How to implement a bitmask in php?

Question

I'm not sure if bitmask is the correct term. Let me explain:

In php, the error_reporting function can be called multiple ways:

// Report simple running errors
error_reporting(E_ERROR | E_WARNING | E_PARSE);

// Reporting E_NOTICE can be good too (to report uninitialized
// variables or catch variable name misspellings ...)
error_reporting(E_ERROR | E_WARNING | E_PARSE | E_NOTICE);

// Report all errors except E_NOTICE
// This is the default value set in php.ini
error_reporting(E_ALL ^ E_NOTICE);

I got the term bitmask from the php.net page here

Anyway the point of this is, I have implemented a SIMPLE method called ls which returns the contents of a directory.

This function takes 3 args... ( $include_hidden = false, $return_absolute = false, $ext = false )

So when i call the function, i set how i want the results. Whether i want the results to return hidden directories, whether i want basenames only etc.

so when i call the function i'm writing

ls(true, false, true)
ls(false, false, true)
ls(true, true, true)
etc...

I thought it would be much more readable if i could just flag how i want the data returned?

so something like:

ls( INCLUDE_HIDDEN | HIDE_EXTS );
ls( SHOW_ABSOLUTE_PATHS | HIDE_EXTS );

etc...

How would i implement this in terms of testing which flags have been called?

Answer 1

It's quite simple actually. First a bit of code to demonstrate how it can be implemented. If you don't understand anything about what this code is doing or how it works, feel free to ask additional questions in the comments:

const FLAG_1 = 0b0001; // 1
const FLAG_2 = 0b0010; // 2
const FLAG_3 = 0b0100; // 4
const FLAG_4 = 0b1000; // 8
// Can you see the pattern? ;-)

function show_flags ($flags) {
  if ($flags & FLAG_1) {
    echo "You passed flag 1!<br>\n";
  }
  if ($flags & FLAG_2) {
    echo "You passed flag 2!<br>\n";
  }
  if ($flags & FLAG_3) {
    echo "You passed flag 3!<br>\n";
  }
  if ($flags & FLAG_4) {
    echo "You passed flag 4!<br>\n";
  }
}

show_flags(FLAG_1 | FLAG_3);

Demo

---

Because the flags are integers, on a 32-bit platform you define up to 32 flags. On a 64-bit platform, it's 64. It is also possible to define the flags as strings, in which case the number of available flags is more or less infinite (within the bounds of system resources, of course). Here's how it works in binary (cut down to 8-bit integers for simplicity).

FLAG_1
Dec:    1
Binary: 00000001

FLAG_2
Dec:    2
Binary: 00000010

FLAG_3
Dec:    4
Binary: 00000100

// And so on...

When you combine the flags to pass them to the function, you OR them together. Let's take a look at what happens when we pass FLAG_1 | FLAG_3

  00000001
| 00000100
= 00000101

And when you want to see which flags were set, you AND the bitmask with the flag. So, lets take the result above and see if FLAG_3 was set:

  00000101
& 00000100
= 00000100

...we get the value of the flag back, a non-zero integer - but if we see if FLAG_2 was set:

  00000101
& 00000010
= 00000000

...we get zero. This means that you can simply evaluate the result of the AND operation as a boolean when checking if the value was passed.

Answer 2

define( "INCLUDE_HIDDEN", 0x1 );
define( "HIDE_EXTS", 0x2 );
define( "SHOW_ABSOLUTE_PATHS", 0x4 );
//And so on, 0x8, 0x10, 0x20, 0x40, 0x80, 0x100, 0x200, 0x400, 0x800 etc..

You can then check for individual flags in your ls function:

if( $flags & INCLUDE_HIDDEN ) { //<-- note just a single &, bitwise and
    //$flags have INCLUDE_HIDDEN
}

Answer 3

The others have offered good suggestions, but these days it's much more common to pass in associative arrays instead of bitmasks. It's much more readable and allows you to pass in other variables other than just true/false values. Something like this:

myFunction(['includeHidden' => true, 'fileExts' => false, 'string' => 'Xyz']);

function myFunction($options) {
    // Set the default options
    $options += [
        'includeHidden' => false,
        'fileExts' => true,
        'string' => 'Abc',
    ];

    if ($options['includeHidden']) {
        ...
    }
    ...
}

Answer 4

I had an API I was working with - the documentation was pretty thin. It was giving us a single integer and said:

"Information is encoded in these values as a bit field. Bit fields work by storing multiple true/false values in the same integer, instead of having multiple integers for each value. To look up a value in a bitfield, you'll want to make use of Bit Masks."

Bit     Decimal Value    Setting
0       1                Display
1       2                Sell
2       4                Kiosk Display
3       8                No Passes
4       16               Dolby Digital
5       32               THX
6       64               DLP
[etc]

I figured out how to solve it based on the other answers here, and though I would share the function I came up with.

I ended up writing this function:

<?php
function bitmap_decode( $label_array, $value, $return_item ) {
    $label_array = array_flip( $label_array ); // swap the keys and values
    $i = $label_array[ $return_item ]; // get the decimal key value for the item on the list
    $i =  2**$i; // use 2 to the nth power to get the decimal bitmap value for the item
    return $value & $i ? 1 : 0; // use the & operator to determine if the value is true or false. return 1 or 0 accordingly
}

... which would parse out the values

$info1 = array(
    'Display',
    'Sell',
    'Kiosk Display',
    'No Passes',
    'Dolby Digital',
    'THX',
    'DLP',
);


$api_value = 5;
echo bitmap_decode( $info1, $api_value, 'Sell' ); // 0

$api_value = 35;
echo bitmap_decode( $info1, $api_value, 'Sell' ); // 1

And then we can do the opposite - encode the values without having to think about any non-binary numbers or anything like that:

function bitmap_encode( $items, $label_array ) {
    $return = 0;
    $label_array = array_flip( $label_array ); // swap the keys and values
    foreach ( $items as $item ) {
        $i = $label_array[ $item ]; // get the decimal key value for the item on the list
        $i = 2**$i; // use 2 to the nth power to get the decimal bitmap value for the item
        $return += $i; // add $i to the return value
    }
    return $return;
}

// set these flags as true
$flags = array( 'Display', 'Sell', 'THX' );
echo bitmap_encode( $flags, $info1 ); // 35

I know this does not precisely answer the OP question, but it's another approach to making a bitmask system in PHP.