PowerShell Diff implementation

Get-Diff.ps1

<#
  Diff implementation based off of https://gist.github.com/adamnew123456/37923cf53f51d6b9af32a539cdfa7cc4
  Written by Jason Rush (https://github.com/jasonrush)

  Returns an arraylist of hashtables. Each hashtable has two properties.
  The "Action" property will have a value of "Keep", "Insert", or "Remove".
  The "Line" property will have the value originally passed via $a_lines or $b_lines.

  While I haven't tested it, technically $a_lines and $b_lines should just be arrays
  and the values should be able to be anything, not just strings from files as you would
  typically use diff for.
  
  Example:
  Get-Diff (Get-Content .\file_a.txt) (Get-Content .\file_b.txt)
  
  Example:
  Get-Diff $oldStrings $newStrings | Foreach-Object {
    if( "Keep" -eq $_.Action ){
        "  $($_.Line)"
    }elseif( "Insert" -eq $_.Action ){
        "+ $($_.Line)"
    }elseif( "Remove" -eq $_.Action ){
        "- $($_.Line)"
    }
  }

#>
function Get-Diff( $a_lines, $b_lines ){
    function DiffKeep( $line ){
        @{
            Action = "Keep"
            Line = $line
        }
    }
    function DiffInsert( $line ){
        @{
            Action = "Insert"
            Line = $line
        }
    }
    function DiffRemove( $line ){
        @{
            Action = "Remove"
            Line = $line
        }
    }
    function Frontier( $x, $history ){
        @{
            x = $x
            history = $history # arraylist
        }
    }
    function DiffOne( [int32] $i ){
        $i - 1
    }

    $a_max = ($a_lines | Measure-Object).Count
    $b_max = ($b_lines | Measure-Object).Count

    $combined_max = [math]::Max(0, $a_max + $b_max)

    $frontier = @{}
    for( $i = -$combined_max; $i -le $combined_max; $i++ ){
        $frontier[$i] = (Frontier 0 (New-Object -TypeName "System.Collections.ArrayList") )
    }

    for( $d=0; $d -le ($a_max + $b_max + 1); $d++ ){
        for( $k=-$d; $k -le $d+1; $k=$k+2 ){
            $go_down = (
                ($k -eq -$d) -or
                (
                    ($k -ne $d) -and
                    ($frontier[$k-1].x -lt $frontier[$k+1].x)
                )
            )
            if( $go_down ){
                $old_x = $frontier[$k+1].x
                $history = $frontier[$k+1].history
                $x = $old_x
            }else{
                $old_x = $frontier[$k-1].x
                $history = $frontier[$k-1].history
                $x = $old_x + 1
            }
            $history = $history.clone()
            $y = $x - $k

            if(
                (1 -le $y) -and
                ($y -le $b_max) -and
                $go_down
            ){
                $history.Add(
                    (DiffInsert $b_lines[ (DiffOne $y) ])
                ) | Out-Null
            }elseif (
                (1 -le $x) -and
                ($x -le $a_max)
            ) {
                $history.Add(
                    (DiffRemove $a_lines[ (DiffOne $x) ])
                ) | Out-Null
            }
            while(
                ($x -lt $a_max) -and
                ($y -lt $b_max) -and
                ($a_lines[ (DiffOne (1+$x) )] -eq $b_lines[ (DiffOne (1+$y) ) ])
            ){
                $x = $x+1
                $y = $y+1
                $history.Add(
                    (DiffKeep $a_lines[ (DiffOne $x) ])
                ) | Out-Null
            }
            if(
                ($x -ge $a_max) -and
                ($y -ge $b_max)
            ){
                return $history
            }else{
                $frontier[$k] = (Frontier $x $history)
            }
        }
    }
}