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Teach hashmap to allow rehashes to be suppressed.
This is useful when hashmaps are accessed by multiple
threads. It still requires the caller to properly
manage their locking. This just prevents unexpected
rehashing during inserts and deletes.
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Add variant of memihash() to allow the hash computation to
be continued. There are times when we compute the hash on
a full path and then the hash on just the path to the parent
directory. This can be expensive on large repositories.
With this, we can hash the parent directory first. And then
continue the computation to include the "/filename".
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Using FLEX_ARRAY macros reduces the amount of manual
computation size we have to do. It also ensures we don't
overflow size_t, and it makes sure we write the same number
of bytes that we allocated.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Interning short strings with high probability of duplicates can reduce the
memory footprint and speed up comparisons.
Add strintern() and memintern() APIs that use a hashmap to manage the pool
of unique, interned strings.
Note: strintern(getenv()) could be used to sanitize git's use of getenv(),
in case we ever encounter a platform where a call to getenv() invalidates
previous getenv() results (which is allowed by POSIX).
Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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The existing hashtable implementation (in hash.[ch]) uses open addressing
(i.e. resolve hash collisions by distributing entries across the table).
Thus, removal is difficult to implement with less than O(n) complexity.
Resolving collisions of entries with identical hashes (e.g. via chaining)
is left to the client code.
Add a hashtable implementation that supports O(1) removal and is slightly
easier to use due to builtin entry chaining.
Supports all basic operations init, free, get, add, remove and iteration.
Also includes ready-to-use hash functions based on the public domain FNV-1
algorithm (http://www.isthe.com/chongo/tech/comp/fnv).
The per-entry data structure (hashmap_entry) is piggybacked in front of
the client's data structure to save memory. See test-hashmap.c for usage
examples.
The hashtable is resized by a factor of four when 80% full. With these
settings, average memory consumption is about 2/3 of hash.[ch], and
insertion is about twice as fast due to less frequent resizing.
Lookups are also slightly faster, because entries are strictly confined to
their bucket (i.e. no data of other buckets needs to be traversed).
Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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