src.java.org.codehaus.jackson.sym.SymbolTable Maven / Gradle / Ivy
package org.codehaus.jackson.sym;
/**
* This class is a kind of specialized type-safe Map, from char array to
* String value. Specialization means that in addition to type-safety
* and specific access patterns (key char array, Value optionally interned
* String; values added on access if necessary), and that instances are
* meant to be used concurrently, but by using well-defined mechanisms
* to obtain such concurrently usable instances. Main use for the class
* is to store symbol table information for things like compilers and
* parsers; especially when number of symbols (keywords) is limited.
*
* For optimal performance, usage pattern should be one where matches
* should be very common (esp. after "warm-up"), and as with most hash-based
* maps/sets, that hash codes are uniformly distributed. Also, collisions
* are slightly more expensive than with HashMap or HashSet, since hash codes
* are not used in resolving collisions; that is, equals() comparison is
* done with all symbols in same bucket index.
* Finally, rehashing is also more expensive, as hash codes are not
* stored; rehashing requires all entries' hash codes to be recalculated.
* Reason for not storing hash codes is reduced memory usage, hoping
* for better memory locality.
*
* Usual usage pattern is to create a single "master" instance, and either
* use that instance in sequential fashion, or to create derived "child"
* instances, which after use, are asked to return possible symbol additions
* to master instance. In either case benefit is that symbol table gets
* initialized so that further uses are more efficient, as eventually all
* symbols needed will already be in symbol table. At that point no more
* Symbol String allocations are needed, nor changes to symbol table itself.
*
* Note that while individual SymbolTable instances are NOT thread-safe
* (much like generic collection classes), concurrently used "child"
* instances can be freely used without synchronization. However, using
* master table concurrently with child instances can only be done if
* access to master instance is read-only (ie. no modifications done).
*/
public final class SymbolTable
{
/**
* Default initial table size. Shouldn't be miniscule (as there's
* cost to both array realloc and rehashing), but let's keep
* it reasonably small nonetheless. For systems that properly
* reuse factories it doesn't matter either way; but when
* recreating factories often, initial overhead may dominate.
*/
protected static final int DEFAULT_TABLE_SIZE = 64;
/**
* Config setting that determines whether Strings to be added need to be
* interned before being added or not. Forcing intern()ing will add
* some overhead when adding new Strings, but may be beneficial if such
* Strings are generally used by other parts of system. Note that even
* without interning, all returned String instances are guaranteed
* to be comparable with equality (==) operator; it's just that such
* guarantees are not made for Strings other classes return.
*/
protected static final boolean INTERN_STRINGS = true;
/**
* Let's limit max size to 3/4 of 8k; this corresponds
* to 32k main hash index. This should allow for enough distinct
* names for almost any case.
*/
final static int MAX_SYMBOL_TABLE_SIZE = 6000;
final static SymbolTable sBootstrapSymbolTable;
static {
sBootstrapSymbolTable = new SymbolTable(DEFAULT_TABLE_SIZE);
}
/*
////////////////////////////////////////
// Configuration:
////////////////////////////////////////
*/
/**
* Sharing of learnt symbols is done by optional linking of symbol
* table instances with their parents. When parent linkage is
* defined, and child instance is released (call to release),
* parent's shared tables may be updated from the child instance.
*/
protected SymbolTable mParent;
/*
////////////////////////////////////////
// Actual symbol table data:
////////////////////////////////////////
*/
/**
* Primary matching symbols; it's expected most match occur from
* here.
*/
protected String[] mSymbols;
/**
* Overflow buckets; if primary doesn't match, lookup is done
* from here.
*
* Note: Number of buckets is half of number of symbol entries, on
* assumption there's less need for buckets.
*/
protected Bucket[] mBuckets;
/**
* Current size (number of entries); needed to know if and when
* rehash.
*/
protected int mSize;
/**
* Limit that indicates maximum size this instance can hold before
* it needs to be expanded and rehashed. Calculated using fill
* factor passed in to constructor.
*/
protected int mSizeThreshold;
/**
* Mask used to get index from hash values; equal to
* mBuckets.length - 1, when mBuckets.length is
* a power of two.
*/
protected int mIndexMask;
/*
////////////////////////////////////////
// Information about concurrency
////////////////////////////////////////
*/
/**
* Flag that indicates if any changes have been made to the data;
* used to both determine if bucket array needs to be copied when
* (first) change is made, and potentially if updated bucket list
* is to be resync'ed back to master instance.
*/
protected boolean mDirty;
/*
////////////////////////////////////////
// Life-cycle:
////////////////////////////////////////
*/
public static SymbolTable createRoot()
{
return sBootstrapSymbolTable.makeOrphan();
}
/**
* Main method for constructing a master symbol table instance; will
* be called by other public constructors.
*
* @param initialSize Minimum initial size for bucket array; internally
* will always use a power of two equal to or bigger than this value.
*/
public SymbolTable(int initialSize)
{
// And we'll also set flags so no copying of buckets is needed:
mDirty = true;
// No point in requesting funny initial sizes...
if (initialSize < 1) {
throw new IllegalArgumentException("Can not use negative/zero initial size: "+initialSize);
}
/* Initial size has to be a power of two. And it shouldn't
* be ridiculously small either
*/
{
int currSize = 4;
while (currSize < initialSize) {
currSize += currSize;
}
initialSize = currSize;
}
initTables(initialSize);
}
private void initTables(int initialSize)
{
mSymbols = new String[initialSize];
mBuckets = new Bucket[initialSize >> 1];
// Mask is easy to calc for powers of two.
mIndexMask = initialSize - 1;
mSize = 0;
// Hard-coded fill factor is 75%
mSizeThreshold = (initialSize - (initialSize >> 2));
}
/**
* Internal constructor used when creating child instances.
*/
private SymbolTable(SymbolTable parent,
String[] symbols, Bucket[] buckets, int size)
{
mParent = parent;
mSymbols = symbols;
mBuckets = buckets;
mSize = size;
// Hard-coded fill factor, 75%
int arrayLen = (symbols.length);
mSizeThreshold = arrayLen - (arrayLen >> 2);
mIndexMask = (arrayLen - 1);
// Need to make copies of arrays, if/when adding new entries
mDirty = false;
}
/**
* "Factory" method; will create a new child instance of this symbol
* table. It will be a copy-on-write instance, ie. it will only use
* read-only copy of parent's data, but when changes are needed, a
* copy will be created.
*
* Note: while this method is synchronized, it is generally not
* safe to both use makeChild/mergeChild, AND to use instance
* actively. Instead, a separate 'root' instance should be used
* on which only makeChild/mergeChild are called, but instance itself
* is not used as a symbol table.
*/
public synchronized SymbolTable makeChild()
{
return new SymbolTable(this, mSymbols, mBuckets, mSize);
}
private SymbolTable makeOrphan()
{
return new SymbolTable(null, mSymbols, mBuckets, mSize);
}
/**
* Method that allows contents of child table to potentially be
* "merged in" with contents of this symbol table.
*
* Note that caller has to make sure symbol table passed in is
* really a child or sibling of this symbol table.
*/
private synchronized void mergeChild(SymbolTable child)
{
/* One caveat: let's try to avoid problems with
* degenerate cases of documents with generated "random"
* names: for these, symbol tables would bloat indefinitely.
* One way to do this is to just purge tables if they grow
* too large, and that's what we'll do here.
*/
if (child.size() > MAX_SYMBOL_TABLE_SIZE) {
/* Should there be a way to get notified about this
* event, to log it or such? (as it's somewhat abnormal
* thing to happen)
*/
// At any rate, need to clean up the tables, then:
initTables(DEFAULT_TABLE_SIZE);
} else {
/* Otherwise, we'll merge changed stuff in, if there are
* more entries (which may not be the case if one of siblings
* has added symbols first or such)
*/
if (child.size() <= size()) { // nothing to add
return;
}
// Okie dokie, let's get the data in!
mSymbols = child.mSymbols;
mBuckets = child.mBuckets;
mSize = child.mSize;
mSizeThreshold = child.mSizeThreshold;
mIndexMask = child.mIndexMask;
}
/* Dirty flag... well, let's just clear it, to force copying just
* in case. Shouldn't really matter, for master tables.
* (which this is, given something is merged to it etc)
*/
mDirty = false;
}
public void release()
{
// If nothing has been added, nothing to do
if (!maybeDirty()) {
return;
}
if (mParent != null) {
mParent.mergeChild(this);
/* Let's also mark this instance as dirty, so that just in
* case release was too early, there's no corruption
* of possibly shared data.
*/
mDirty = false;
}
}
/*
////////////////////////////////////////////////////
// Public API, generic accessors:
////////////////////////////////////////////////////
*/
public int size() { return mSize; }
public boolean maybeDirty() { return mDirty; }
/*
////////////////////////////////////////////////////
// Public API, accessing symbols:
////////////////////////////////////////////////////
*/
public String findSymbol(char[] buffer, int start, int len, int hash)
{
if (len < 1) { // empty Strings are simplest to handle up front
return "";
}
hash &= mIndexMask;
String sym = mSymbols[hash];
// Optimal case; checking existing primary symbol for hash index:
if (sym != null) {
// Let's inline primary String equality checking:
if (sym.length() == len) {
int i = 0;
do {
if (sym.charAt(i) != buffer[start+i]) {
break;
}
} while (++i < len);
// Optimal case; primary match found
if (i == len) {
return sym;
}
}
// How about collision bucket?
Bucket b = mBuckets[hash >> 1];
if (b != null) {
sym = b.find(buffer, start, len);
if (sym != null) {
return sym;
}
}
}
if (!mDirty) { //need to do copy-on-write?
copyArrays();
mDirty = true;
} else if (mSize >= mSizeThreshold) { // Need to expand?
rehash();
/* Need to recalc hash; rare occurence (index mask has been
* recalculated as part of rehash)
*/
hash = calcHash(buffer, start, len) & mIndexMask;
}
++mSize;
String newSymbol = new String(buffer, start, len);
if (INTERN_STRINGS) {
newSymbol = newSymbol.intern();
}
// Ok; do we need to add primary entry, or a bucket?
if (mSymbols[hash] == null) {
mSymbols[hash] = newSymbol;
} else {
int bix = hash >> 1;
mBuckets[bix] = new Bucket(newSymbol, mBuckets[bix]);
}
return newSymbol;
}
/**
* Similar to to {@link #findSymbol(char[],int,int,int)}; used to either
* do potentially cheap intern() (if table already has intern()ed version),
* or to pre-populate symbol table with known values.
*/
/* 26-Nov-2008, tatu: not used currently; if not used in near future,
* let's just delete it.
*/
/*
public String findSymbol(String str)
{
int len = str.length();
// Sanity check:
if (len < 1) {
return "";
}
int index = calcHash(str) & mIndexMask;
String sym = mSymbols[index];
// Optimal case; checking existing primary symbol for hash index:
if (sym != null) {
// Let's inline primary String equality checking:
if (sym.length() == len) {
int i = 0;
for (; i < len; ++i) {
if (sym.charAt(i) != str.charAt(i)) {
break;
}
}
// Optimal case; primary match found
if (i == len) {
return sym;
}
}
// How about collision bucket?
Bucket b = mBuckets[index >> 1];
if (b != null) {
sym = b.find(str);
if (sym != null) {
return sym;
}
}
}
// Need to expand?
if (mSize >= mSizeThreshold) {
rehash();
// Need to recalc hash; rare occurence (index mask has been
// recalculated as part of rehash)
index = calcHash(str) & mIndexMask;
} else if (!mDirty) {
// Or perhaps we need to do copy-on-write?
copyArrays();
mDirty = true;
}
++mSize;
if (INTERN_STRINGS) {
str = str.intern();
}
// Ok; do we need to add primary entry, or a bucket?
if (mSymbols[index] == null) {
mSymbols[index] = str;
} else {
int bix = index >> 1;
mBuckets[bix] = new Bucket(str, mBuckets[bix]);
}
return str;
}
*/
/**
* Implementation of a hashing method for variable length
* Strings. Most of the time intention is that this calculation
* is done by caller during parsing, not here; however, sometimes
* it needs to be done for parsed "String" too.
*
* @param len Length of String; has to be at least 1 (caller guarantees
* this pre-condition)
*/
public static int calcHash(char[] buffer, int start, int len) {
int hash = (int) buffer[0];
for (int i = 1; i < len; ++i) {
hash = (hash * 31) + (int) buffer[i];
}
return hash;
}
public static int calcHash(String key) {
int hash = (int) key.charAt(0);
for (int i = 1, len = key.length(); i < len; ++i) {
hash = (hash * 31) + (int) key.charAt(i);
}
return hash;
}
/*
//////////////////////////////////////////////////////////
// Internal methods
//////////////////////////////////////////////////////////
*/
/**
* Method called when copy-on-write is needed; generally when first
* change is made to a derived symbol table.
*/
private void copyArrays() {
String[] oldSyms = mSymbols;
int size = oldSyms.length;
mSymbols = new String[size];
System.arraycopy(oldSyms, 0, mSymbols, 0, size);
Bucket[] oldBuckets = mBuckets;
size = oldBuckets.length;
mBuckets = new Bucket[size];
System.arraycopy(oldBuckets, 0, mBuckets, 0, size);
}
/**
* Method called when size (number of entries) of symbol table grows
* so big that load factor is exceeded. Since size has to remain
* power of two, arrays will then always be doubled. Main work
* is really redistributing old entries into new String/Bucket
* entries.
*/
private void rehash()
{
int size = mSymbols.length;
int newSize = size + size;
String[] oldSyms = mSymbols;
Bucket[] oldBuckets = mBuckets;
mSymbols = new String[newSize];
mBuckets = new Bucket[newSize >> 1];
// Let's update index mask, threshold, now (needed for rehashing)
mIndexMask = newSize - 1;
mSizeThreshold += mSizeThreshold;
int count = 0; // let's do sanity check
/* Need to do two loops, unfortunately, since spillover area is
* only half the size:
*/
for (int i = 0; i < size; ++i) {
String symbol = oldSyms[i];
if (symbol != null) {
++count;
int index = calcHash(symbol) & mIndexMask;
if (mSymbols[index] == null) {
mSymbols[index] = symbol;
} else {
int bix = index >> 1;
mBuckets[bix] = new Bucket(symbol, mBuckets[bix]);
}
}
}
size >>= 1;
for (int i = 0; i < size; ++i) {
Bucket b = oldBuckets[i];
while (b != null) {
++count;
String symbol = b.getSymbol();
int index = calcHash(symbol) & mIndexMask;
if (mSymbols[index] == null) {
mSymbols[index] = symbol;
} else {
int bix = index >> 1;
mBuckets[bix] = new Bucket(symbol, mBuckets[bix]);
}
b = b.getNext();
}
}
if (count != mSize) {
throw new Error("Internal error on SymbolTable.rehash(): had "+mSize+" entries; now have "+count+".");
}
}
/*
//////////////////////////////////////////////////////////
// Bucket class
//////////////////////////////////////////////////////////
*/
/**
* This class is a symbol table entry. Each entry acts as a node
* in a linked list.
*/
static final class Bucket {
private final String mSymbol;
private final Bucket mNext;
public Bucket(String symbol, Bucket next) {
mSymbol = symbol;
mNext = next;
}
public String getSymbol() { return mSymbol; }
public Bucket getNext() { return mNext; }
public String find(char[] buf, int start, int len) {
String sym = mSymbol;
Bucket b = mNext;
while (true) { // Inlined equality comparison:
if (sym.length() == len) {
int i = 0;
do {
if (sym.charAt(i) != buf[start+i]) {
break;
}
} while (++i < len);
if (i == len) {
return sym;
}
}
if (b == null) {
break;
}
sym = b.getSymbol();
b = b.getNext();
}
return null;
}
/* 26-Nov-2008, tatu: not used currently; if not used in near future,
* let's just delete it.
*/
/*
public String find(String str) {
String sym = mSymbol;
Bucket b = mNext;
while (true) {
if (sym.equals(str)) {
return sym;
}
if (b == null) {
break;
}
sym = b.getSymbol();
b = b.getNext();
}
return null;
}
*/
}
}