<#
.SYNOPSIS
    Windows DHCP Server Prometheus Exporter
.DESCRIPTION
    Prometheus exporter for Windows DHCP Server - scope utilization, active leases,
    free addresses, reservations, failover status, DHCP statistics, and lease
    expiration tracking via PowerShell. Uses the built-in DhcpServer module.
.PARAMETER Mode
    Output mode: 'stdout' (default), 'textfile', or 'http'
.PARAMETER Port
    HTTP port for http mode (default: 9534)
.PARAMETER TextfileDir
    Directory for textfile collector output (default: C:\ProgramData\node_exporter)
.PARAMETER InstallScheduledTask
    Switch to create a scheduled task for automatic execution
.PARAMETER TaskIntervalMinutes
    Interval in minutes for the scheduled task (default: 2)
.NOTES
    Author:  Phil Connor
    Contact: contact@mylinux.work
    Website: https://mylinux.work
    License: MIT
    Version: 1.0

    Metrics Exported:
      Core:
        - windows_dhcp_up
        - windows_dhcp_exporter_info{version}

      Scope:
        - windows_dhcp_scope_pool_total{scope_id,name}
        - windows_dhcp_scope_pool_in_use{scope_id,name}
        - windows_dhcp_scope_pool_free{scope_id,name}
        - windows_dhcp_scope_pool_reserved{scope_id,name}
        - windows_dhcp_scope_pool_utilization{scope_id,name}
        - windows_dhcp_scope_pool_pending{scope_id,name}
        - windows_dhcp_scope_state{scope_id,name,state}
        - windows_dhcp_scope_leases_expiring{scope_id,name,within}
        - windows_dhcp_scope_lease_longest_seconds{scope_id,name}
        - windows_dhcp_scope_lease_shortest_seconds{scope_id,name}

      Failover:
        - windows_dhcp_failover_state{name,partner,mode,state}
        - windows_dhcp_failover_scope_count{name}

      Statistics:
        - windows_dhcp_discovers_total
        - windows_dhcp_offers_total
        - windows_dhcp_requests_total
        - windows_dhcp_acks_total
        - windows_dhcp_naks_total
        - windows_dhcp_declines_total
        - windows_dhcp_releases_total

      Totals:
        - windows_dhcp_scopes_total
        - windows_dhcp_leases_active_total
        - windows_dhcp_exporter_duration_seconds
        - windows_dhcp_exporter_last_run_timestamp
#>

param(
    [ValidateSet('stdout','textfile','http')]
    [string]$Mode = 'stdout',

    [int]$Port = 9534,

    [string]$TextfileDir = 'C:\ProgramData\node_exporter',

    [switch]$InstallScheduledTask,

    [int]$TaskIntervalMinutes = 2
)

$ErrorActionPreference = 'SilentlyContinue'
$Version = '1.0'

# ============================================================================
# SCHEDULED TASK INSTALLER
# ============================================================================

if ($InstallScheduledTask) {
    $scriptPath = $MyInvocation.MyCommand.Path
    $action = New-ScheduledTaskAction -Execute 'powershell.exe' `
        -Argument "-NoProfile -ExecutionPolicy Bypass -File `"$scriptPath`" -Mode textfile"
    $trigger = New-ScheduledTaskTrigger -RepetitionInterval (New-TimeSpan -Minutes $TaskIntervalMinutes) `
        -Once -At (Get-Date)
    $settings = New-ScheduledTaskSettingsSet -AllowStartIfOnBatteries -DontStopIfGoingOnBatteries `
        -StartWhenAvailable -ExecutionTimeLimit (New-TimeSpan -Minutes 5)
    Register-ScheduledTask -TaskName 'WindowsDhcpExporter' -Action $action -Trigger $trigger `
        -Settings $settings -RunLevel Highest -User 'SYSTEM' -Force
    Write-Host 'Scheduled task "WindowsDhcpExporter" installed successfully.'
    exit 0
}

# ============================================================================
# HELPER FUNCTIONS
# ============================================================================

function Get-PrometheusEscape {
    param([string]$Value)
    $Value -replace '\\', '\\\\' -replace '"', '\"' -replace "`n", '\n'
}

function Write-MetricHeader {
    param([string]$Name, [string]$Type, [string]$Help)
    "# HELP $Name $Help"
    "# TYPE $Name $Type"
}

# ============================================================================
# METRIC COLLECTION
# ============================================================================

function Get-DhcpMetrics {
    $startTime = Get-Date
    $metrics = [System.Collections.Generic.List[string]]::new()

    # --- Exporter status ---
    $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_up' 'gauge' 'Exporter status (1=up, 0=down)'))

    try {
        Import-Module DhcpServer -ErrorAction Stop
        $metrics.Add('windows_dhcp_up 1')
    } catch {
        $metrics.Add('windows_dhcp_up 0')
        return ($metrics -join "`n")
    }

    $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_exporter_info' 'gauge' 'Exporter version information'))
    $metrics.Add("windows_dhcp_exporter_info{version=`"$Version`"} 1")

    # --- Scope metrics ---
    try {
        $scopes = Get-DhcpServerv4Scope -ErrorAction Stop
        $totalScopes = ($scopes | Measure-Object).Count
        $totalActive = 0

        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_scope_pool_total' 'gauge' 'Total addresses in the DHCP scope'))
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_scope_pool_in_use' 'gauge' 'Currently leased addresses'))
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_scope_pool_free' 'gauge' 'Available addresses in the scope'))
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_scope_pool_reserved' 'gauge' 'Number of reservations in the scope'))
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_scope_pool_utilization' 'gauge' 'Scope utilization percentage'))
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_scope_pool_pending' 'gauge' 'Pending offers'))
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_scope_state' 'gauge' 'Scope state (1=current state)'))
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_scope_leases_expiring' 'gauge' 'Leases expiring within threshold'))
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_scope_lease_longest_seconds' 'gauge' 'Remaining time on the longest lease'))
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_scope_lease_shortest_seconds' 'gauge' 'Remaining time on the shortest lease'))

        foreach ($scope in $scopes) {
            $scopeId = $scope.ScopeId.ToString()
            $scopeName = Get-PrometheusEscape $scope.Name

            try {
                $stats = Get-DhcpServerv4ScopeStatistics -ScopeId $scope.ScopeId -ErrorAction Stop
                $total = $stats.AddressesAvailable + $stats.AddressesInUse
                $inUse = $stats.AddressesInUse
                $free = $stats.AddressesAvailable
                $pending = $stats.PendingOffers
                $reserved = (Get-DhcpServerv4Reservation -ScopeId $scope.ScopeId -ErrorAction SilentlyContinue | Measure-Object).Count
                $util = if ($total -gt 0) { [math]::Round(($inUse / $total) * 100, 2) } else { 0 }

                $totalActive += $inUse

                $metrics.Add("windows_dhcp_scope_pool_total{scope_id=`"$scopeId`",name=`"$scopeName`"} $total")
                $metrics.Add("windows_dhcp_scope_pool_in_use{scope_id=`"$scopeId`",name=`"$scopeName`"} $inUse")
                $metrics.Add("windows_dhcp_scope_pool_free{scope_id=`"$scopeId`",name=`"$scopeName`"} $free")
                $metrics.Add("windows_dhcp_scope_pool_reserved{scope_id=`"$scopeId`",name=`"$scopeName`"} $reserved")
                $metrics.Add("windows_dhcp_scope_pool_utilization{scope_id=`"$scopeId`",name=`"$scopeName`"} $util")
                $metrics.Add("windows_dhcp_scope_pool_pending{scope_id=`"$scopeId`",name=`"$scopeName`"} $pending")
            } catch {
                # Skip scope stats if unavailable
            }

            # Scope state
            $stateStr = $scope.State.ToString()
            $metrics.Add("windows_dhcp_scope_state{scope_id=`"$scopeId`",name=`"$scopeName`",state=`"$stateStr`"} 1")

            # Lease expiration tracking
            try {
                $leases = Get-DhcpServerv4Lease -ScopeId $scope.ScopeId -ErrorAction Stop |
                    Where-Object { $_.AddressState -match 'Active' -and $_.LeaseExpiryTime }
                $now = Get-Date
                $exp1h = 0; $exp4h = 0; $exp24h = 0
                $longest = 0; $shortest = [int]::MaxValue

                foreach ($lease in $leases) {
                    $remaining = ($lease.LeaseExpiryTime - $now).TotalSeconds
                    if ($remaining -gt 0) {
                        if ($remaining -le 3600)  { $exp1h++ }
                        if ($remaining -le 14400) { $exp4h++ }
                        if ($remaining -le 86400) { $exp24h++ }
                        if ($remaining -gt $longest)  { $longest = [math]::Round($remaining) }
                        if ($remaining -lt $shortest) { $shortest = [math]::Round($remaining) }
                    }
                }
                if ($shortest -eq [int]::MaxValue) { $shortest = 0 }

                $metrics.Add("windows_dhcp_scope_leases_expiring{scope_id=`"$scopeId`",name=`"$scopeName`",within=`"1h`"} $exp1h")
                $metrics.Add("windows_dhcp_scope_leases_expiring{scope_id=`"$scopeId`",name=`"$scopeName`",within=`"4h`"} $exp4h")
                $metrics.Add("windows_dhcp_scope_leases_expiring{scope_id=`"$scopeId`",name=`"$scopeName`",within=`"24h`"} $exp24h")
                $metrics.Add("windows_dhcp_scope_lease_longest_seconds{scope_id=`"$scopeId`",name=`"$scopeName`"} $longest")
                $metrics.Add("windows_dhcp_scope_lease_shortest_seconds{scope_id=`"$scopeId`",name=`"$scopeName`"} $shortest")
            } catch {
                # Skip lease details if unavailable
            }
        }

        # Scope totals
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_scopes_total' 'gauge' 'Total number of configured scopes'))
        $metrics.Add("windows_dhcp_scopes_total $totalScopes")
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_leases_active_total' 'gauge' 'Total active leases across all scopes'))
        $metrics.Add("windows_dhcp_leases_active_total $totalActive")
    } catch {
        # DHCP scope enumeration failed
    }

    # --- Failover ---
    try {
        $failovers = Get-DhcpServerv4Failover -ErrorAction Stop
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_failover_state' 'gauge' 'Failover relationship state (1=Normal)'))
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_failover_scope_count' 'gauge' 'Number of scopes in the failover relationship'))

        foreach ($fo in $failovers) {
            $foName = Get-PrometheusEscape $fo.Name
            $partner = Get-PrometheusEscape $fo.PartnerServer
            $foMode = $fo.Mode.ToString()
            $foState = $fo.State.ToString()
            $stateVal = if ($foState -eq 'Normal') { 1 } else { 0 }
            $scopeCount = ($fo.ScopeId | Measure-Object).Count

            $metrics.Add("windows_dhcp_failover_state{name=`"$foName`",partner=`"$partner`",mode=`"$foMode`",state=`"$foState`"} $stateVal")
            $metrics.Add("windows_dhcp_failover_scope_count{name=`"$foName`"} $scopeCount")
        }
    } catch {
        # No failover configured - skip
    }

    # --- DHCP server statistics ---
    try {
        $stats = Get-DhcpServerv4Statistics -ErrorAction Stop
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_discovers_total' 'counter' 'Total DHCPDISCOVER packets received'))
        $metrics.Add("windows_dhcp_discovers_total $($stats.Discovers)")
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_offers_total' 'counter' 'Total DHCPOFFER packets sent'))
        $metrics.Add("windows_dhcp_offers_total $($stats.Offers)")
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_requests_total' 'counter' 'Total DHCPREQUEST packets received'))
        $metrics.Add("windows_dhcp_requests_total $($stats.Requests)")
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_acks_total' 'counter' 'Total DHCPACK packets sent'))
        $metrics.Add("windows_dhcp_acks_total $($stats.Acks)")
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_naks_total' 'counter' 'Total DHCPNAK packets sent'))
        $metrics.Add("windows_dhcp_naks_total $($stats.Naks)")
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_declines_total' 'counter' 'Total DHCPDECLINE packets received'))
        $metrics.Add("windows_dhcp_declines_total $($stats.Declines)")
        $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_releases_total' 'counter' 'Total DHCPRELEASE packets received'))
        $metrics.Add("windows_dhcp_releases_total $($stats.Releases)")
    } catch {
        # Statistics unavailable
    }

    # --- Duration and timestamp ---
    $duration = [math]::Round(((Get-Date) - $startTime).TotalSeconds, 2)
    $timestamp = [math]::Round((Get-Date -UFormat %s), 0)

    $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_exporter_duration_seconds' 'gauge' 'Script execution time'))
    $metrics.Add("windows_dhcp_exporter_duration_seconds $duration")
    $metrics.AddRange([string[]](Write-MetricHeader 'windows_dhcp_exporter_last_run_timestamp' 'gauge' 'Unix timestamp of last successful run'))
    $metrics.Add("windows_dhcp_exporter_last_run_timestamp $timestamp")

    return ($metrics -join "`n")
}

# ============================================================================
# OUTPUT
# ============================================================================

switch ($Mode) {
    'stdout' {
        Get-DhcpMetrics
    }
    'textfile' {
        if (-not (Test-Path $TextfileDir)) { New-Item -ItemType Directory -Path $TextfileDir -Force | Out-Null }
        $tempFile = Join-Path $TextfileDir "windows-dhcp-metrics.tmp"
        $finalFile = Join-Path $TextfileDir "windows-dhcp-metrics.prom"
        Get-DhcpMetrics | Out-File -FilePath $tempFile -Encoding utf8 -NoNewline
        Move-Item -Path $tempFile -Destination $finalFile -Force
        Write-Host "Wrote metrics to $finalFile"
    }
    'http' {
        $prefix = "http://+:$Port/metrics/"
        $listener = [System.Net.HttpListener]::new()
        $listener.Prefixes.Add($prefix)
        $listener.Start()
        Write-Host "Listening on port $Port..."
        try {
            while ($listener.IsListening) {
                $context = $listener.GetContext()
                $response = $context.Response
                $metricsOutput = Get-DhcpMetrics
                $buffer = [System.Text.Encoding]::UTF8.GetBytes($metricsOutput)
                $response.ContentType = 'text/plain; version=0.0.4; charset=utf-8'
                $response.ContentLength64 = $buffer.Length
                $response.OutputStream.Write($buffer, 0, $buffer.Length)
                $response.OutputStream.Close()
            }
        } finally {
            $listener.Stop()
        }
    }
}