Convert hex string to a base36 string - powershell

Question

In AWS each instance has an ID that looks something like this: i-1234567890abcdef ; the last 16 characters being a hexadecimal number. I would like to treat the "1234567890abcdef" part as a hex number and convert it to base36, so a-z0-9. This way I can use is as the computer's name and not go over the 15 character limit. How is that done in Powershell ?

Answer 1

Converting the input from hex is easy enough: skip the first two characters, and convert to UInt64:
[convert]::ToUInt64($text.Substring(2), 16)
but PowerShell (.Net) has no built-in way to convert to base 36. You'll need to implement it yourself, e.g. this code taken from https://ss64.com/ps/syntax-base36.html and adjusted for larger numbers:
function convertTo-Base36
{
[CmdletBinding()]
param (
[parameter(valuefrompipeline=$true, HelpMessage="Integer number to convert")]
[uint64]$DecimalNumber=""
)
$alphabet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
do
{
$remainder = ($DecimalNumber % 36)
$char = $alphabet.Substring($remainder, 1)
$base36Num = "$char$base36Num"
$DecimalNumber = ($DecimalNumber - $remainder) / 36
}
while ($DecimalNumber -gt 0)
$base36Num
}
Then:
$x='i-1234567890abcdef'
$hexPart = $x.Substring(2)
$decimal = [convert]::ToUInt64($hexPart, 16)
convertTo-Base36 $decimal
# -> 9YS742MX86WF
or:
[convert]::ToUInt64('i-1234567890abcdef'.Substring(2), 16) | Convertto-Base36