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 <tclHash.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
Address of hash table structure (for all procedures but Tcl_InitHashTable, this must have been initialized by previous call to Tcl_InitHashTable). 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. Key to use for probe into table. Exact form depends on keyType used to create table. The word at *newPtr is set to 1 if a new entry was created and 0 if there was already an entry for key. Pointer to hash table entry. New value to assign to hash table entry. Need not have type ClientData, but must fit in same space as ClientData. 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 susequent 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 tclHash.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