<#
.Synopsis
   Gathers hotfixes for all computers in a list then returns a list of hotfixes that are only installed on some of the nodes.
.DESCRIPTION
   Given a list of computers, this cmdlet verifies each computer is reachable and then gathers a list of hotfixes installed on each computer. It will then either display a table of all hotfixes
   that are only installed on a subset of the computers specified or, if the DisplayAllHotfixes switch is specified, a table of all hotfixes installed on all computers. The first column
   is the HotfixID and each other column (property) is the computer name; if the hotfix is installed on that computer it's marked "True", if not it's marked "False".
.EXAMPLE
   $computerlist = @("ServerA", "ServerB", "ServerC")
   Get-HotfixDiscrepancyReport -ComputerList $serverlist

   Generates a hotfix report for three servers.
.EXAMPLE
   Get-HotfixDiscrepancyReport -Computerlist ComputerA, ComputerB, ComputerC -DisplayAllHotfixes | Export-CSV .\hfreport.csv

   Lists all hotfixes and whether or not they're installed for three computers and exports the list to a CSV file.
#>
function Get-HotfixDiscrepancyReport
{
    [CmdletBinding()]
    [OutputType([PSObject[]])]
    Param
    (
        # A list of computers - can be an array variable or a comma delimited list of computers
        [Parameter(Mandatory=$true,
                   ValueFromPipeline=$true,
                   Position=0)]
        [Array]$ComputerList,

        # If you want to display all hotfixes instead of just hotfixes that aren't installed on every computer
        [Switch]$DisplayAllHotfixes
    )

    Process
    {
        Write-Verbose ("Verifying computer list")
        $VerifiedComputers = @()
        foreach ($computer in $ComputerList)
        {
            if(Test-Connection $computer -Count 1 -ErrorAction SilentlyContinue)
            {
                $VerifiedComputers += $computer
            }
            else
            {
                Write-Verbose ("$computer failed ping; skipping")
            }
        }
        If ($VerifiedComputers.Count -lt 2)
        {
            Write-Error -Message "Not enough computers to generate a report. If your initial list had more than one computer, enable verbose mode to see which computers are failing."
            return
        }
        [hashtable]$InstalledHotfixes = @{}
        Write-Verbose ("Getting hotfixes for " + $VerifiedComputers[0])
        Get-WmiObject Win32_QuickFixEngineering -Computername $VerifiedComputers[0] | % {
            $InstalledHotfixes.Add($_.HotfixId, @($VerifiedComputers[0]))
        }
        for ($i = 1; $i -lt $VerifiedComputers.Count; $i++)
        {
            Write-Verbose ("Getting hotfixes for " + $VerifiedComputers[$i])
            Get-WmiObject Win32_QuickFixEngineering -ComputerName $VerifiedComputers[$i] | % {

                if ($InstalledHotfixes.ContainsKey($_.HotfixID))
                {
                    $InstalledHotfixes.Set_Item($_.HotfixID,($InstalledHotfixes.Get_Item($_.HotfixId) + $VerifiedComputers[$i]))
                }
                else
                {
                    $InstalledHotfixes.Add($_.HotfixId, @($VerifiedComputers[$i]))
                }
            }
        }
        if (!$DisplayAllHotfixes)
        {
            Write-Verbose ("Filtering list to only hotfixes not on all nodes")
            $NewHfList = @{}
            $InstalledHotfixes.GetEnumerator() | % {if ($_.Value.Count -lt $VerifiedComputers.count) {$NewHfList.Add($_.Key,$_.Value)}}
        }
        else
        {
            $NewHfList = $InstalledHotfixes
        }
        $hfdiscrepancylist = @()
        Write-Verbose ("Creating hotfix list")
        $NewHfList.GetEnumerator() | % {
            $hfditem = "" | Select HotfixID
            $hfditem.HotfixID = $_.Key
            foreach ($computer in $VerifiedComputers)
            {
                $hfditem | Add-Member -NotePropertyName $computer -NotePropertyValue ([boolean]($_.Value -contains $computer))
            }
            $hfdiscrepancylist += $hfditem
        }
        return $hfdiscrepancylist | Sort-Object HotfixID
    }
}