* data, std::size_t key_size, std::size_t prefix_size)
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{
|
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if (nodes.empty())
|
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{
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nodes.emplace_back();
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table.resize(16);
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chain.emplace_back();
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}
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|
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if (data != last_key)
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{
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// keep the nodes within the beginning that this key has in common
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// with the key they were found for; a prefix that ends inside a
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// token of this key is not one of its prefixes
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const std::size_t same = common_prefix_length(data, last_key, (std::min)(key_size, last_key_size));
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std::size_t keep = chain.size();
|
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while (keep > 1 && (chain[keep - 1].end > same || (chain[keep - 1].end == same && same < key_size && data[same] != '/')))
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{
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--keep;
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}
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chain.resize(keep);
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last_key = data;
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last_key_size = key_size;
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}
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// walk the tokens that are not walked yet
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while (chain.back().end < prefix_size && chain.back().end < key_size)
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{
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const std::size_t start = chain.back().end + 1;
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const void* const slash = std::memchr(data + start, '/', key_size - start);
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const std::size_t end = slash == nullptr ? key_size : static_cast<std::size_t>(static_cast<const char*>(slash) - data);
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step next;
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next.node = child(chain.back().node, data + start, end - start);
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next.end = end;
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chain.push_back(next);
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}
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|
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if (chain.back().end == prefix_size)
|
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{
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return chain.back().node;
|
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}
|
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// an earlier call walked further
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const auto it = std::lower_bound(chain.begin(), chain.end(), prefix_size, [](const step & s, std::size_t size)
|
||||
{
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return s.end < size;
|
||||
});
|
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return it->node;
|
||||
}
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||||
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||||
std::size_t slot_of(std::size_t parent, std::size_t hash) const noexcept
|
||||
{
|
||||
std::uint64_t x = static_cast<std::uint64_t>(hash) ^ (static_cast<std::uint64_t>(parent) * 0x9E3779B97F4A7C15ULL);
|
||||
x ^= x >> 32u;
|
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x *= 0xD6E8FEB86659FD93ULL;
|
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x ^= x >> 32u;
|
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return static_cast<std::size_t>(x) & (table.size() - 1);
|
||||
}
|
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|
||||
/// @brief the node of the prefix of the node @a parent followed by the token, which is added if it is not there
|
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std::size_t child(std::size_t parent, const char* token, std::size_t token_size)
|
||||
{
|
||||
const std::size_t hash = hash_bytes(token, token_size, seed);
|
||||
std::size_t i = slot_of(parent, hash);
|
||||
while (table[i].index != 0)
|
||||
{
|
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const node& n = nodes[table[i].index];
|
||||
if (n.parent == parent && n.hash == hash && n.token_size == token_size && std::memcmp(n.token, token, token_size) == 0)
|
||||
{
|
||||
return table[i].index;
|
||||
}
|
||||
i = (i + 1) & (table.size() - 1);
|
||||
}
|
||||
|
||||
node created;
|
||||
created.parent = parent;
|
||||
created.hash = hash;
|
||||
created.token = token;
|
||||
created.token_size = token_size;
|
||||
nodes.push_back(created);
|
||||
table[i].index = nodes.size() - 1;
|
||||
|
||||
// keep the load factor below 1/2
|
||||
if (nodes.size() * 2 > table.size())
|
||||
{
|
||||
std::vector<bucket> old(table.size() * 2);
|
||||
old.swap(table);
|
||||
for (std::size_t k = 1; k < nodes.size(); ++k)
|
||||
{
|
||||
std::size_t j = slot_of(nodes[k].parent, nodes[k].hash);
|
||||
while (table[j].index != 0)
|
||||
{
|
||||
j = (j + 1) & (table.size() - 1);
|
||||
}
|
||||
table[j].index = k;
|
||||
}
|
||||
}
|
||||
return nodes.size() - 1;
|
||||
}
|
||||
|
||||
std::uint64_t seed = 0;
|
||||
/// @brief the nodes; the first is the root, the empty prefix
|
||||
std::vector<node> nodes{};
|
||||
/// @brief the nodes (not the root) in a hash table
|
||||
std::vector<bucket> table{};
|
||||
/// @brief the root, and the node after each token of the key that was walked last
|
||||
std::vector<step> chain{};
|
||||
const char* last_key = nullptr;
|
||||
std::size_t last_key_size = 0;
|
||||
bool any_marked = false;
|
||||
};
|
||||
|
||||
/*!
|
||||
@brief the pointer prefixes of a flattened object that denote arrays
|
||||
|
||||
@ref unflatten creates arrays exactly below the prefixes that have a
|
||||
reference token 0 among their children, and objects everywhere else.
|
||||
Deciding this up front keeps the result independent of the order in which
|
||||
the flattened object is iterated, which is unspecified for some object
|
||||
types.
|
||||
|
||||
A prefix is identified by the text of the key it is the beginning of:
|
||||
escaping is unambiguous, so equal prefixes of valid keys are equal texts.
|
||||
The set refers to the keys, which must outlive it. Short prefixes are
|
||||
kept in a hash table and compared as texts, which costs at most the length
|
||||
of the bound. Longer prefixes are kept in a @ref prefix_tree, because
|
||||
hashing and comparing every one of the prefixes of a key with many tokens
|
||||
from its beginning would take quadratic time.
|
||||
|
||||
The hash function depends on a seed that is derived from the address of a
|
||||
local variable, which address space layout randomization changes between
|
||||
processes, so keys cannot be crafted to collide in advance; the result
|
||||
does not depend on the seed. Where the addresses are not randomized, or
|
||||
when the seed is known, keys can collide: k short prefixes then cost
|
||||
k^2 / 2 comparisons of fewer than 64 bytes each. The long prefixes are not
|
||||
hashed as a whole.
|
||||
*/
|
||||
class array_prefix_set
|
||||
{
|
||||
public:
|
||||
/// @brief choose the seed of the hash function, before anything is added: it is derived from the address that is passed
|
||||
void reseed(const void* address) noexcept
|
||||
{
|
||||
// NOLINTNEXTLINE(cppcoreguidelines-pro-type-reinterpret-cast): the address is only a source of entropy
|
||||
std::uint64_t x = static_cast<std::uint64_t>(reinterpret_cast<std::uintptr_t>(address)) ^ 0x9E3779B97F4A7C15ULL;
|
||||
x *= 0xBF58476D1CE4E5B9ULL;
|
||||
seed = x ^ (x >> 31u);
|
||||
tree.reseed(seed);
|
||||
}
|
||||
|
||||
/// @brief add the first @a prefix_size bytes of the key of @a key_size bytes at @a data
|
||||
void insert(const char* data, std::size_t key_size, std::size_t prefix_size)
|
||||
{
|
||||
if (prefix_size >= short_limit)
|
||||
{
|
||||
tree.mark(data, key_size, prefix_size);
|
||||
return;
|
||||
}
|
||||
|
||||
// flattened objects are usually sorted, so most keys that have a
|
||||
// 0 yield the prefix of the previous one
|
||||
if (last_data != nullptr && last_size == prefix_size && std::memcmp(last_data, data, prefix_size) == 0)
|
||||
{
|
||||
return;
|
||||
}
|
||||
last_data = data;
|
||||
last_size = prefix_size;
|
||||
|
||||
const std::size_t hash = prefix_tree::hash_bytes(data, prefix_size, seed);
|
||||
if (table.empty())
|
||||
{
|
||||
table.resize(16);
|
||||
}
|
||||
const std::size_t mask = table.size() - 1;
|
||||
std::size_t i = hash & mask;
|
||||
while (table[i].data != nullptr)
|
||||
{
|
||||
if (table[i].hash == hash && table[i].size == prefix_size && std::memcmp(table[i].data, data, prefix_size) == 0)
|
||||
{
|
||||
return;
|
||||
}
|
||||
i = (i + 1) & mask;
|
||||
}
|
||||
table[i] = slot(hash, data, prefix_size);
|
||||
|
||||
// keep the load factor below 1/2
|
||||
if (++used * 2 > table.size())
|
||||
{
|
||||
grow();
|
||||
}
|
||||
}
|
||||
|
||||
bool empty() const noexcept
|
||||
{
|
||||
return used == 0 && tree.empty();
|
||||
}
|
||||
|
||||
/// @brief whether the first @a prefix_size bytes of the key of @a key_size bytes at @a data were added
|
||||
bool contains(const char* data, std::size_t key_size, std::size_t prefix_size)
|
||||
{
|
||||
if (prefix_size >= short_limit)
|
||||
{
|
||||
return tree.is_marked(data, key_size, prefix_size);
|
||||
}
|
||||
if (used == 0)
|
||||
{
|
||||
return false;
|
||||
}
|
||||
const std::size_t hash = prefix_tree::hash_bytes(data, prefix_size, seed);
|
||||
const std::size_t mask = table.size() - 1;
|
||||
for (std::size_t i = hash & mask; table[i].data != nullptr; i = (i + 1) & mask)
|
||||
{
|
||||
if (table[i].hash == hash && table[i].size == prefix_size && std::memcmp(table[i].data, data, prefix_size) == 0)
|
||||
{
|
||||
return true;
|
||||
}
|
||||
}
|
||||
return false;
|
||||
}
|
||||
|
||||
private:
|
||||
/// @brief prefixes of fewer bytes are kept in the hash table
|
||||
static constexpr std::size_t short_limit = 64;
|
||||
|
||||
struct slot
|
||||
{
|
||||
slot() noexcept = default;
|
||||
|
||||
slot(std::size_t h, const char* d, std::size_t s) noexcept
|
||||
: hash(h), data(d), size(s)
|
||||
{}
|
||||
|
||||
std::size_t hash = 0;
|
||||
const char* data = nullptr; // nullptr: the slot is empty
|
||||
std::size_t size = 0;
|
||||
};
|
||||
|
||||
void grow()
|
||||
{
|
||||
std::vector<slot> old(table.size() * 2);
|
||||
old.swap(table);
|
||||
const std::size_t mask = table.size() - 1;
|
||||
for (const auto& s : old)
|
||||
{
|
||||
if (s.data != nullptr)
|
||||
{
|
||||
std::size_t i = s.hash & mask;
|
||||
while (table[i].data != nullptr)
|
||||
{
|
||||
i = (i + 1) & mask;
|
||||
}
|
||||
table[i] = s;
|
||||
}
|
||||
}
|
||||
}
|
||||
|
||||
std::uint64_t seed = 0;
|
||||
prefix_tree tree{};
|
||||
std::vector<slot> table{};
|
||||
std::size_t used = 0;
|
||||
const char* last_data = nullptr;
|
||||
std::size_t last_size = 0;
|
||||
};
|
||||
|
||||
/*!
|
||||
@brief check that @a key is a valid JSON pointer, and add the prefixes of
|
||||
it that denote arrays to @a arrays
|
||||
@@ -1052,7 +1267,7 @@ class json_pointer
|
||||
// a token 0 makes the pointer before it an array
|
||||
if (i - begin == 1 && data[begin] == '0')
|
||||
{
|
||||
arrays.insert(data, begin - 1);
|
||||
arrays.insert(data, size, begin - 1);
|
||||
}
|
||||
begin = i + 1;
|
||||
}
|
||||
@@ -2062,6 +2277,7 @@ class json_pointer
|
||||
// token 0 among their children; the values below them are arrays,
|
||||
// all others are objects (see array_prefix_set)
|
||||
array_prefix_set arrays;
|
||||
arrays.reseed(&arrays);
|
||||
const string_t* previous_key = nullptr;
|
||||
for (const auto& element : *value.m_data.m_value.object)
|
||||
{
|
||||
@@ -2153,7 +2369,7 @@ class json_pointer
|
||||
case detail::value_t::null:
|
||||
{
|
||||
// a first child 0 makes the value an array without a look-up
|
||||
if ((token_size == 1 && data[pos + 1] == '0') || arrays.contains(data, pos))
|
||||
if ((token_size == 1 && data[pos + 1] == '0') || arrays.contains(data, key.size(), pos))
|
||||
{
|
||||
// some reference token below this position is 0, so the
|
||||
// value is an array
|
||||
|
||||
Reference in new issue
Block a user