<#
.SYNOPSIS
  Create or list Scheduled Tasks on local or remote servers under SYSTEM account.

.DESCRIPTION
  Streamlines task creation and management across multiple servers:
  - Always runs as SYSTEM
  - Supports Once/Daily/Weekly/Monthly triggers
  - Custom action command + arguments
  - Custom settings (start when available, wake to run, restart on failure)
  - Optionally list existing tasks
  - Can target a remote server via CIM session

.PARAMETER ServerName
  (Optional) Remote server to connect to. If omitted, actions run locally.

.PARAMETER ListTasks
  Switch. If specified, lists current scheduled tasks and exits.

.PARAMETER TaskName
  Name of the scheduled task (required unless -ListTasks).

.PARAMETER Description
  Task description (appears in Task Scheduler UI).

.PARAMETER Frequency
  Trigger frequency: Once, Daily, Weekly, Monthly.

.PARAMETER At
  DateTime for the trigger (for Once) or time of day (for recurring triggers).

.PARAMETER DaysOfWeek
  When Frequency = Weekly, specify days (e.g. Monday,Wednesday).

.PARAMETER Months
  When Frequency = Monthly, specify months (e.g. January,June).

.PARAMETER DaysOfMonth
  When Frequency = Monthly, specify days of month (e.g. 1,15,28).

.PARAMETER Command
  Executable to run (default: powershell.exe).

.PARAMETER Arguments
  Command arguments (e.g. '-File C:\Scripts\MyScript.ps1').

.PARAMETER StartWhenAvailable
  If set, task will start if missed.

.PARAMETER WakeToRun
  If set, will wake the machine to run.

.PARAMETER RestartIntervalMinutes
  On failure, how often (minutes) to retry. Zero = no retry.

.PARAMETER RestartCount
  How many retry attempts on failure.
#>
[CmdletBinding()]
Param(
    [string]$ServerName,
    [switch]$ListTasks,
    [Parameter(Mandatory = $false, Position = 0)][string]$TaskName,
    [string]$Description,
    [ValidateSet('Once','Daily','Weekly','Monthly')][string]$Frequency,
    [datetime]$At,
    [string[]]$DaysOfWeek,
    [string[]]$Months,
    [int[]]$DaysOfMonth,
    [string]$Command    = 'powershell.exe',
    [string]$Arguments  = '',
    [switch]$StartWhenAvailable,
    [switch]$WakeToRun,
    [int]$RestartIntervalMinutes = 0,
    [int]$RestartCount = 0
)

# Create CIM session if remote
if ($ServerName) {
    Write-Host "Creating CIM session to $ServerName..."
    $CimSession = New-CimSession -ComputerName $ServerName
}

# List tasks and exit
if ($ListTasks) {
    Write-Host "Listing Scheduled Tasks on" ($ServerName ? $ServerName : 'local')
    Get-ScheduledTask -CimSession $CimSession |
      Select-Object TaskName, State, Actions |
      Format-Table -AutoSize
    return
}

# Validate required parameters for task creation
if (-not $TaskName -or -not $Description -or -not $Frequency -or -not $At) {
    Throw 'Parameters -TaskName, -Description, -Frequency, and -At are required when creating a task.'
}

# Build trigger
switch ($Frequency) {
    'Once' {
        $trigger = New-ScheduledTaskTrigger -Once -At $At
    }
    'Daily' {
        $trigger = New-ScheduledTaskTrigger -Daily -At $At.TimeOfDay
    }
    'Weekly' {
        if (-not $DaysOfWeek) { Throw 'Specify -DaysOfWeek for Weekly triggers.' }
        $trigger = New-ScheduledTaskTrigger -Weekly -DaysOfWeek $DaysOfWeek -At $At.TimeOfDay
    }
    'Monthly' {
        if (-not $Months -or -not $DaysOfMonth) { Throw 'Specify -Months and -DaysOfMonth for Monthly triggers.' }
        $trigger = New-ScheduledTaskTrigger -Monthly -MonthsOfYear $Months -DaysOfMonth $DaysOfMonth -At $At.TimeOfDay
    }
}

# Build action
$action = New-ScheduledTaskAction -Execute $Command -Argument $Arguments

# Build settings
$settings = New-ScheduledTaskSettingsSet `
    -AllowStartIfOnBatteries:$false `
    -DontStopOnIdleEnd `
    -StartWhenAvailable:$StartWhenAvailable.IsPresent `
    -WakeToRun:$WakeToRun.IsPresent

if ($RestartIntervalMinutes -gt 0) {
    $interval = New-TimeSpan -Minutes $RestartIntervalMinutes
    $settings = $settings | Set-ScheduledTaskSettings -RestartOnFailureInterval $interval -RestartCount $RestartCount
}

# Register task as SYSTEM
Write-Host "Registering task '$TaskName' on" ($ServerName ? $ServerName : 'local')
Register-ScheduledTask `
  -TaskName $TaskName `
  -Action $action `
  -Trigger $trigger `
  -Settings $settings `
  -Description $Description `
  -User 'NT AUTHORITY\SYSTEM' `
  -RunLevel Highest `
  -CimSession $CimSession `
  -Force

Write-Host "Task '$TaskName' created successfully."