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.TH atom 3 "21 January 2001" "Straylight/Edgeware" "mLib utilities library"
.SH NAME
atom \- atom table manager
.\" @atom_createtable
.\" @atom_destroytable
.\" @atom_intern
.\" @atom_nintern
.\" @atom_gensym
.\" @atom_name
.\" @atom_len
.\" @atom_hash
.\" @atom_mkiter
.\" @atom_next
.\"
.\" @ATOM_GLOBAL
.\" @INTERN
.\" @GENSYM
.\" @ATOM_NAME
.\" @ATOM_LEN
.\" @ATOM_HASH
.\"
.SH SYNOPSIS
.nf
.B "#include <mLib/atom.h>"

.BI "void atom_createtable(atom_table *" t );
.BI "void atom_destroytable(atom_table *" t );

.BI "atom *atom_intern(atom_table *" t ", const char *" p );
.BI "atom *atom_nintern(atom_table *" t ", const char *" p ", size_t " n );
.BI "atom *atom_gensym(atom_table *" t );
.BI "atom *INTERN(const char *" p );
.BI "atom *GENSYM;"

.BI "const char *atom_name(const atom *" a );
.BI "size_t atom_len(const atom *" a );
.BI "uint32 atom_hash(const atom *" a );
.BI "const char *ATOM_NAME(const atom *" a );
.BI "size_t ATOM_LEN(const atom *" a );
.BI "uint32 ATOM_HASH(const atom *" a );

.BI "void atom_mkiter(atom_iter *" i ", atom_table *" t );
.BI "atom *atom_next(atom_iter *" i );

.BI "extern atom_table *ATOM_GLOBAL;"
.fi
.SH DESCRIPTION
The
.B atom
functions and macros implement a data type similar to immutable strings,
with an additional property: that the addresses of two atoms from the
same table are equal if and only if they contain the same text.  Atom
strings don't have to be null-terminated, although the interface is
simpler if you know that all of your atoms are null-terminated.  It's
also possible to make
.IR "uninterned atoms" :
see below.
.PP
If a necessary memory allocation fails during an atom operation, the
exception
.B EXC_NOMEM
is raised.
.PP
Atoms are useful for speeding up string comparisons, and for saving
memory when many possibly-identical strings need storing.
.PP
There is a global atom table, named
.BR ATOM_GLOBAL ,
available for general use.  You can initialize your own atom table if
either you want to ensure that the atoms are not shared with some other
table, or if you want to be able to free the atoms later.  Private atom
tables have the type
.BR atom_table ;
initialize it using the function
.B atom_createtable
and free it when you're finished using
.BR atom_destroytable .
.SS "Creating atoms from strings"
The process of making atoms from strings is called
.IR interning .
The function
.B atom_nintern
takes an atom table, a string, and a length, and returns an atom with
the same text.  If your string is null-terminated, you can instead use
.B atom_intern
which has no length argument; if, in addition, you want to use the
global atom table, you can use the single-argument
.B INTERN
macro, which takes just a null-terminated string.
.PP
A terminating null byte is always appended to an atom name.  This is not
considered to be a part of the name itself, and does not contribute to
the atom's length as reported by the
.B ATOM_LEN
macro.
.SS "Uninterned atoms"
You can make an atom which is guaranteed to be distinct from every other
atom, and has no sensible text string, by calling
.BR atom_gensym ,
passing it the address of your atom table.  The macro
.B GENSYM
(which doesn't look like a function, and has no parentheses following
it!) will return a unique atom from the global table.  Uninterned atoms
have a generated name of the form
.RB ` *gen- \fInnn * '
where
.I nnn
is an atom-table-specific sequence number.  This text is there as a
debugging convenience, and doesn't mean that the uninterned atom has the
same address as an interned atom with the same text.
.SS "Other enquiries about atoms"
Atoms can be interrogated to find their names and hashes.  The macro
.B ATOM_NAME
returns a pointer to the atom's name (its text);
.B ATOM_LEN
returns the length of the atom's name, excluding the terminating null;
and
.B ATOM_HASH
returns the atom's hash value, which is useful if you want to use the
atom as a key in some other structure.  There are lower-case function
versions of these, which have the same effect.  There is little point in
using the functions.
.SS "Enumerating atoms"
You can iterate over the atoms in an atom table.  The
.B atom_mkiter
function initializes an iterator object to iterate over a particular
atom table;
.B atom_next
returns the next atom from the iterator.  Atoms are not returned in any
particular order.
.SH SEE ALSO
.BR assoc (3),
.BR hash (3),
.BR mLib (3).
.SH AUTHOR
Mark Wooding, <mdw@distorted.org.uk>