in reply to Practical Example of Converting Packed Value

I'd probably use just bitwise math, but as you've asked for pack, here's one way: 

$s = chr( 0xa8 ) . chr( 0x38 );;

print unpack 'b*', $s;;
0001010100011100

print unpack 'A5A4A7', unpack 'b*', $s;;
00010 1010 0011100

print map{ unpack 'C', pack 'b*', $_ } unpack 'A5A4A7', unpack 'b*', $
+s;;
8 5 28

( $day, $month, $year ) = map{ 
    unpack 'C', pack 'b*', $_ 
} unpack 'A5A4A7', unpack 'b*', $
printf "%4d-%02d-%02d\n", 1980+$year, $month, $day;;

2008-05-08

[download]

Updated: Or using bitwise math: 

$s = chr( 0xa8 ) . chr( 0x38 );;

$n = unpack 's', $s;;

( $day, $month, $year ) = ( 
     $n & 0x1f, 
   ( $n & 0x1e0 ) >> 5, 
   ( $n & 0xfe00 ) >> 9 
);;

printf "%4d-%02d-%02d\n", 1980+$year, $month, $day;;
2008-05-08

[download]
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

Replies are listed 'Best First'.
Re^2: Practical Example of Converting Packed Value
by Jim (Curate) on Dec 01, 2008 at 00:30 UTC
    Thank you, ++BrowserUk!

    I actually didn't ask for solutions using pack/unpack; I only described the representation format as "packed". Please feel free to show me how you would do it using bitwise math.

    Jim

[reply]

      Please feel free to show me how you would do it using bitwise math.

      That's what the second snippet does. First, unpack 's' is used to convert the bytes representing a number into something Perl understands to be a number. Then bit math is performed to extract each of the fields.

[reply]
[d/l]
        Yeah, BrowserUk was adding that last bit while I was too hastily replying to his very helpful post.

        I understand the bitmask and the right-shift operations much better now. This example had the concreteness and immediacy I needed. It helps me a lot to display the bitmask AND operation in binary.

        my $day   = ( $short_int & 0b0000000_0000_11111 )      ;
my $month = ( $short_int & 0b0000000_1111_00000 ) >> 5 ;
my $year  = ( $short_int & 0b1111111_0000_00000 ) >> 9 ;

my $date  = sprintf "%04d-%02d-%02d", 1980 + $year, $month, $day;

        [download]
        Is there any reason not to use these binary constants instead of hexadecimal constants in the real program?

        I suppose in a real program, the binary/hexadecimal constants and the year 1980 would be symbolic constants; e.g., DAY_MASK and EPOCH_YEAR. Or not.

[reply]
[d/l]

In Section Seekers of Perl Wisdom