Bit Shifting in PowerShell, Redux

I do bit shifting in PowerShell all the time. It's quite necessary when you are working with option values in Active Directory, WMI, and various Win32 interfaces. .NET tends to use enumerations, which are tad easier to deal with in PowerShell.

I had never felt it necessary to break my routines out before, but I saw a posting on this topic by another individual today and I just thought he was making it harder than it had to be. I wanted to present another option to the community for these types of routines. I'm not putting down his work in any way - it was quite ingenious. I never would've thought of doing it that way.

Just FYI, 'shr' is an abbreviation for 'shift right' and correspondingly 'shl' is an abbreviation for 'shift left'. When I was in college (in the stone age) those function names were used in "Pascal extension libraries" and in "Fortran libraries" for performing shift operations. Shift operations are built-in as native operations to many (most?) compiled languages such as C and its various dialects.

Enjoy.

###
### Bit-shift operations
### bit-shift.ps1
###
### for bytes and short integers
###
### Michael B. Smith
### michael at TheEssentialExchange.com
### May, 2012
###

$bitsPerByte = 8	## [byte]
$bitsperWord = 16	## [int16] or [uint16]

function shift-left( [int]$valuesize, [int]$mask, $val, [int]$bits )
{
	if( $bits -ge $valuesize )
	{
		return 0
	}
	if( $bits -eq 0 )
	{
		return $val
	}
	if( $bits -lt 0 )
	{
		write-error "Can't shift by a negative value of bits"
		return -1
	}

	### it's possible to write this so that you never
	### overshift and generate an overflow. it's easier
	### to use a larger variable and mask at the end.

	[int]$result = $val
	for( $i = 0; $i -lt $bits; $i++ )
	{
		$result *= 2
	}

	return ( $result -band $mask )
}

function shift-right( [int]$valuesize, [int]$mask, $val, [int]$bits )
{
	if( $bits -ge $valuesize )
	{
		return 0
	}
	if( $bits -eq 0 )
	{
		return $val
	}
	if( $bits -lt 0 )
	{
		write-error "Can't shift by a negative value of bits"
		return -1
	}

	for( $i = 0; $i -lt $bits; $i++ )
	{
		## normally PowerShell does banker's rounding (well, .NET does)
		## we have to override that here to get true integer division.
		$val = [Math]::Floor( $val / 2 )
	}

	return $val
}

function shl-byte( [byte]$val, [int]$bits )
{
	$result = shift-left $bitsPerByte 0xff $val $bits
	if( $result -lt 0 )
	{
		return $result
	}

	return ( [byte]$result )
}

function shr-byte( [byte]$val, [int]$bits )
{
	$result = shift-right $bitsPerByte 0xff $val $bits
	if( $result -lt 0 )
	{
		return $result
	}

	return ( [byte]$result )
}

function shl-word( [uint16]$val, [int]$bits )
{
	$result = shift-left $bitsPerWord 0xffff $val $bits
	if( $result -lt 0 )
	{
		return $result
	}

	return ( [uint16]$result )
}

function shr-word( [uint16]$val, [int]$bits )
{
	$result = shift-right $bitsPerWord 0xffff $val $bits
	if( $result -lt 0 )
	{
		return $result
	}

	return ( [uint16]$result )
}

function shl( $val, [int]$bits )
{
	if( $val -is [byte] )
	{
		return ( shl-byte $val $bits )
	}
	elseif( $val -is [int16] )
	{
		return [int16]( shl-word $val $bits )
	}
	elseif( $val -is [uint16] )
	{
		return ( shl-word $val $bits )
	}
	elseif( ( $val -lt 65536 ) -and ( $val -ge 0 ) ) ### pretend it's uint16
	{
		return ( shl-word ( [uint16]$val ) $bits )
	}

	write-error "value is an invalid type"
	return -1
}

function shr( $val, [int]$bits )
{
	if( $val -is [byte] )
	{
		return ( shr-byte $val $bits )
	}
	elseif( $val -is [int16] )
	{
		return [int16]( shr-word $val $bits )
	}
	elseif( $val -is [uint16] )
	{
		return ( shr-word $val $bits )
	}
	elseif( ( $val -lt 65536 ) -and ( $val -ge 0 ) ) ### pretend it's uint16
	{
		return ( shr-word ( [uint16]$val ) $bits )
	}

	write-error "value is an invalid type"
	return -1
}
Published Thursday, May 10, 2012 9:27 AM by michael
Filed under: ,

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