Git-like Diff Tool in PowerShell

Show-GitLikeDiff.ps1

# Git-like Diff Tool in PowerShell

function Compare-Files {
    param (
        [string]$OldFilePath,
        [string]$NewFilePath,
        [int]$ContextLines = 3
    )

    $oldFile = Get-Content -Path $OldFilePath
    $newFile = Get-Content -Path $NewFilePath

    $diffs = [System.Collections.Generic.List[PSObject]]::new()

    $oldIndex = 0
    $newIndex = 0

    while ($oldIndex -lt $oldFile.Count -or $newIndex -lt $newFile.Count) {
        $oldLine = if ($oldIndex -lt $oldFile.Count) { $oldFile[$oldIndex] } else { $null }
        $newLine = if ($newIndex -lt $newFile.Count) { $newFile[$newIndex] } else { $null }

        if ($oldLine -eq $newLine) {
            $oldIndex++
            $newIndex++
        }
        else {
            $diffs.Add([pscustomobject]@{
                    ChangeType    = if ($null -eq $oldLine) { 'Added' } elseif ($newLine -eq $null) { 'Deleted' } else { 'Modified' }
                    OldLineNumber = if ($null -ne $oldLine) { $oldIndex + 1 } else { $null }
                    NewLineNumber = if ($null -ne $newLine) { $newIndex + 1 } else { $null }
                    OldLine       = $oldLine
                    NewLine       = $newLine
                })
            if ($null -ne $oldLine) { $oldIndex++ }
            if ($null -ne $newLine) { $newIndex++ }
        }
    }

    return $diffs
}

function Write-Diff {
    param (
        [psobject[]]$Diffs,
        [string[]]$OldFile,
        [string[]]$NewFile,
        [int]$ContextLines = 3
    )

    foreach ($diff in $Diffs) {
        if ($diff.ChangeType -eq 'Modified') {
            Write-Host "Line $($diff.OldLineNumber),$($diff.NewLineNumber) Modified:" -ForegroundColor Yellow
            for ($i = [Math]::Max(0, $diff.OldLineNumber - $ContextLines - 1); $i -lt [Math]::Min($OldFile.Count, $diff.OldLineNumber + $ContextLines); $i++) {
                Write-Host "  $($OldFile[$i])"
            }
            Write-Host "- $($diff.OldLine)" -ForegroundColor Red
            Write-Host "+ $($diff.NewLine)" -ForegroundColor Green
        }
        elseif ($diff.ChangeType -eq 'Added') {
            Write-Host "Line $($diff.NewLineNumber) Added:" -ForegroundColor Green
            Write-Host "+ $($diff.NewLine)" -ForegroundColor Green
        }
        elseif ($diff.ChangeType -eq 'Deleted') {
            Write-Host "Line $($diff.OldLineNumber) Deleted:" -ForegroundColor Red
            Write-Host "- $($diff.OldLine)" -ForegroundColor Red
        }
    }
}

function Show-GitLikeDiff {
    param (
        [string]$OldFilePath,
        [string]$NewFilePath,
        [int]$ContextLines = 3
    )

    $oldFile = Get-Content -Path $OldFilePath
    $newFile = Get-Content -Path $NewFilePath

    $diffs = Compare-Files -OldFilePath $OldFilePath -NewFilePath $NewFilePath -ContextLines $ContextLines
    Write-Diff -Diffs $diffs -OldFile $oldFile -NewFile $newFile -ContextLines $ContextLines
}

# Example usage:
Show-GitLikeDiff -OldFilePath './old.ps1' -NewFilePath './new.ps1'