Class SymbolTable
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).
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Nested Class Summary
Nested ClassesModifier and TypeClassDescription(package private) static final class
This class is a symbol table entry. -
Field Summary
FieldsModifier and TypeFieldDescriptionprotected static final float
protected static final int
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 final String
protected SymbolTable.Bucket[]
Overflow buckets; if primary doesn't match, lookup is done from here.protected boolean
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
Mask used to get index from hash values; equal tomBuckets.length - 1
, when mBuckets.length is a power of two.protected boolean
Flag that determines whether Strings to be added need to be interned before being added or not.protected int
Current size (number of entries); needed to know if and when rehash.protected int
Limit that indicates maximum size this instance can hold before it needs to be expanded and rehashed.protected String[]
Primary matching symbols; it's expected most match occur from here.protected int
Version of this table instance; used when deriving new concurrently used versions from existing 'master' instance. -
Constructor Summary
ConstructorsModifierConstructorDescriptionMethod 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.private
SymbolTable
(boolean internStrings, String[] symbols, SymbolTable.Bucket[] buckets, int size, int sizeThreshold, int indexMask, int version) Internal constructor used when creating child instances. -
Method Summary
Modifier and TypeMethodDescriptiondouble
static int
calcHash
(char[] buffer, int start, int len) Implementation of a hashing method for variable length Strings.static int
private void
Method called when copy-on-write is needed; generally when first change is made to a derived symbol table.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.findSymbol
(String str) Similar to tofindSymbol(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.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
boolean
isDirty()
"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.private void
rehash()
Method called when size (number of entries) of symbol table grows so big that load factor is exceeded.void
setInternStrings
(boolean state) int
size()
int
version()
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Field Details
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DEFAULT_TABLE_SIZE
protected static final int DEFAULT_TABLE_SIZEDefault 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:
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DEFAULT_FILL_FACTOR
protected static final float DEFAULT_FILL_FACTOR- See Also:
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EMPTY_STRING
- See Also:
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mInternStrings
protected boolean mInternStringsFlag 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
Primary matching symbols; it's expected most match occur from here. -
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.
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mSize
protected int mSizeCurrent size (number of entries); needed to know if and when rehash. -
mSizeThreshold
protected int mSizeThresholdLimit 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 mIndexMaskMask used to get index from hash values; equal tomBuckets.length - 1
, when mBuckets.length is a power of two. -
mThisVersion
protected int mThisVersionVersion of this table instance; used when deriving new concurrently used versions from existing 'master' instance. -
mDirty
protected boolean mDirtyFlag 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.
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Constructor Details
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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 notinitialSize
- 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.
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SymbolTable
private SymbolTable(boolean internStrings, String[] symbols, SymbolTable.Bucket[] buckets, int size, int sizeThreshold, int indexMask, int version) Internal constructor used when creating child instances.
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Method Details
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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.
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mergeChild
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.
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setInternStrings
public void setInternStrings(boolean state) -
size
public int size() -
version
public int version() -
isDirty
public boolean isDirty() -
isDirectChildOf
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findSymbol
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
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findSymbolIfExists
Similar to {link #findSymbol}, but will not add passed in symbol if it is not in symbol table yet. -
findSymbol
Similar to tofindSymbol(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)
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calcHash
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copyArrays
private void copyArrays()Method called when copy-on-write is needed; generally when first change is made to a derived symbol table. -
rehash
private void rehash()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. -
calcAvgSeek
public double calcAvgSeek()
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