#requires -Version 5.1
<#
Normalize hostname by replacing underscore (_) with hyphen (-).
Only applies Rename-Computer when an underscore exists.

Requirements:
- Run as Administrator.

Notes:
- Computer name change takes effect after restart.
#>

[CmdletBinding(SupportsShouldProcess = $true)]
param(
    [switch]$Restart
)

Set-StrictMode -Version Latest
$ErrorActionPreference = 'Stop'

function Test-IsAdministrator {
    try {
        $currentIdentity = [Security.Principal.WindowsIdentity]::GetCurrent()
        $principal = New-Object Security.Principal.WindowsPrincipal($currentIdentity)
        return $principal.IsInRole([Security.Principal.WindowsBuiltInRole]::Administrator)
    }
    catch {
        return $false
    }
}

if (-not (Test-IsAdministrator)) {
    throw 'Please run this script as Administrator.'
}

$currentName = $env:COMPUTERNAME
if ([string]::IsNullOrWhiteSpace($currentName)) {
    throw 'Could not read current hostname.'
}

$newName = $currentName.Replace('_', '-')

Write-Host ("Current hostname: {0}" -f $currentName)
Write-Host ("Normalized hostname: {0}" -f $newName)

if ($currentName -eq $newName) {
    Write-Host 'No underscore found. No change is required.' -ForegroundColor Yellow
    exit 0
}

# Check NetBIOS computer name length limit.
if ($newName.Length -gt 15) {
    throw ("Normalized hostname exceeds 15 characters: {0}" -f $newName)
}

if ($PSCmdlet.ShouldProcess($currentName, "Rename-Computer to $newName")) {
    Rename-Computer -NewName $newName -Force
    Write-Host 'Hostname changed. It will take effect after restart.' -ForegroundColor Green

    if ($Restart) {
        Write-Host 'Restarting now.' -ForegroundColor Yellow
        Restart-Computer -Force
    }
    else {
        Write-Host 'Please restart manually when ready.' -ForegroundColor Yellow
    }
}