gecko-dev/js/js2/hash.cpp

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C++

// -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
//
// The contents of this file are subject to the Netscape Public
// License Version 1.1 (the "License"); you may not use this file
// except in compliance with the License. You may obtain a copy of
// the License at http://www.mozilla.org/NPL/
//
// Software distributed under the License is distributed on an "AS
// IS" basis, WITHOUT WARRANTY OF ANY KIND, either express oqr
// implied. See the License for the specific language governing
// rights and limitations under the License.
//
// The Original Code is the JavaScript 2 Prototype.
//
// The Initial Developer of the Original Code is Netscape
// Communications Corporation. Portions created by Netscape are
// Copyright (C) 1998 Netscape Communications Corporation. All
// Rights Reserved.
#include "hash.h"
#include <new>
namespace JS = JavaScript;
//
// Hash Codes
//
// General-purpose null-terminated C string hash function
JS::HashNumber JS::hashString(const char *s)
{
HashNumber h = 0;
uchar ch;
while ((ch = (uchar)*s++) != 0)
h = (h >> 28) ^ (h << 4) ^ ch;
return h;
}
// General-purpose String hash function
JS::HashNumber JS::hashString(const String &s)
{
HashNumber h = 0;
String::const_iterator p = s.begin();
String::size_type n = s.size();
if (n < 16)
// Hash every character in a short string.
while (n--)
h = (h >> 28) ^ (h << 4) ^ *p++;
else
// Sample a la java.lang.String.hash().
for (String::size_type m = n / 8; n >= m; p += m, n -= m)
h = (h >> 28) ^ (h << 4) ^ *p;
return h;
}
//
// Hash Tables
//
const uint minLgNBuckets = 4;
JS::GenericHashTableIterator::GenericHashTableIterator(GenericHashTable &ht):
ht(ht), entry(0), nextBucket(ht.buckets)
{
DEBUG_ONLY(++ht.nReferences);
operator++();
}
JS::GenericHashTableIterator &
JS::GenericHashTableIterator::operator++()
{
GenericHashEntry *e = entry;
if (e) {
backpointer = &e->next;
e = e->next;
}
if (!e) {
GenericHashEntry **const bucketsEnd = ht.bucketsEnd;
GenericHashEntry **bucket = nextBucket;
while (bucket != bucketsEnd) {
e = *bucket++;
if (e) {
backpointer = bucket-1;
break;
}
}
nextBucket = bucket;
}
entry = e;
return *this;
}
JS::GenericHashTable::GenericHashTable(uint32 nEntriesDefault):
nEntries(0)
{
DEBUG_ONLY(nReferences = 0);
uint lgNBuckets = ceilingLog2(nEntriesDefault);
if (lgNBuckets < minLgNBuckets)
lgNBuckets = minLgNBuckets;
defaultLgNBuckets = lgNBuckets;
recomputeMinMaxNEntries(lgNBuckets);
uint32 nBuckets = JS_BIT(lgNBuckets);
buckets = new GenericHashEntry*[nBuckets];
// No exceptions after this point unless buckets is deleted.
bucketsEnd = buckets + nBuckets;
zero(buckets, bucketsEnd);
}
// Initialize shift, minNEntries, and maxNEntries based on the lg2 of the
// number of buckets.
void JS::GenericHashTable::recomputeMinMaxNEntries(uint lgNBuckets)
{
uint32 nBuckets = JS_BIT(lgNBuckets);
shift = 32 - lgNBuckets;
maxNEntries = nBuckets; // Maximum ratio is 100%
minNEntries = lgNBuckets <= defaultLgNBuckets ? 0 : 3*(nBuckets>>3); // Minimum ratio is 37.5%
}
// Rehash the table. This method cannot throw out-of-memory exceptions, so it is
// safe to call from a destructor.
void JS::GenericHashTable::rehash()
{
uint32 newLgNBuckets = ceilingLog2(nEntries);
if (newLgNBuckets < defaultLgNBuckets)
newLgNBuckets = defaultLgNBuckets;
uint32 newNBuckets = JS_BIT(newLgNBuckets);
try {
GenericHashEntry **newBuckets = new GenericHashEntry*[newNBuckets];
// No exceptions after this point.
GenericHashEntry **newBucketsEnd = newBuckets + newNBuckets;
zero(newBuckets, newBucketsEnd);
recomputeMinMaxNEntries(newLgNBuckets);
GenericHashEntry **be = bucketsEnd;
for (GenericHashEntry **b = buckets; b != be; b++) {
GenericHashEntry *e = *b;
while (e) {
GenericHashEntry *next = e->next;
// Place e in the new set of buckets.
GenericHashEntry **nb = newBuckets + (e->keyHash*goldenRatio >> shift);
e->next = *nb;
*nb = e;
e = next;
}
}
buckets = newBuckets;
bucketsEnd = newBucketsEnd;
} catch (std::bad_alloc) {
// Out of memory. Ignore the error and just relax the resizing boundaries.
if (buckets + JS_BIT(newLgNBuckets) > bucketsEnd)
maxNEntries >>= 1;
else
minNEntries <<= 1;
}
}