Gets recursive unique combinations of characters in a string

Get-StringPermutations.ps1

function Get-StringPermutations {
    <# .SYNOPSIS
    Calculates the permutations of an input string #>
    [Alias('permutate')]
    Param(
        # String to calculate permutations from
        [Parameter(Mandatory)]
        [String]$String,
        # Return only unique permutations
        [Parameter()]
        [Switch]$Unique
    )

    function _Permutate($String, $Stub = '') {
        if ($String.length -eq 2) {
            return ($Stub + $String[0] + $String[1]), ($Stub + $String[1] + $String[0])
        }
        for ($i = 0; $i -lt $String.Length; $i++) {
            _Permutate -String $String.remove($i, 1) -Stub ($Stub + $String[$i])
        }
    }
    if ($String.length -eq 1) { return $String }
    if (!$Unique) { _Permutate -String $String } else {
        [Collections.Generic.HashSet[string]]::new([string[]](_Permutate -String $String))
    }
}

$needs_work = @'
function Get-StringPermutations {
    <# .SYNOPSIS
    Calculates the permutations of an input string #>
    [Alias('permutate')]
    Param(
        # String to calculate permutations from
        [Parameter(Mandatory)]
        [String]$String,
        # Return only unique permutations
        [Parameter()]
        [Switch]$Unique
    )

    function _Permutate($String, $Stub, $Level) {
        for ($i = 0; $i -lt $String.Length; $i++) {
            if ($Level -eq 1) { return $Stub + $String[$i] }
            _Permutate -String $String.remove($i, 1) -Stub ($Stub + $String[$i]) -Level ($Level - 1)
        }
    }
    function _UPermutate($String, $Stub, $Level) {
        $Pool = [Collections.Generic.HashSet[char]]::new($String.length)
        foreach ($i in (0..($String.length - 1) | & { process { if ($Pool.add([char]$String[$_])) { $_ } } })) {
            if ($Level -eq 1) { return $Stub + $String[$i] }
            _UPermutate -String $String.remove($i, 1) -Stub ($Stub + $String[$i]) -Level ($Level - 1)
        }
    }
    # function _UPermutate($String, $Stub, $Level) {
    #     foreach ($i in [Collections.Generic.HashSet[char]]::new([char[]]$String)) {
    #         if ($Level -eq 1) { return $Stub + $i }
    #         _UPermutate -String $String.remove($String.LastIndexOf($i), 1) -Stub ($Stub + $i) -Level ($Level - 1)
    #     }
    # }
    if (!$Unique) { _Permutate -String $String -Stub '' -Level $String.length }
    else { _UPermutate -String $String -Stub '' -Level $String.length }
}
'@

Now part of my HelperFunctions module.