param(
    [Parameter(Position = 0)]
    [string]$Ip,
    [Parameter(Position = 1, ValueFromRemainingArguments = $true)]
    [string[]]$HostName,
    [string]$Path = "$env:SystemRoot\System32\drivers\etc\hosts",
    [switch]$DryRun,
    [switch]$Check,
    [switch]$List,
    [switch]$Remove,
    [switch]$Help
)

$ErrorActionPreference = 'Stop'
$ScriptName = Split-Path -Leaf $PSCommandPath

if ($Help) {
    @'
Usage:
  hosts-add.ps1 [-Path PATH] -List
  hosts-add.ps1 [-Path PATH] [-DryRun] [-Check] [-Remove] IP HOSTNAME...

Manage hosts entries with duplicate checks and timestamped backups.
'@
    exit 0
}

function Test-Admin {
    $identity = [Security.Principal.WindowsIdentity]::GetCurrent()
    $principal = [Security.Principal.WindowsPrincipal]::new($identity)
    return $principal.IsInRole([Security.Principal.WindowsBuiltInRole]::Administrator)
}

function Get-HostsLines {
    param([string]$File)
    if (Test-Path -LiteralPath $File) {
        return @(Get-Content -LiteralPath $File)
    }
    return @()
}

if ($List) {
    if ($Ip -or $HostName) { throw '-List does not take IP or hostname arguments.' }
    Get-HostsLines -File $Path | Where-Object { $_ -match '\S' -and $_ -notmatch '^\s*#' }
    exit 0
}

if (-not $Ip -or -not $HostName -or $HostName.Count -eq 0) {
    throw 'IP and at least one hostname are required.'
}
if ($Ip -notmatch '^[0-9A-Fa-f:.]+$') {
    throw "Invalid IP address format: $Ip"
}
foreach ($hostEntry in $HostName) {
    if ($hostEntry -notmatch '^[A-Za-z0-9._-]+$') {
        throw "Invalid hostname: $hostEntry"
    }
}
if ($Check -and $Remove) {
    throw '-Check and -Remove cannot be used together.'
}

$lines = Get-HostsLines -File $Path

if ($Check) {
    $missing = $false
    foreach ($hostEntry in $HostName) {
        $found = $false
        foreach ($line in $lines) {
            if ($line -match '^\s*#' -or $line -notmatch '\S') { continue }
            $parts = $line -split '\s+'
            if ($parts.Count -ge 2 -and $parts[0] -eq $Ip -and $parts[1..($parts.Count - 1)] -contains $hostEntry) {
                $found = $true
                break
            }
        }
        if ($found) {
            Write-Host "[$ScriptName] OK $hostEntry -> $Ip"
        } else {
            Write-Host "[$ScriptName] MISSING $hostEntry -> $Ip"
            $missing = $true
        }
    }
    if ($missing) { exit 1 }
    exit 0
}

$target = New-Object System.Collections.Generic.List[string]
foreach ($line in $lines) {
    if ($line -match '^\s*#' -or $line -notmatch '\S') {
        $target.Add($line)
        continue
    }
    $parts = @($line -split '\s+' | Where-Object { $_ })
    if ($parts.Count -lt 2) {
        $target.Add($line)
        continue
    }
    $keptHosts = @($parts[1..($parts.Count - 1)] | Where-Object { $HostName -notcontains $_ })
    if ($keptHosts.Count -gt 0) {
        $target.Add(($parts[0] + ' ' + ($keptHosts -join ' ')))
    }
}

if (-not $Remove) {
    $missingHosts = @()
    foreach ($hostEntry in $HostName) {
        $exists = $false
        foreach ($line in $target) {
            if ($line -match '^\s*#' -or $line -notmatch '\S') { continue }
            $parts = @($line -split '\s+' | Where-Object { $_ })
            if ($parts.Count -ge 2 -and $parts[1..($parts.Count - 1)] -contains $hostEntry) {
                $exists = $true
                break
            }
        }
        if (-not $exists) { $missingHosts += $hostEntry }
    }
    if ($missingHosts.Count -gt 0) {
        $target.Add("$Ip $($missingHosts -join ' ')")
    }
}

$currentText = ($lines -join "`n")
$targetText = ($target.ToArray() -join "`n")
if ($currentText -eq $targetText) {
    Write-Host "[$ScriptName] Hosts file is already current: $Path"
    exit 0
}

if ($DryRun) {
    Write-Host "[$ScriptName] Would update: $Path"
    $target.ToArray()
    exit 0
}

$defaultHosts = "$env:SystemRoot\System32\drivers\etc\hosts"
if (([IO.Path]::GetFullPath($Path) -eq [IO.Path]::GetFullPath($defaultHosts)) -and -not (Test-Admin)) {
    throw 'Administrator privileges are required to edit the system hosts file.'
}

if (Test-Path -LiteralPath $Path) {
    Copy-Item -LiteralPath $Path -Destination "$Path.bak.$(Get-Date -Format yyyyMMddHHmmss)" -Force
} else {
    New-Item -ItemType Directory -Force -Path (Split-Path -Parent $Path) | Out-Null
}
Set-Content -LiteralPath $Path -Value $target.ToArray() -Encoding ASCII
Write-Host "[$ScriptName] Updated: $Path"