hash coding
hash bucket
hashing
hash table

    (Or "hashing") A scheme for providing
   rapid access to data items which are distinguished by some
   key.  Each data item to be stored is associated with a key,
   e.g. the name of a person.  A hash function is applied to
   the item's key and the resulting hash value is used as an
   index to select one of a number of "hash buckets" in a hash
   table.  The table contains pointers to the original items.

   If, when adding a new item, the hash table already has an
   entry at the indicated location then that entry's key must be
   compared with the given key to see if it is the same.  If two
   items' keys hash to the same value (a "hash collision") then
   some alternative location is used (e.g. the next free location
   cyclically following the indicated one).  For best
   performance, the table size and hash function must be
   tailored to the number of entries and range of keys to be
   used.  The hash function usually depends on the table size so
   if the table needs to be enlarged it must usually be
   completely rebuilt.

   When you look up a name in the phone book (for example), you
   typically hash it by extracting its first letter; the hash
   buckets are the alphabetically ordered letter sections.

   See also: btree, checksum, CRC, pseudorandom number,
   random, random number, soundex.

   (1997-08-03)