_DiffFolders

#https://serverfault.com/questions/532065/how-do-i-diff-two-folders-in-windows-powershell/1041111#1041111

Function DirDiff { Param($a, $b, [switch]$Force, [switch]$Verbose)
	$a,$b `
	| DiffFolders -Force:$Force -Verbose:$Verbose `
	| %{
		$item = $_
		switch -Exact ($item.event) {
			'Added'   { ">`t$($item.value)" }
			'Deleted' { "<`t$($item.value)" }
			'Changed' { "~`t$($item.value)" }
			''        { "=`t$($item.value)" }
		}
	}
}

linux.ps1

Function DiffFolders { Param([switch]$Force, [switch]$Verbose)
	Begin {
		$current = $last = $NULL
		
		Function Output { Param($event, $value)
			[PSCustomObject]@{
				parent = $current
				event  = $event
				value  = (
					$value | Resolve-Path -Relative -RelativeBasePath (($current,$last)[$event -eq 'Deleted'])
				) + (([IO.Path]::DirectorySeparatorChar,'')[!$value.Parent])
			}
		}
	}
	Process {
		$last      = $current
		$current   = $_
		$unchanged = 0
		if (!$last) {
			return
		}

		$queue = [System.Collections.Queue]::new()
		$queue.Enqueue(($last,$current | %{ Get-Item $_ }))
		while ($queue.Count) {
			$x,$y = $queue.Dequeue()

			if ($x) {
				$x = @($x | Get-ChildItem | Sort-Object -Property Name)
			}
			else {
				$x = @()
			}
			if ($y) {
				$y = @($y | Get-ChildItem | Sort-Object -Property Name)
			}
			else {
				$y = @()
			}
			
			$i = $j = 0
			$m = $n = $NULL
			while ($True) {
				$m = $x[$i]
				$n = $y[$j]
				if (!($m -or $n)) {
					break
				}
			
				if (
					$m.Parent -or $n.Parent -and
					$m.Name -ceq $n.Name
				) {
					if (!$m.Parent) {
						$m = $NULL
					}
					elseif (!$n.Parent) {
						$n = $NULL
					}
					else {
						$queue.Enqueue(($m,$n))
						++$i
						++$j
						continue
					}
				}
				
				if ($m -and (
					(!$n) -or ($m.Name -lt $n.Name)
				)) {
					Output 'Deleted' $m
					if ($Force -and $m.Parent) {
						$queue.Enqueue(($m,$NULL))
					}
					++$i
				}
				elseif ($n -and (
					(!$m) -or ($n.Name -lt $m.Name)
				)) {
					Output 'Added' $n
					if ($Force -and $n.Parent) {
						$queue.Enqueue(($NULL,$n))
					}
					++$j
				}
				else {
					++$i
					++$j
					if (
						$m.Length -ne $n.Length -or
						(cmp -s $m.FullName $n.FullName) -or 
						!$?
					) {
						Output 'Changed' $n
					}
					elseif ($Verbose) {
						Output '' $n
					}
					else {
						++$unchanged
					}
				}
			}
		}

		if ($unchanged) {
			[PSCustomObject]@{
				parent = $current
				event  = 'Unchanged'
				value  = $unchanged
			}
		}
	}
}

win_pwsh5.ps1

Function DiffFolders { Param([switch]$Force, [switch]$Verbose)
	Begin {
		$parent = $current = $last = $NULL
		
		Function Output { Param($event, $value)
			[PSCustomObject]@{
				parent = $parent
				event  = $event
				value  = $value
			}
		}
		
		$hash = { (Get-FileHash -Algorithm MD5 -LiteralPath $Input).Hash }
		
		Function Differ { Param($name)
			$old = $last.Item($name)
			$new = $current.Item($name)
			$last.Remove($name)
			
			if ((!$old) -and (!$new)) {
				return $False
			}
			if ($old[1] -ne $new[1]) {
				return $True
			}
			
			$new[2] = Start-Job $hash -InputObject $new[0]
			if (!$old[2]) {
				$old[2] = $old[0] | &$hash
			}
			$new[2] = Receive-Job -Wait $new[2]
			return $old[2] -ne $new[2]
		}
	}
	Process {
		$last      = $current
		$current   = @{}
		$unchanged = 0
		
		$parent   = $_
		$item     = Get-Item $parent
		$relative = "^$([regex]::escape($item.FullName))"
		$item `
		| Get-ChildItem -Recurse -Force:$Force `
		| %{
			$name = $_.FullName -replace $relative,''
			if ($_.Parent) {
				$name += [IO.Path]::DirectorySeparatorChar
				$current.Add($name, $NULL)
			}
			else {
				$current.Add($name, ($_.FullName, $_.Length, $NULL))
			}
			
			if (!$last) {
			}
			elseif (!$last.ContainsKey($name)) {
				Output 'Added' $name
			}
			elseif (Differ $name) {
				Output 'Changed' $name
			}
			elseif ($Verbose) {
				Output '' $name
			}
			else {
				++$unchanged
			}
		}
		
		if ($last) {
			$last.Keys | %{ Output 'Deleted' $_ }
			if ($unchanged) {
				Output 'Unchanged' $unchanged
			}
		}
	}
}

TODO create our own hashing function that streams two files at once, stopping at the first difference, and continuable later when comparing a third file to the second.