Get-Entropy with hotspot implemented via Add-Type

Get-Entropy.ps1

## Not measurably faster. Doing this via Add-Type or direct PowerShell still takes about 4 seconds per megabyte.

function Get-Entropy
{
<#
.SYNOPSIS

    Calculate the entropy of a byte array.

    Derived from Get-Entropy by Matthew Graeber (@mattifestation)

.PARAMETER ByteArray

    Specifies the byte array containing the data from which entropy will be calculated.

.EXAMPLE

    C:\PS> $RandArray = New-Object Byte[](10000)
    C:\PS> foreach ($Offset in 0..9999) { $RandArray[$Offset] = [Byte] (Get-Random -Min 0 -Max 256) }
    C:\PS> $RandArray | Get-Entropy

    Description
    -----------
    Calculates the entropy of a large array containing random bytes.

.EXAMPLE

    C:\PS> 0..255 | Get-Entropy

    Description
    -----------
    Calculates the entropy of 0-255. This should equal exactly 8.

.INPUTS

    System.Byte[]

    Get-Entropy accepts a byte array from the pipeline

.OUTPUTS

    System.Double

    Get-Entropy outputs a double representing the entropy of the byte array.

.LINK

    http://www.exploit-monday.com
#>

    [CmdletBinding()] Param (
        [Parameter(Mandatory = $True, Position = 0, ValueFromPipeline = $True)]
        [Byte[]]
        $ByteArray
    )

    BEGIN
    {
        $FrequencyTable = @{}
        $ByteArrayLength = 0

        Add-Type -Name EntropyCalculator -Namespace Kansa -MemberDefinition '
            public static void ProcessBytes(System.Collections.Hashtable frequencyTable, Byte[] byteArray)
            {
                foreach(Byte inputByte in byteArray)
                {
                    if(! frequencyTable.ContainsKey(inputByte))
                    {
                        frequencyTable[inputByte] = 0;
                    }

                    frequencyTable[inputByte] = (int) frequencyTable[inputByte] + 1;
                }
            }
        '
    }

    PROCESS
    {
        $ByteArrayLength += $ByteArray.Length
        [Kansa.EntropyCalculator]::ProcessBytes($FrequencyTable, $ByteArray)
    }

    END
    {
        $Entropy = 0.0

        foreach ($Byte in 0..255)
        {
            $ByteProbability = ([Double] $FrequencyTable[[Byte]$Byte]) / $ByteArrayLength
            if ($ByteProbability -gt 0)
            {
                $Entropy += -$ByteProbability * [Math]::Log($ByteProbability, 2)
            }
        }

        Write-Output $Entropy
    }
}