<#
.SYNOPSIS
    Sends a reminder on or before a date in a sharepoint
    list date field
.DESCRIPTION
    Sends a reminding message to a user and some cc recipients
    on or before a specific date. Recipient and date must be field
    in the same sharepoint list.
.PARAMETER WebUri
    Specifies the full URI of the (sub)site.
.PARAMETER ListName
    Specifies the list's name.
.PARAMETER DateFieldName
    Specifies the name of the field containing the desired due date.
    No space chars. Special characters must be escaped, example:
    "Datum-zurück" becomes "Datum_x002d_zur_x00fc_ck"
.PARAMETER UserFieldName
    Specifies the name of the field containing the user name.
    Must be a sharepoint user lookup field. Special characters must
    be escaped, see DateFieldName.
.PARAMETER ActiveFieldName
    Specifies the name of the field which will be used to decide if
    the current item is active. If the field is empty, non-
    existent or contains the letters [Nn] the item will be assumed as
    non-active. Special characters must be escaped.
.PARAMETER DaysBefore
    Specifies how many days before the due date this reminder will
    fire.
.PARAMETER EmailTemplate
    Specifies either a path to a email template or the template as
    string. The template is a notmal text file. You can use the
    notation {item:fieldname} for looking up values from the list item.
    Two special items ca be used: {LookedUpName} and {DueDate}.
.PARAMETER EmailSubject
    Specifies the email subject line
.PARAMETER EmailFrom
    Specifies the sender's email address
.PARAMETER AlwaysSendCC
    Specifies an email address which will be used as cc address for
    every message
.PARAMETER WhatIf
    If set, a dry run will be performed
.PARAMETER EventLog
    If set the error messages are written to the event log. Event id
    666 are script errors, id 667 are template engine errors.
.NOTES
    Author: kurator | lagerhaus128.ch
    Changelog:
    Revision 3:
        Code optimization, added some explict type casts.
    Revision 2:
        Added event log write functionality.
    Revision 1:
        ActiveFieldName has been added, minor code changes.
    Revision 0:
        Initial revision
.EXAMPLE
    SP-SendReminder -WebUri "https://sps-dev/oks/testlists" -ListName "List01" -DateFieldName "DateField" -UserFieldName "UserNameField" -DaysBefore "0" -EmailTemplate "C:\temp\sps-dev\test01.templ" -EmailSubject "Due date is coming" -EmailFrom "noreply@domain.tld" -AlwaysSendCC "ccaddress@domain.tld" -WhatIf:$true -EventLog
#>

#Requires -Version 4.0
#Requires -PSSnapin Microsoft.Sharepoint.Powershell -Version 1.0

param(
    [Parameter(Mandatory=$true)][string]$WebUri,
    [Parameter(Mandatory=$true)][string]$ListName,
    [Parameter(Mandatory=$true)][string]$DateFieldName,
    [Parameter(Mandatory=$true)][string]$UserFieldName,
    [Parameter(Mandatory=$false)][string]$ActiveFieldName,
    [Parameter(Mandatory=$true)][int]$DaysBefore,
    [Parameter(Mandatory=$true)][string]$EmailTemplate,
    [Parameter(Mandatory=$true)][string]$EmailSubject,
    [Parameter(Mandatory=$true)][string]$EmailFrom,
    [Parameter(Mandatory=$false)][string]$AlwaysSendCC,
    [Parameter(Mandatory=$false)][bool]$WhatIf=$false,
    [switch]$EventLog
)

Add-PSSnapin Microsoft.Sharepoint.Powershell -ErrorAction SilentlyContinue

Try {
    $SPWeb = Get-SPWeb $WebUri
    $SPWeb.Lists[$ListName].Items | ForEach-Object {
        $item = $_
        if(([datetime]::Today).AddDays(-1*$DaysBefore) -eq $item[$DateFieldName]) {
            $ItemActive = $true
            if($ActiveFieldName.Length -ne 0) {
                if(($item[$ActiveFieldName].Length -eq 0) -or ([string]($item[$ActiveFieldName].toString()) -match '[Nn]')) {
                    $ItemActive = $false
                }
            }
            if(($ItemActive) -and ($item[$UserFieldName])) {
                $user = New-Object Microsoft.SharePoint.SPFieldUserValue($SPWeb, $item[$UserFieldName].ToString())
                if(Test-Path $EmailTemplate -ErrorAction SilentlyContinue) {
                    $EmailBody = [IO.File]::ReadAllText($EmailTemplate)
                } else {
                    $EmailBody = $EmailTemplate
                }
                ($EmailBody | Select-String -Pattern '(\{[a-zA-Z0-9_\:]+\})' -AllMatches).Matches.Value | ForEach-Object {
                    try{
                        switch -Regex ($_) {
                            '\{LookedUpName\}' {
                                $EmailBody = $EmailBody.Replace($_,[string]$user.LookupValue)
                            }
                            '\{DueDate\}' {
                                $EmailBody = $EmailBody.Replace($_,[string](Get-Date -Date $item[$DateFieldName] -UFormat '%d.%m.%Y'))
                            }
                            '\{item\:.*\}' {
                                if($item[($_ -creplace '\{item:(.*)\}','$1')]) {
                                    $EmailBody = $EmailBody.Replace($_,[string]$item[($_ -creplace '\{item:(.*)\}','$1')])
                                } else {
                                    $EmailBody = $EmailBody.Replace($_,"!!List does not contain field ""{0}""" -f $_)
                                }
                            }
                            default {
                                $EmailBody = $EmailBody.Replace($_,"!!Reference ""{0}"" not found" -f $_)
                            }
                        }

                        if(Get-Command "LookupCCUser" -ErrorAction SilentlyContinue) {
                            $CCUser = LookUpCCUser $SPWeb,$item
                        }
                    } catch {
                        if ($EventLog) {
                            Write-EventLog -LogName 'Windows PowerShell' -Source 'PowerShell' -EntryType 'Error' -EventId 667 -Message ('The script engine of{0} whith the following exception: {1}' -f $MyInvocation.MyCommand.Name, $_.Exception.Message) -Category 667
                        } else {
                            Write-Error $_.Exception.Message
                        }
                    }
                }
                if(! $WhatIf) {
                    $EmailHeader = New-Object System.Collections.Specialized.StringDictionary
                    @{"to"=$user.User.Email; "from"=$EmailFrom; "subject"=$EmailSubject; "content-type"="text/html"}.GetEnumerator() | ForEach-Object {
                        $EmailHeader.Add($_.Name, $_.Value)
                    }
                    if($AlwaysSendCC) {$EmailHeader.Add("Cc", $AlwaysSendCC)}
                    [Microsoft.SharePoint.Utilities.SPUtility]::SendEmail($SPWeb, $EmailHeader, $EmailBody)
                    Remove-Variable EmailHeader
                } else {
                    if ($EventLog) {
                        Write-EventLog -LogName 'Windows PowerShell' -Source 'PowerShell' -EntryType 'Information' -EventId 666 -Message ("Would send the following lines to {0}, {1}`n`nSUBJECT: {2}`n`n{3}" -f $user.User.Email, $AlwaysSendCC, $EmailSubject, $EmailBody) -Category 666
                    } else {
                        Write-Host ("Would send the following lines to {0}, {1}`n`nSUBJECT: {2}`n`n{3}" -f $user.User.Email, $AlwaysSendCC, $EmailSubject, $EmailBody)
                    }
                }
                Remove-Variable EmailBody
            }
            Remove-Variable ItemActive
        }
        Remove-Variable item
    }
    Remove-Variable SPWeb
} Catch {
    if ($EventLog) {
        Write-EventLog -LogName 'Windows PowerShell' -Source 'PowerShell' -EntryType 'Error' -EventId 666 -Message ('{0} failed on line {1} whith the following exception: {2}' -f $MyInvocation.MyCommand.Name, $_.InvocationInfo.ScriptLineNumber, $_.Exception.Message) -Category 666
    } else {
        Write-Error $_.Exception.Message
    }
    exit 1
}