m4_comment([$Id: bt_compare.so,v 10.25 2004/03/05 21:39:33 bostic Exp $])

m4_ref_title(Access Methods, Btree comparison,
    specifying a Btree @comparison function, am_conf/malloc, am_conf/bt_prefix)

m4_p([dnl
The Btree data structure is a sorted, balanced tree structure storing
associated key/data pairs.  By default, the sort order is lexicographical,
with shorter keys collating before longer keys.  The user can specify the
sort order for the Btree by using the m4_refT(dbh_set_bt_compare).])

m4_p([dnl
Sort routines are passed pointers to keys as arguments.  The keys are
represented as m4_ref(Dbt) structures.  The routine must return an integer
less than, equal to, or greater than zero if the first argument is
considered to be respectively less than, equal to, or greater than the
second argument.  The only fields that the routines may examine in the
m4_ref(Dbt) structures are m4_arg(data) and m4_arg(size) fields.])

m4_p([dnl
An example routine that might be used to sort integer keys in the database
is as follows:])

include(ref/am_conf/bt_compare1.cs)

m4_p([dnl
Note that the data must first be copied into memory that is appropriately
aligned, as m4_db does not guarantee any kind of alignment of the
underlying data, including for comparison routines.  When writing
comparison routines, remember that databases created on machines of
different architectures may have different integer byte orders, for which
your code may need to compensate.])

m4_p([dnl
An example routine that might be used to sort keys based on the first
five bytes of the key (ignoring any subsequent bytes) is as follows:])

include(ref/am_conf/bt_compare2.cs)

m4_p([dnl
All comparison functions must cause the keys in the database to be
well-ordered. The most important implication of being well-ordered is
that the key relations must be transitive, that is, if key A is less
than key B, and key B is less than key C, then the comparison routine
must also return that key A is less than key C.])

m4_p([dnl
It is reasonable for a comparison function to not examine an entire key
in some applications, which implies that partial keys may be specified
to the m4_db interfaces.  When partial keys are specified to m4_db,
interfaces which retrieve data items based on a user-specified key (for
example, m4_ref(dbh_get) and m4_ref(dbc_get) with the m4_ref(DB_SET)
flag), will not modify the user-specified key by returning the actual
key stored in the database.  The actual key can be retrieved by calling
the m4_refT(dbc_get) with the m4_ref(DB_CURRENT) flag.])

m4_page_footer