pjs/js/js2/hash.cpp

// -*- 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;
	}
}