com.ctc.wstx.util

Class SymbolTable

java.lang.Object

com.ctc.wstx.util.SymbolTable

public class SymbolTable
extends Object

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).

Field Summary

Fields

Modifier and Type	Field and Description
protected static float	DEFAULT_FILL_FACTOR
protected static int	DEFAULT_TABLE_SIZE Default initial table size; no need to make it miniscule, due to couple of things: first, overhead of array reallocation is significant, and second, overhead of rehashing is also non-negligible.
protected static String	EMPTY_STRING
protected com.ctc.wstx.util.SymbolTable.Bucket[]	mBuckets Overflow buckets; if primary doesn't match, lookup is done from here.
protected boolean	mDirty 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 int	mIndexMask Mask used to get index from hash values; equal to mBuckets.length - 1, when mBuckets.length is a power of two.
protected boolean	mInternStrings Flag that determines whether Strings to be added need to be interned before being added or not.
protected int	mSize Current size (number of entries); needed to know if and when rehash.
protected int	mSizeThreshold Limit that indicates maximum size this instance can hold before it needs to be expanded and rehashed.
protected String[]	mSymbols Primary matching symbols; it's expected most match occur from here.
protected int	mThisVersion Version of this table instance; used when deriving new concurrently used versions from existing 'master' instance.

Constructor Summary

Constructors

Constructor and Description
SymbolTable() Method for constructing a master symbol table instance; this one will create master instance with default size, and with interning enabled.
SymbolTable(boolean internStrings) Method for constructing a master symbol table instance.
SymbolTable(boolean internStrings, int initialSize) Method for constructing a master symbol table instance.
SymbolTable(boolean internStrings, int initialSize, float fillFactor) Main method for constructing a master symbol table instance; will be called by other public constructors.

Method Summary

Modifier and Type	Method and Description
double	calcAvgSeek()
static int	calcHash(char[] buffer, int start, int len) Implementation of a hashing method for variable length Strings.
static int	calcHash(String key)
String	findSymbol(char[] buffer, int start, int len, int hash) Main access method; will check if actual symbol String exists; if so, returns it; if not, will create, add and return it.
String	findSymbol(String str) Similar to to 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.
String	findSymbolIfExists(char[] buffer, int start, int len, int hash) Similar to {link #findSymbol}, but will not add passed in symbol if it is not in symbol table yet.
boolean	isDirectChildOf(SymbolTable t)
boolean	isDirty()
SymbolTable	makeChild() "Factory" method; will create a new child instance of this symbol table.
void	mergeChild(SymbolTable child) Method that allows contents of child table to potentially be "merged in" with contents of this symbol table.
void	setInternStrings(boolean state)
int	size()
int	version()

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

DEFAULT_TABLE_SIZE

protected static final int DEFAULT_TABLE_SIZE

Default initial table size; no need to make it miniscule, due to couple of things: first, overhead of array reallocation is significant, and second, overhead of rehashing is also non-negligible.

Let's use 128 as the default; it allows for up to 96 symbols, and uses about 512 bytes on 32-bit machines.

See Also:

Constant Field Values

DEFAULT_FILL_FACTOR

protected static final float DEFAULT_FILL_FACTOR

See Also:

Constant Field Values

EMPTY_STRING

protected static final String EMPTY_STRING

See Also:

Constant Field Values

mInternStrings

protected boolean mInternStrings

Flag 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.

mSymbols

protected String[] mSymbols

Primary matching symbols; it's expected most match occur from here.

mBuckets

protected com.ctc.wstx.util.SymbolTable.Bucket[] mBuckets

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.

mSize

protected int mSize

Current size (number of entries); needed to know if and when rehash.

mSizeThreshold

protected int mSizeThreshold

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.

mIndexMask

protected int mIndexMask

Mask used to get index from hash values; equal to mBuckets.length - 1, when mBuckets.length is a power of two.

mThisVersion

protected int mThisVersion

Version of this table instance; used when deriving new concurrently used versions from existing 'master' instance.

mDirty

protected boolean mDirty

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.

Constructor Detail

SymbolTable

public SymbolTable()

Method for constructing a master symbol table instance; this one will create master instance with default size, and with interning enabled.

SymbolTable

public SymbolTable(boolean internStrings)

Method for constructing a master symbol table instance.

SymbolTable

public SymbolTable(boolean internStrings,
                   int initialSize)

Method for constructing a master symbol table instance.

SymbolTable

public SymbolTable(boolean internStrings,
                   int initialSize,
                   float fillFactor)

Main method for constructing a master symbol table instance; will be called by other public constructors.

Parameters:

internStrings - Whether Strings to add are intern()ed or not

initialSize - Minimum initial size for bucket array; internally will always use a power of two equal to or bigger than this value.

fillFactor - Maximum fill factor allowed for bucket table; when more entries are added, table will be expanded.

Method Detail

makeChild

public SymbolTable makeChild()

"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 data access part of 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.

mergeChild

public void mergeChild(SymbolTable child)

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.

setInternStrings

public void setInternStrings(boolean state)

size

public int size()

version

public int version()

isDirty

public boolean isDirty()

isDirectChildOf

public boolean isDirectChildOf(SymbolTable t)

findSymbol

public String findSymbol(char[] buffer,
                         int start,
                         int len,
                         int hash)

Main access method; will check if actual symbol String exists; if so, returns it; if not, will create, add and return it.

Returns:

The symbol matching String in input array

findSymbolIfExists

public String findSymbolIfExists(char[] buffer,
                                 int start,
                                 int len,
                                 int hash)

Similar to {link #findSymbol}, but will not add passed in symbol if it is not in symbol table yet.

findSymbol

public String findSymbol(String str)

Similar to to 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.

calcHash

public static int calcHash(char[] buffer,
                           int start,
                           int len)

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.

Parameters:

len - Length of String; has to be at least 1 (caller guarantees this pre-condition)

calcHash

public static int calcHash(String key)

calcAvgSeek

public double calcAvgSeek()