PerlIO::via::MD5 - PerlIO layer for creating an MD5 digest of a file

 use PerlIO::via::MD5;

 PerlIO::via::MD5->method( 'hexdigest' ); # default, return 32 hex digits
 PerlIO::via::MD5->method( 'digest' );    # return 16-byte binary value
 PerlIO::via::MD5->method( 'b64digest' ); # return 22-byte base64 (MIME) value

 open( my $in,'<:via(MD5)','file' )
  or die "Can't open file for digesting: $!\n";
 my $digest = <$in>;

 $method = PerlIO::via::MD5->method;  # obtain current setting
 PerlIO::via::MD5->method( $method ); # set new digest method

 Digest::MD5 (any)

package PerlIO::via::MD5;

# Set the version info
# Make sure we do things by the book from now on

$VERSION = '0.06';
use strict;

# Make sure the encoding/decoding stuff is available

use Digest::MD5 (); # no need to pollute this namespace

# Initialize the hash with allowable methods
# Set the default method to be used

my %allowed = (digest => 1, hexdigest => 1, b64digest => 1);
my $method = 'hexdigest';

# Satisfy -require-

1;

#-----------------------------------------------------------------------

# Methods for settings that will be used by the objects

#-----------------------------------------------------------------------

#  IN: 1 class (ignored)
#      2 new setting for method (
# OUT: 1 current setting for eol

sub method {

# Lose the class
# If we have a new value
#  Die now if invalid method name
#  Set the new value
# Return whatever we have now

    shift;
    if (@_) {
        die "Invalid digest method '$_[0]'" unless $allowed{$_[0]};
        $method = shift;
    }
    $method;
} #method

#-----------------------------------------------------------------------

# Methods for the actual layer implementation

#-----------------------------------------------------------------------
#  IN: 1 class
#      2 mode string (ignored)
#      3 file handle of PerlIO layer below (ignored)
# OUT: 1 blessed object

sub PUSHED { 

# Return now if we're not reading
# Create Digest::MD5 object and bless it as ourself

  return -1 if $_[1] ne 'r';
  bless [Digest::MD5->new,$method],$_[0];
} #PUSHED

#-----------------------------------------------------------------------
#  IN: 1 instantiated object
#      2 handle to read from
# OUT: 1 empty string (when still busy) or the digest string (when done)

sub FILL {

# Read the line from the handle
# If there is something to be added
#  Add it
#  Indicate nothing really to be returned yet

    my $line = readline( $_[1] );
    if (defined($line)) {
        $_[0]->[0]->add( $line );
	return '';

# Elsif we still have an MD5 object (and end of data reached)
#  Obtain the MD5 object and method name
#  Remove MD5 object from PerlIO::via::MD5 object (so we'll really exit next)
#  Return the result of the digest

    } elsif ($_[0]->[0]) {
        my ($object,$method) = @{$_[0]};
        $_[0]->[0] = '';
        return $object->$method;

# Else (end of data really reached)
#  Return signalling end of data reached

    } else {
        return undef;
    }
} #FILL

#-----------------------------------------------------------------------

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