CharsToNameCanonicalizer (Jackson-core 2.0.2 API)
Date: 2012-05-14

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## com.fasterxml.jackson.core.sym
Class CharsToNameCanonicalizer

    java.lang.Object
      com.fasterxml.jackson.core.sym.CharsToNameCanonicalizer

    public final class CharsToNameCanonicalizer

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
`protected com.fasterxml.jackson.core.sym.CharsToNameCanonicalizer.Bucket[]` | `_buckets` Overflow buckets; if primary doesn't match, lookup is done from here.
`protected boolean` | `_canonicalize` Whether any canonicalization should be attempted (whether using intern or not)
`protected boolean` | `_dirty` 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` | `_indexMask` Mask used to get index from hash values; equal to `_buckets.length - 1`, when _buckets.length is a power of two.
`protected boolean` | `_intern` Whether canonical symbol Strings are to be intern()ed before added to the table or not
`protected CharsToNameCanonicalizer` | `_parent` Sharing of learnt symbols is done by optional linking of symbol table instances with their parents.
`protected int` | `_size` Current size (number of entries); needed to know if and when rehash.
`protected int` | `_sizeThreshold` Limit that indicates maximum size this instance can hold before it needs to be expanded and rehashed.
`protected String[]` | `_symbols` Primary matching symbols; it's expected most match occur from here.
`protected static int` | `DEFAULT_TABLE_SIZE` Default initial table size.
`protected static int` | `MAX_TABLE_SIZE` Let's not expand symbol tables past some maximum size; this should protected against OOMEs caused by large documents with uniquer (~= random) names.

Method Summary
`static int` | `calcHash(char[] buffer, int start, int len)` Implementation of a hashing method for variable length Strings.
`static int` | `calcHash(String key)`
`static CharsToNameCanonicalizer` | `createRoot()` Method called to create root canonicalizer for a `JsonFactory` instance.
`String` | `findSymbol(char[] buffer, int start, int len, int hash)`
`CharsToNameCanonicalizer` | `makeChild(boolean canonicalize, boolean intern)` "Factory" method; will create a new child instance of this symbol table.
`boolean` | `maybeDirty()`
`void` | `release()`
`int` | `size()`

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. 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.

  See Also:
    Constant Field Values

### MAX_TABLE_SIZE

    protected static final int MAX_TABLE_SIZE

  Let's not expand symbol tables past some maximum size; this should protected against OOMEs caused by large documents with uniquer (~= random) names.

  See Also:
    Constant Field Values

### _parent

    protected CharsToNameCanonicalizer _parent

  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.

### _intern

    protected final boolean _intern

  Whether canonical symbol Strings are to be intern()ed before added to the table or not

### _canonicalize

    protected final boolean _canonicalize

  Whether any canonicalization should be attempted (whether using intern or not)

### _symbols

    protected String[] _symbols

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

### _buckets

    protected com.fasterxml.jackson.core.sym.CharsToNameCanonicalizer.Bucket[] _buckets

  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.

### _size

    protected int _size

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

### _sizeThreshold

    protected int _sizeThreshold

  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.

### _indexMask

    protected int _indexMask

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

### _dirty

    protected boolean _dirty

  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.

Method Detail

### createRoot

    public static CharsToNameCanonicalizer createRoot()

  Method called to create root canonicalizer for a `JsonFactory` instance. Root instance is never used directly; its main use is for storing and sharing underlying symbol arrays as needed.

### makeChild

    public CharsToNameCanonicalizer makeChild(boolean canonicalize,
                                              boolean intern)

  "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.

### release

    public void release()

### size

    public int size()

### maybeDirty

    public boolean maybeDirty()

### findSymbol

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

### 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)

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