Going with a little-endian order: 

{
    my %prev;
    sub c_out {
        my( $fh, $c )= @_;
        if(  ! defined $c  ) {
            print $fh pack "CC", $c & 0xff, $c >> 8
                if  exists $prev{$fh};
        } elsif(  exists $prev{$fh}  ) {
            print $fh pack "CC", $prev{$fh} | (($c&0xf)<<4), $c>>4;
        } else {
            print $fh pack "C", $c & 0xff;
            $prev{$fh}= ( $c>>8 ) & 0xf;
            return;
        }
        delete $prev{$fh};
    }
}
{
    my %prev;
    sub c_in {
        my( $fh )= @_;
        my $c;
        if(  exists $prev{$fh}  ) {
            if(  ! read( $fh, $c, 1 )  ) {
                die "Invalid trailing nybble ($prev{$fh})"
                    unless  0 == delete $prev{$fh};
                return;
            }
            return delete($prev{$fh})<<8 | unpack("C",$c);
        } else {
            my $len= read( $fh, $c, 2 );
            die "Extra trailing byte (",unpack("C",$c),")"
                if  1 == $len;
            return   if  0 == $len;
            ( $c, $prev{$fh} )= unpack "CC", $c;
            $c |= ($prev{$fh}&0xf)<<8;
            $prev{$fh} >>= 4;
            return $c;
        }
    }
}

[download]

I have you pass the file handle to the routine and don't handle bareword file handles so use c_out(\*FILE,$sesqui) and finish up by doing c_out(\*FILE) to flush the final nybble, if any. (Also, don't intermix *FILE and \*FILE as I didn't bother to make the code robust in the face of that either.)

        - tye (but my friends call me "Tye")

In reply to (tye)Re: 12 bit ints pairs into 3 bytes by tye
in thread 12 bit ints pairs into 3 bytes by jhanna

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.