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 (especially 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 (i.e. no modifications done).

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `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 int` | `_flags`
    `protected boolean` | `_hashShared` Flag that indicates whether underlying data structures for the main hash area are shared or not.
    `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 int` | `_longestCollisionList` We need to keep track of the longest collision list; this is needed both to indicate problems with attacks and to allow flushing for other cases.
    `protected BitSet` | `_overflows` Lazily constructed structure that is used to keep track of collision buckets that have overflowed once: this is used to detect likely attempts at denial-of-service attacks that uses hash collisions.
    `protected CharsToNameCanonicalizer` | `_parent` Sharing of learnt symbols is done by optional linking of symbol table instances with their parents.
    `protected int` | `_seed` Seed value we use as the base to make hash codes non-static between different runs, but still stable for lifetime of a single symbol table instance.
    `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 AtomicReference<com.fasterxml.jackson.core.sym.CharsToNameCanonicalizer.TableInfo>` | `_tableInfo` Member that is only used by the root table instance: root passes immutable state info child instances, and children may return new state if they add entries to the table.
    `static int` | `HASH_MULT`

  - ### Method Summary

    All Methods Static Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `int` | `_hashToIndex(int rawHash)` Helper method that takes in a "raw" hash value, shuffles it as necessary, and truncates to be used as the index.
    `int` | `bucketCount()` Method for checking number of primary hash buckets this symbol table uses.
    `int` | `calcHash(char[] buffer, int start, int len)` Implementation of a hashing method for variable length Strings.
    `int` | `calcHash(String key)`
    `int` | `collisionCount()` Method mostly needed by unit tests; calculates number of entries that are in collision list.
    `static CharsToNameCanonicalizer` | `createRoot()` Method called to create root canonicalizer for a `JsonFactory` instance.
    `protected static CharsToNameCanonicalizer` | `createRoot(int seed)`
    `String` | `findSymbol(char[] buffer, int start, int len, int h)`
    `int` | `hashSeed()`
    `CharsToNameCanonicalizer` | `makeChild(int flags)` "Factory" method; will create a new child instance of this symbol table.
    `int` | `maxCollisionLength()` Method mostly needed by unit tests; calculates length of the longest collision chain.
    `boolean` | `maybeDirty()`
    `void` | `release()` Method called by the using code to indicate it is done with this instance.
    `protected void` | `reportTooManyCollisions(int maxLen)`
    `int` | `size()`
    `protected void` | `verifyInternalConsistency()` Diagnostics method that will verify that internal data structures are consistent; not meant as user-facing method but only for test suites and possible troubleshooting.

    - ### Methods inherited from class java.lang.Object

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

- - ### Field Detail

    - #### HASH_MULT

          public static final int HASH_MULT

      See Also:
        Constant Field Values

    - #### _parent

          protected final 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.

    - #### _tableInfo

          protected final AtomicReference<com.fasterxml.jackson.core.sym.CharsToNameCanonicalizer.TableInfo> _tableInfo

      Member that is only used by the root table instance: root passes immutable state info child instances, and children may return new state if they add entries to the table. Child tables do NOT use the reference.

    - #### _seed

          protected final int _seed

      Seed value we use as the base to make hash codes non-static between different runs, but still stable for lifetime of a single symbol table instance. This is done for security reasons, to avoid potential DoS attack via hash collisions.
      Since:
        2.1

    - #### _flags

          protected final int _flags

    - #### _canonicalize

          protected boolean _canonicalize

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

      NOTE: non-final since we may need to disable this with overflow.

    - #### _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.

    - #### _longestCollisionList

          protected int _longestCollisionList

      We need to keep track of the longest collision list; this is needed both to indicate problems with attacks and to allow flushing for other cases.
      Since:
        2.1

    - #### _hashShared

          protected boolean _hashShared

      Flag that indicates whether underlying data structures for the main hash area are shared or not. If they are, then they need to be handled in copy-on-write way, i.e. if they need to be modified, a copy needs to be made first; at this point it will not be shared any more, and can be modified.

      This flag needs to be checked both when adding new main entries, and when adding new collision list queues (i.e. creating a new collision list head entry)

    - #### _overflows

          protected BitSet _overflows

      Lazily constructed structure that is used to keep track of collision buckets that have overflowed once: this is used to detect likely attempts at denial-of-service attacks that uses hash collisions.
      Since:
        2.4

  - ### 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.
      Returns:
        Root instance to use for constructing new child instances

    - #### createRoot

          protected static CharsToNameCanonicalizer createRoot(int seed)

    - #### makeChild

          public CharsToNameCanonicalizer makeChild(int flags)

      "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.

      Parameters:
        `flags` - Bit flags of active `JsonFactory.Feature`s enabled.
      Returns:
        Actual canonicalizer instance that can be used by a parser

    - #### release

          public void release()

      Method called by the using code to indicate it is done with this instance. This lets instance merge accumulated changes into parent (if need be), safely and efficiently, and without calling code having to know about parent information.

    - #### size

          public int size()

      Returns:
        Number of symbol entries contained by this canonicalizer instance

    - #### bucketCount

          public int bucketCount()

      Method for checking number of primary hash buckets this symbol table uses.
      Returns:
        number of primary slots table has currently

    - #### maybeDirty

          public boolean maybeDirty()

    - #### hashSeed

          public int hashSeed()

    - #### collisionCount

          public int collisionCount()

      Method mostly needed by unit tests; calculates number of entries that are in collision list. Value can be at most (`size()` - 1), but should usually be much lower, ideally 0.
      Returns:
        Number of collisions in the primary hash area
      Since:
        2.1

    - #### maxCollisionLength

          public int maxCollisionLength()

      Method mostly needed by unit tests; calculates length of the longest collision chain. This should typically be a low number, but may be up to `size()` - 1 in the pathological case
      Returns:
        Length of the collision chain
      Since:
        2.1

    - #### findSymbol

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

    - #### _hashToIndex

          public int _hashToIndex(int rawHash)

      Helper method that takes in a "raw" hash value, shuffles it as necessary, and truncates to be used as the index.
      Parameters:
        `rawHash` - Raw hash value to use for calculating index
      Returns:
        Index value calculated

    - #### calcHash

          public 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:
        `buffer` - Input buffer that contains name to decode
        `start` - Pointer to the first character of the name
        `len` - Length of String; has to be at least 1 (caller guarantees)
      Returns:
        Hash code calculated

    - #### calcHash

          public int calcHash(String key)

    - #### reportTooManyCollisions

          protected void reportTooManyCollisions(int maxLen)

      Parameters:
        `maxLen` - Maximum allowed length of collision chain
      Since:
        2.1

    - #### verifyInternalConsistency

          protected void verifyInternalConsistency()

      Diagnostics method that will verify that internal data structures are consistent; not meant as user-facing method but only for test suites and possible troubleshooting.
      Since:
        2.10

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