Re: [PATCH @13746] tied hashes: memoization

Message-ID: <20011231202845.A1323@math.ohio-state.edu>

p4raw-id: //depot/perl@14686

1 files changed, 127 insertions, 0 deletions

diff --git a/lib/Tie/Memoize.pm b/lib/Tie/Memoize.pm
new file mode 100644
index 0000000000..0b3d320bc0
--- /dev/null
+++ b/lib/Tie/Memoize.pm
@@ -0,0 +1,127 @@
+use strict;
+package Tie::Memoize;
+use Tie::Hash;
+our @ISA = 'Tie::ExtraHash';
+
+our $exists_token = \undef;
+
+sub croak {require Carp; goto &Carp::croak}
+
+# Format: [0: STORAGE, 1: EXISTS-CACHE, 2: FETCH_function;
+#	   3: EXISTS_function, 4: DATA, 5: EXISTS_different ]
+
+sub FETCH {
+  my ($h,$key) = ($_[0][0], $_[1]);
+  my $res = $h->{$key};
+  return $res if defined $res;	# Shortcut if accessible
+  return $res if exists $h->{$key}; # Accessible, but undef
+  my $cache = $_[0][1]{$key};
+  return if defined $cache and not $cache; # Known to not exist
+  my @res = $_[0][2]->($key, $_[0][4]);	# Autoload
+  $_[0][1]{$key} = 0, return unless @res; # Cache non-existence
+  delete $_[0][1]{$key};	# Clear existence cache, not needed any more
+  $_[0][0]{$key} = $res[0];	# Store data and return
+}
+
+sub EXISTS   {
+  my ($a,$key) = (shift, shift);
+  return 1 if exists $a->[0]{$key}; # Have data
+  my $cache = $a->[1]{$key};
+  return $cache if defined $cache; # Existence cache
+  my @res = $a->[3]($key,$a->[4]);
+  $_[0][1]{$key} = 0, return unless @res; # Cache non-existence
+  # Now we know it exists
+  return ($_[0][1]{$key} = 1) if $a->[5]; # Only existence reported
+  # Now know the value
+  $_[0][0]{$key} = $res[0];	# Store data
+  return 1
+}
+
+sub TIEHASH  {
+  croak 'syntax: tie %hash, \'Tie::AutoLoad\', \&fetch_subr' if @_ < 2;
+  croak 'syntax: tie %hash, \'Tie::AutoLoad\', \&fetch_subr, $data, \&exists_subr, \%data_cache, \%existence_cache' if @_ > 6;
+  push @_, undef if @_ < 3;	# Data
+  push @_, $_[1] if @_ < 4;	# exists
+  push @_, {} while @_ < 6;	# initial value and caches
+  bless [ @_[4,5,1,3,2], $_[1] ne $_[3]], $_[0]
+}
+
+1;
+
+=head1 NAME
+
+Tiel::Memoize - add data to hash when needed
+
+=head1 SYNOPSIS
+
+  require Tie::Memoize;
+  tie %hash, 'Tie::Memoize',
+      \&fetch,			# The rest is optional
+      $DATA, \&exists,
+      {%ini_value}, {%ini_existence};
+
+=head1 DESCRIPTION
+
+This package allows a tied hash to autoload its values on the first access,
+and to use the cached value on the following accesses.
+
+Only read-accesses (via fetching the value or C<exists>) result in calls to
+the functions; the modify-accesses are performed as on a normal hash.
+
+The required arguments during C<tie> are the hash, the package, and
+the reference to the C<FETCH>ing function.  The optional arguments are
+an arbitrary scalar $data, the reference to the C<EXISTS> function,
+and initial values of the hash and of the existence cache.
+
+Both the C<FETCH>ing function and the C<EXISTS> functions have the
+same signature: the arguments are C<$key, $data>; $data is the same
+value as given as argument during tie()ing.  Both functions should
+return an empty list if the value does not exist.  If C<EXISTS>
+function is different from the C<FETCH>ing function, it should return
+a TRUE value on success.  The C<FETCH>ing function should return the
+intended value if the key is valid.
+
+=head1 Inheriting from B<Tie::Memoize>
+
+The structure of the tied() data is an array reference with elements
+
+  0:  cache of known values
+  1:  cache of known existence of keys
+  2:  FETCH  function
+  3:  EXISTS function
+  4:  $data
+
+The rest is for internal usage of this package.  In particular, if
+TIEHASH is overwritten, it should call SUPER::TIEHASH.
+
+=head1 EXAMPLE
+
+  sub slurp {
+    my ($key, $dir) = shift;
+    open my $h, '<', "$dir/$key" or return;
+    local $/; <$h>			# slurp it all
+  }
+  sub exists { my ($key, $dir) = shift; return -f "$dir/$key" }
+
+  tie %hash, 'Tie::Memoize', \&slurp, $directory, \&exists,
+      { fake_file1 => $content1, fake_file2 => $content2 },
+      { pretend_does_not_exists => 0, known_to_exist => 1 };
+
+This example treats the slightly modified contents of $directory as a
+hash.  The modifications are that the keys F<fake_file1> and
+F<fake_file2> fetch values $content1 and $content2, and
+F<pretend_does_not_exists> will never be accessed.  Additionally, the
+existence of F<known_to_exist> is never checked (so if it does not
+exists when its content is needed, the user of %hash may be confused).
+
+=head1 BUGS
+
+FIRSTKEY and NEXTKEY methods go through the keys which were already read,
+not all the possible keys of the hash.
+
+=head1 AUTHOR
+
+Ilya Zakharevich L<mailto:perl-module-hash-memoize@ilyaz.org>.
+
+=cut
+