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Short strings always use all bytes in the hash

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Collisions in short strings occurr just by their existence, when
internalizing them. (Collisions in long strings is caused/controlled
by the program, when adding them as keys to the same table.)
This commit is contained in:
Roberto Ierusalimschy
2020-04-01 10:52:41 -03:00
parent 513559cc47
commit 7288528a1e
3 changed files with 10 additions and 7 deletions
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12
lstring.c
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@@ -23,7 +23,7 @@
/*
** Lua will use at most ~(2^LUAI_HASHLIMIT) bytes from a string to
** Lua will use at most ~(2^LUAI_HASHLIMIT) bytes from a long string to
** compute its hash
*/
#if !defined(LUAI_HASHLIMIT)
@@ -50,9 +50,9 @@ int luaS_eqlngstr (TString *a, TString *b) {
}
unsigned int luaS_hash (const char *str, size_t l, unsigned int seed) {
unsigned int luaS_hash (const char *str, size_t l, unsigned int seed,
size_t step) {
unsigned int h = seed ^ cast_uint(l);
size_t step = (l >> LUAI_HASHLIMIT) + 1;
for (; l >= step; l -= step)
h ^= ((h<<5) + (h>>2) + cast_byte(str[l - 1]));
return h;
@@ -62,7 +62,9 @@ unsigned int luaS_hash (const char *str, size_t l, unsigned int seed) {
unsigned int luaS_hashlongstr (TString *ts) {
lua_assert(ts->tt == LUA_VLNGSTR);
if (ts->extra == 0) { /* no hash? */
ts->hash = luaS_hash(getstr(ts), ts->u.lnglen, ts->hash);
size_t len = ts->u.lnglen;
size_t step = (len >> LUAI_HASHLIMIT) + 1;
ts->hash = luaS_hash(getstr(ts), len, ts->hash, step);
ts->extra = 1; /* now it has its hash */
}
return ts->hash;
@@ -199,7 +201,7 @@ static TString *internshrstr (lua_State *L, const char *str, size_t l) {
TString *ts;
global_State *g = G(L);
stringtable *tb = &g->strt;
unsigned int h = luaS_hash(str, l, g->seed);
unsigned int h = luaS_hash(str, l, g->seed, 1);
TString **list = &tb->hash[lmod(h, tb->size)];
lua_assert(str != NULL); /* otherwise 'memcmp'/'memcpy' are undefined */
for (ts = *list; ts != NULL; ts = ts->u.hnext) {