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Tcl_Hash(3)			      Tcl Library Procedures			      Tcl_Hash(3)

_________________________________________________________________________________________________

NAME
       Tcl_InitHashTable,   Tcl_InitCustomHashTable,  Tcl_InitObjHashTable,  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_InitCustomHashTable(tablePtr, keyType, typePtr)

       Tcl_InitObjHashTable(tablePtr)

       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  proce-
						dures  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,
						TCL_CUSTOM_TYPE_KEYS,  TCL_CUSTOM_PTR_KEYS, or an
						integer value greater than 1.

       Tcl_HashKeyType *typePtr (in)		Address of structure which defines the	behaviour
						of the hash table.

       const 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 four forms: strings, one-word
       values, integer arrays, or custom keys defined by a Tcl_HashKeyType structure (See section
       THE TCL_HASHKEYTYPE STRUCTURE below). 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  regard-
       less of the number of entries in a table.

       The   core   provides   three   functions   for	 the   initialization	of  hash  tables,
       Tcl_InitHashTable, Tcl_InitObjHashTable and Tcl_InitCustomHashTable.

       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.	All  of  the  key
       types described later are allowed, with the exception of TCL_CUSTOM_TYPE_KEYS and TCL_CUS-
       TOM_PTR_KEYS.

       Tcl_InitObjHashTable is a wrapper around Tcl_InitCustomHashTable and  initializes  a  hash
       table whose keys are Tcl_Obj *.

       Tcl_InitCustomHashTable 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 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 does not) actually
				point to a string.

       TCL_CUSTOM_TYPE_KEYS	Keys are of arbitrary type, and are stored in the entry.  Hashing
				and  comparison  is  determined  by  typePtr. The Tcl_HashKeyType
				structure is described in the section THE TCL_HASHKEYTYPE  STRUC-
				TURE below.

       TCL_CUSTOM_PTR_KEYS	Keys  are  pointers  to  an arbitrary type, and are stored in the
				entry. Hashing and  comparison	is  determined	by  typePtr.  The
				Tcl_HashKeyType   structure  is  described  in	the  section  THE
				TCL_HASHKEYTYPE STRUCTURE below.

       other			If keyType is not one of the above, 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 was not 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 associ-
       ated with the entry's value, such as freeing a structure that it points to.

       Tcl_FindHashEntry  is  similar to Tcl_CreateHashEntry except that it does not 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, respec-
       tively.	 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  inte-
       gers,  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_NextHashEn-
       try will return each of the entries in the table exactly once, in an arbitrary order.   It
       is  inadvisable	to  modify  the  structure  of	the  table, e.g.  by creating or deleting
       entries, while the search is in	progress,  with  the  exception  of  deleting  the  entry
       returned by Tcl_FirstHashEntry or Tcl_NextHashEntry.

       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 ckfree.

       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  rou-
       tines  should  never  refer  directly to any of the fields of any of the hash-related data
       structures; use the procedures and macros defined here.

THE TCL_HASHKEYTYPE STRUCTURE
       Extension writers can define new hash key types by defining four procedures,  initializing
       a Tcl_HashKeyType structure to describe the type, and calling Tcl_InitCustomHashTable. The
       Tcl_HashKeyType structure is defined as follows:
	      typedef struct Tcl_HashKeyType {
		  int version;
		  int flags;
		  Tcl_HashKeyProc *hashKeyProc;
		  Tcl_CompareHashKeysProc *compareKeysProc;
		  Tcl_AllocHashEntryProc *allocEntryProc;
		  Tcl_FreeHashEntryProc *freeEntryProc;
	      } Tcl_HashKeyType;

       The version member is the version of the table. If this structure is  extended  in  future
       then the version can be used to distinguish between different structures. It should be set
       to TCL_HASH_KEY_TYPE_VERSION.

       The flags member is 0 or one or more of the following values OR'ed together:

       TCL_HASH_KEY_RANDOMIZE_HASH
				There are some things, pointers for example  which  do	not  hash
				well  because they do not use the lower bits. If this flag is set
				then the hash table will attempt to rectify this  by  randomizing
				the  bits  and	then using the upper N bits as the index into the
				table.

       TCL_HASH_KEY_SYSTEM_HASH This flag forces Tcl to use the memory allocation procedures pro- |
				vided  by the operating system when allocating and freeing memory |
				used to store the hash table data  structures,	and  not  any  of |
				Tcl's  own  customized memory allocation routines. This is impor- |
				tant if the hash table is to be used in the implementation  of	a |
				custom set of allocation routines, or something that a custom set |
				of allocation routines might depend on, in  order  to  avoid  any |
				circular dependency.

       The hashKeyProc member contains the address of a function called to calculate a hash value
       for the key.
	      typedef unsigned int (Tcl_HashKeyProc) (
		      Tcl_HashTable *tablePtr,
		      void *keyPtr);
       If this is NULL then keyPtr is used and TCL_HASH_KEY_RANDOMIZE_HASH is assumed.

       The compareKeysProc member contains the address of a function called to compare two keys.
	      typedef int (Tcl_CompareHashKeysProc) (
		      void *keyPtr,
		      Tcl_HashEntry *hPtr);
       If this is NULL then the keyPtr pointers are compared. If the keys do not match	then  the
       function returns 0, otherwise it returns 1.

       The  allocEntryProc member contains the address of a function called to allocate space for
       an entry and initialize the key and clientData.
	      typedef Tcl_HashEntry *(Tcl_AllocHashEntryProc) (
		      Tcl_HashTable *tablePtr,
		      void *keyPtr);
       If this is NULL then Tcl_Alloc is used to allocate enough space for a  Tcl_HashEntry,  the
       key pointer is assigned to key.oneWordValue and the clientData is set to NULL. String keys
       and array keys use this function to allocate enough space for the entry and the key in one
       block,  rather than doing it in two blocks. This saves space for a pointer to the key from
       the entry and another memory allocation. Tcl_Obj*  keys	use  this  function  to  allocate
       enough space for an entry and increment the reference count on the object.

       The  freeEntryProc  member  contains the address of a function called to free space for an
       entry.
	      typedef void (Tcl_FreeHashEntryProc) (Tcl_HashEntry *hPtr);
       If this is NULL then Tcl_Free is used to free the space for the entry.  Tcl_Obj* keys  use
       this function to decrement the reference count on the object.

KEYWORDS
       hash table, key, lookup, search, value

Tcl										      Tcl_Hash(3)
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