If the utilization of a HashTable (size vs capacity) goes below 20%,
we'll now shrink the table down to capacity = (size * 2).
This fixes an issue where tables would grow infinitely when inserting
and removing keys repeatedly. Basically, we would accumulate deleted
buckets with nothing reclaiming them, and eventually deciding that we
needed to grow the table (because we grow if used+deleted > limit!)
I found this because HashTable iteration was taking a suspicious amount
of time in Core::EventLoop::get_next_timer_expiration(). Turns out the
timer table kept growing in capacity over time. That made iteration
slower and slower since HashTable iterators visit every bucket.
Just walk the table from start to finish, deleting buckets as we go.
This removes the need for remove() to return an iterator, which is
preventing me from implementing hash table auto-shrinking.
This was used in `HashMap::try_ensure_capacity`, but was missing from
`HashTable`s implementation. No one had used
`HashMap::try_ensure_capacity` before so it went unnoticed!
This will allow us to avoid some potentially expensive type conversion
during lookup, like form String to StringView, which would allocate
memory otherwise.
Example failure:
IDAllocator.h only pulls in AK/Hashtable.h, so any compilation unit that
includes AK/IDAllocator.h without including AK/Traits.h before it used
to be doomed to fail with the cryptic error message "In instantiation of
'AK::HashTable<T, TraitsForT, IsOrdered>::Iterator AK::HashTable<T,
TraitsForT, IsOrdered>::find(const T&) [with T = int; TraitsForT =
AK::Traits: incomplete type 'AK::Traits<int>' used in nested name
specifier".
This implements the macOS API malloc_good_size() which returns the
true allocation size for a given requested allocation size. This
allows us to make use of all the available memory in a malloc chunk.
For example, for a malloc request of 35 bytes our malloc would
internally use a chunk of size 64, however the remaining 29 bytes
would be unused.
Knowing the true allocation size allows us to request more usable
memory that would otherwise be wasted and make that available for
Vector, HashTable and potentially other callers in the future.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
The old approach was more complex and also had a very bad edge case
with lots of collisions. This approach eliminates that possiblility.
It also makes both reading and writing lookups a little bit faster.
(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
Problem:
- Using regular functions rather than function templates results in
the arguments not being deduced. This then requires the same
function to be written multiple times and for `move` to be used
rather than `forward`.
Solution:
- Collapse multiple function overloads to a single function template
with a deduced argument. This allows the argument to be a forwarding
reference and bind to either an l-value or r-value and forward the
value.
Problem:
- Many constructors are defined as `{}` rather than using the ` =
default` compiler-provided constructor.
- Some types provide an implicit conversion operator from `nullptr_t`
instead of requiring the caller to default construct. This violates
the C++ Core Guidelines suggestion to declare single-argument
constructors explicit
(https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c46-by-default-declare-single-argument-constructors-explicit).
Solution:
- Change default constructors to use the compiler-provided default
constructor.
- Remove implicit conversion operators from `nullptr_t` and change
usage to enforce type consistency without conversion.
Double the capacity when used+deleted buckets crosses 60% of capacity.
This appears to be a sweet spot for performance based on some ad-hoc
testing with test-js. :^)
Instead of each hash bucket being a SinglyLinkedList, switch to using
closed hashing (open addressing). Buckets are chained together via
double hashing (hashing the hash until we find an unused bucket.)
This greatly reduces malloc traffic, since each added element no longer
allocates a new linked list node.
Appears performance neutral on test-js. Can definitely be tuned and
could use proper management of load factor, etc.
This allows performing an action based on whether something
was actually added or removed without having to look it up
prior to calling set() or remove().
This commit replaces SinglyLinkedListIterator::universal_end() with an
empty SinglyLinkedListIterator(). Piano needs this in order to
initialize a member array of iterators without 84 lines of
universal_end().
This was only used by HashTable::dump() which I used when doing the
first HashTable implementation. Removing this allows us to also remove
most includes of <AK/kstdio.h>.
As suggested by Joshua, this commit adds the 2-clause BSD license as a
comment block to the top of every source file.
For the first pass, I've just added myself for simplicity. I encourage
everyone to add themselves as copyright holders of any file they've
added or modified in some significant way. If I've added myself in
error somewhere, feel free to replace it with the appropriate copyright
holder instead.
Going forward, all new source files should include a license header.
It doesn't seem sane to try to iterate over a HashTable while it's in
the middle of being cleared. Since this might cause strange problems,
this patch adds an assertion if an iterator is constructed during
clear() or rehash() of a HashTable.
Solve this by adding find() overloads to HashTable and SinglyLinkedList
that take a templated functor for comparing the values.
This allows HashMap to call HashTable::find() without having to create
a temporary Entry for use as the table key. :^)
