Validate string replacement changes that affect large amount of files

Check-StringReplacementChange.ps1

##
## Get the diff content of a Github PR, and analyze if the diffs are only caused by the string replacement.
## [Interesting exercise]
##

param(
    [Parameter(Mandatory=$true)]
    [string]
    $PRUrl,

    [Parameter(Mandatory=$true)]
    [string]
    $OriginalString,

    [Parameter(Mandatory=$true)]
    [string]
    $CurrentString,

    [switch]
    $PassThru
)

enum DiffState {
    None
    FileInfo
    DiffContent
}

[DiffState]$state = [DiffState]::None
$Log = @{}

$diffFile = Join-Path -Path $PSScriptRoot -ChildPath "diff.txt"
if (Test-Path -Path $diffFile) { Remove-Item -Path $diffFile -Force -ErrorAction Stop }

##
## .diff and .patch can be append in the end to get some raw content
##
$PRUrl += ".diff"
Invoke-WebRequest -Uri $PRUrl -OutFile $diffFile

$Script:ScanResult = $null
switch -Wildcard -File $diffFile
{    
    "diff --git *"
        {
            $state = [DiffState]::FileInfo
            $fileName = ($_ -split " ")[-1].Substring(2)

            if ($Script:ScanResult -ne $null -and 
                $Script:ScanResult["OriginalStrs"].Count -ne $Script:ScanResult["NewStrs"].Count)
            {
                throw "$($Script:ScanResult['FileName']) contains mismatched + and -"
            }
            
            if ($Log.ContainsKey($fileName)) 
            {
                throw "$fileName appeared twice in diff file. Something is wrong"
            }

            $Script:ScanResult = @{ FileName = $fileName; OriginalStrs = @(); NewStrs = @() }
            $Log[$fileName] = $Script:ScanResult
        }

    "@@ * @@ *"
        {
            $state = [DiffState]::DiffContent
        }

    "-*"
        {
            if ($state -eq [DiffState]::DiffContent)
            {
                $old = $_.Substring(1)
                if (![string]::IsNullOrWhiteSpace($old))
                {
                    $Script:ScanResult["OriginalStrs"] += $old
                }
            }
        }

    "+*"
        {
            if ($state -eq [DiffState]::DiffContent)
            {
                $new = $_.Substring(1)
                if (![string]::IsNullOrWhiteSpace($new))
                {
                    $Script:ScanResult["NewStrs"] += $new
                }
            }
        }
}


Write-Host -Object "$($Log.Count) files involved in this PR`n"

foreach ($result in $Log.Values)
{
    $newStrs = $result["NewStrs"]
    $originalStrs = $result["OriginalStrs"]

    for ($i = 0; $i -lt $originalStrs.Length; $i++)
    {
        $oldString = $originalStrs[$i]
        $newString = $newStrs[$i]

        $afterReplace = $oldString -replace $OriginalString, $CurrentString
        if ($afterReplace -cne $newString)
        {
            Write-Host @"
$($result['FileName']) - mismatch
Old: $oldString
New: $newString

"@
        }
    }
}

if ($PassThru) { return $Log }