116 lines
4.2 KiB
C
116 lines
4.2 KiB
C
/*
|
|
* String hash computation (interning).
|
|
*
|
|
* String hashing is performance critical because a string hash is computed
|
|
* for all new strings which are candidates to be added to the string table.
|
|
* However, strings actually added to the string table go through a codepoint
|
|
* length calculation which dominates performance because it goes through
|
|
* every byte of the input string (but only for strings added).
|
|
*
|
|
* The string hash algorithm should be fast, but on the other hand provide
|
|
* good enough hashes to ensure both string table and object property table
|
|
* hash tables work reasonably well (i.e., there aren't too many collisions
|
|
* with real world inputs). Unless the hash is cryptographic, it's always
|
|
* possible to craft inputs with maximal hash collisions.
|
|
*
|
|
* NOTE: The hash algorithms must match tools/dukutil.py:duk_heap_hashstring()
|
|
* for ROM string support!
|
|
*/
|
|
|
|
#include "duk_internal.h"
|
|
|
|
#if defined(DUK_USE_STRHASH_DENSE)
|
|
/* Constants for duk_hashstring(). */
|
|
#define DUK__STRHASH_SHORTSTRING 4096L
|
|
#define DUK__STRHASH_MEDIUMSTRING (256L * 1024L)
|
|
#define DUK__STRHASH_BLOCKSIZE 256L
|
|
|
|
DUK_INTERNAL duk_uint32_t duk_heap_hashstring(duk_heap *heap, const duk_uint8_t *str, duk_size_t len) {
|
|
duk_uint32_t hash;
|
|
|
|
/* Use Murmurhash2 directly for short strings, and use "block skipping"
|
|
* for long strings: hash an initial part and then sample the rest of
|
|
* the string with reasonably sized chunks. An initial offset for the
|
|
* sampling is computed based on a hash of the initial part of the string;
|
|
* this is done to (usually) avoid the case where all long strings have
|
|
* certain offset ranges which are never sampled.
|
|
*
|
|
* Skip should depend on length and bound the total time to roughly
|
|
* logarithmic. With current values:
|
|
*
|
|
* 1M string => 256 * 241 = 61696 bytes (0.06M) of hashing
|
|
* 1G string => 256 * 16321 = 4178176 bytes (3.98M) of hashing
|
|
*
|
|
* XXX: It would be better to compute the skip offset more "smoothly"
|
|
* instead of having a few boundary values.
|
|
*/
|
|
|
|
/* note: mixing len into seed improves hashing when skipping */
|
|
duk_uint32_t str_seed = heap->hash_seed ^ ((duk_uint32_t) len);
|
|
|
|
if (len <= DUK__STRHASH_SHORTSTRING) {
|
|
hash = duk_util_hashbytes(str, len, str_seed);
|
|
} else {
|
|
duk_size_t off;
|
|
duk_size_t skip;
|
|
|
|
if (len <= DUK__STRHASH_MEDIUMSTRING) {
|
|
skip = (duk_size_t) (16 * DUK__STRHASH_BLOCKSIZE + DUK__STRHASH_BLOCKSIZE);
|
|
} else {
|
|
skip = (duk_size_t) (256 * DUK__STRHASH_BLOCKSIZE + DUK__STRHASH_BLOCKSIZE);
|
|
}
|
|
|
|
hash = duk_util_hashbytes(str, (duk_size_t) DUK__STRHASH_SHORTSTRING, str_seed);
|
|
off = DUK__STRHASH_SHORTSTRING + (skip * (hash % 256)) / 256;
|
|
|
|
/* XXX: inefficient loop */
|
|
while (off < len) {
|
|
duk_size_t left = len - off;
|
|
duk_size_t now = (duk_size_t) (left > DUK__STRHASH_BLOCKSIZE ? DUK__STRHASH_BLOCKSIZE : left);
|
|
hash ^= duk_util_hashbytes(str + off, now, str_seed);
|
|
off += skip;
|
|
}
|
|
}
|
|
|
|
#if defined(DUK_USE_STRHASH16)
|
|
/* Truncate to 16 bits here, so that a computed hash can be compared
|
|
* against a hash stored in a 16-bit field.
|
|
*/
|
|
hash &= 0x0000ffffUL;
|
|
#endif
|
|
return hash;
|
|
}
|
|
#else /* DUK_USE_STRHASH_DENSE */
|
|
DUK_INTERNAL duk_uint32_t duk_heap_hashstring(duk_heap *heap, const duk_uint8_t *str, duk_size_t len) {
|
|
duk_uint32_t hash;
|
|
duk_size_t step;
|
|
duk_size_t off;
|
|
|
|
/* Slightly modified "Bernstein hash" from:
|
|
*
|
|
* http://eternallyconfuzzled.com/tuts/algorithms/jsw_tut_hashing.aspx
|
|
*
|
|
* Modifications: string skipping and reverse direction similar to
|
|
* Lua 5.1.5, and different hash initializer.
|
|
*
|
|
* The reverse direction ensures last byte it always included in the
|
|
* hash which is a good default as changing parts of the string are
|
|
* more often in the suffix than in the prefix.
|
|
*/
|
|
|
|
hash = heap->hash_seed ^ ((duk_uint32_t) len); /* Bernstein hash init value is normally 5381 */
|
|
step = (len >> DUK_USE_STRHASH_SKIP_SHIFT) + 1;
|
|
for (off = len; off >= step; off -= step) {
|
|
DUK_ASSERT(off >= 1); /* off >= step, and step >= 1 */
|
|
hash = (hash * 33) + str[off - 1];
|
|
}
|
|
|
|
#if defined(DUK_USE_STRHASH16)
|
|
/* Truncate to 16 bits here, so that a computed hash can be compared
|
|
* against a hash stored in a 16-bit field.
|
|
*/
|
|
hash &= 0x0000ffffUL;
|
|
#endif
|
|
return hash;
|
|
}
|
|
#endif /* DUK_USE_STRHASH_DENSE */
|