hash(3Tcl)

NAME
     Tcl_InitHashTable, Tcl_DeleteHashTable, Tcl_CreateHashEntry,
     Tcl_DeleteHashEntry, Tcl_FindHashEntry, Tcl_GetHashValue,
     Tcl_SetHashValue, Tcl_GetHashKey, Tcl_FirstHashEntry, Tcl_NextHashEntry,
     Tcl_HashStats - procedures to manage hash tables

SYNOPSIS
     #include <tcl.h>

     Tcl_InitHashTable(tablePtr, keyType)

     Tcl_DeleteHashTable(tablePtr)

     Tcl_HashEntry *
     Tcl_CreateHashEntry(tablePtr, key, newPtr)

     Tcl_DeleteHashEntry(entryPtr)

     Tcl_HashEntry *
     Tcl_FindHashEntry(tablePtr, key)

     ClientData
     Tcl_GetHashValue(entryPtr)

     Tcl_SetHashValue(entryPtr, value)

     char *
     Tcl_GetHashKey(tablePtr, entryPtr)

     Tcl_HashEntry *
     Tcl_FirstHashEntry(tablePtr, searchPtr)

     Tcl_HashEntry *
     Tcl_NextHashEntry(searchPtr)

     char *
     Tcl_HashStats(tablePtr)

ARGUMENTS
     Tcl_HashTable    *tablePtr    (in)      Address of hash table structure
                                             (for all procedures but
                                             Tcl_InitHashTable, this must have
                                             been initialized by previous call
                                             to Tcl_InitHashTable).

     int              keyType      (in)      Kind of keys to use for new hash
                                             table.  Must be either
                                             TCL_STRING_KEYS,
                                             TCL_ONE_WORD_KEYS, or an integer
                                             value greater than 1.

     char             *key         (in)      Key to use for probe into table.
                                             Exact form depends on keyType
                                             used to create table.

     int              *newPtr      (out)     The word at *newPtr is set to 1
                                             if a new entry was created and 0
                                             if there was already an entry for
                                             key.

     Tcl_HashEntry    *entryPtr    (in)      Pointer to hash table entry.

     ClientData       value        (in)      New value to assign to hash table
                                             entry.  Need not have type
                                             ClientData, but must fit in same
                                             space as ClientData.

     Tcl_HashSearch   *searchPtr   (in)      Pointer to record to use to keep
                                             track of progress in enumerating
                                             all the entries in a hash table.


DESCRIPTION
     A hash table consists of zero or more entries, each consisting of a key
     and a value.  Given the key for an entry, the hashing routines can very
     quickly locate the entry, and hence its value.  There may be at most one
     entry in a hash table with a particular key, but many entries may have
     the same value.  Keys can take one of three forms:  strings, one-word
     values, or integer arrays.  All of the keys in a given table have the
     same form, which is specified when the table is initialized.

     The value of a hash table entry can be anything that fits in the same
     space as a ``char *'' pointer.  Values for hash table entries are managed
     entirely by clients, not by the hash module itself.  Typically each
     entry's value is a pointer to a data structure managed by client code.

     Hash tables grow gracefully as the number of entries increases, so that
     there are always less than three entries per hash bucket, on average.
     This allows for fast lookups regardless of the number of entries in a
     table.

     Tcl_InitHashTable initializes a structure that describes a new hash
     table.  The space for the structure is provided by the caller, not by the
     hash module.  The value of keyType indicates what kinds of keys will be
     used for all entries in the table.  KeyType must have one of the
     following values:

     TCL_STRING_KEYS          Keys are null-terminated ASCII strings.  They
                              are passed to hashing routines using the address
                              of the first character of the string.

     TCL_ONE_WORD_KEYS        Keys are single-word values;  they are passed to
                              hashing routines and stored in hash table
                              entries as ``char *'' values.  The pointer value
                              is the key;  it need not (and usually doesn't)
                              actually point to a string.

     other                    If keyType is not TCL_STRING_KEYS or
                              TCL_ONE_WORD_KEYS, then it must be an integer
                              value greater than 1.  In this case the keys
                              will be arrays of ``int'' values, where keyType
                              gives the number of ints in each key.  This
                              allows structures to be used as keys.  All keys
                              must have the same size.  Array keys are passed
                              into hashing functions using the address of the
                              first int in the array.

     Tcl_DeleteHashTable deletes all of the entries in a hash table and frees
     up the memory associated with the table's bucket array and entries.  It
     does not free the actual table structure (pointed to by tablePtr), since
     that memory is assumed to be managed by the client.  Tcl_DeleteHashTable
     also does not free or otherwise manipulate the values of the hash table
     entries.  If the entry values point to dynamically-allocated memory, then
     it is the client's responsibility to free these structures before
     deleting the table.

     Tcl_CreateHashEntry locates the entry corresponding to a particular key,
     creating a new entry in the table if there wasn't already one with the
     given key.  If an entry already existed with the given key then *newPtr
     is set to zero.  If a new entry was created, then *newPtr is set to a
     non-zero value and the value of the new entry will be set to zero.  The
     return value from Tcl_CreateHashEntry is a pointer to the entry, which
     may be used to retrieve and modify the entry's value or to delete the
     entry from the table.

     Tcl_DeleteHashEntry will remove an existing entry from a table.  The
     memory associated with the entry itself will be freed, but the client is
     responsible for any cleanup associated with the entry's value, such as
     freeing a structure that it points to.

     Tcl_FindHashEntry is similar to Tcl_CreateHashEntry except that it
     doesn't create a new entry if the key doesn't exist; instead, it returns
     NULL as result.

     Tcl_GetHashValue and Tcl_SetHashValue are used to read and write an
     entry's value, respectively.  Values are stored and retrieved as type
     ``ClientData'', which is large enough to hold a pointer value.  On almost
     all machines this is large enough to hold an integer value too.

     Tcl_GetHashKey returns the key for a given hash table entry, either as a
     pointer to a string, a one-word (``char *'') key, or as a pointer to the
     first word of an array of integers, depending on the keyType used to
     create a hash table.  In all cases Tcl_GetHashKey returns a result with
     type ``char *''.  When the key is a string or array, the result of
     Tcl_GetHashKey points to information in the table entry;  this
     information will remain valid until the entry is deleted or its table is
     deleted.

     Tcl_FirstHashEntry and Tcl_NextHashEntry may be used to scan all of the
     entries in a hash table.  A structure of type ``Tcl_HashSearch'',
     provided by the client, is used to keep track of progress through the
     table.  Tcl_FirstHashEntry initializes the search record and returns the
     first entry in the table (or NULL if the table is empty).  Each
     subsequent call to Tcl_NextHashEntry returns the next entry in the table
     or NULL if the end of the table has been reached.  A call to
     Tcl_FirstHashEntry followed by calls to Tcl_NextHashEntry will return
     each of the entries in the table exactly once, in an arbitrary order.  It
     is unadvisable to modify the structure of the table, e.g.  by creating or
     deleting entries, while the search is in progress.

     Tcl_HashStats returns a dynamically-allocated string with overall
     information about a hash table, such as the number of entries it
     contains, the number of buckets in its hash array, and the utilization of
     the buckets.  It is the caller's responsibility to free the result string
     by passing it to free.

     The header file tcl.h defines the actual data structures used to
     implement hash tables.  This is necessary so that clients can allocate
     Tcl_HashTable structures and so that macros can be used to read and write
     the values of entries.  However, users of the hashing routines should
     never refer directly to any of the fields of any of the hash-related data
     structures; use the procedures and macros defined here.


KEYWORDS
     hash table, key, lookup, search, value