Imports System
Imports System.Text
Imports System.Numerics
Module Base36Converter
''' <summary>
''' Encodes an arbitrary string into its Base-36 representation (Big-Endian).
''' </summary>
Function EncodeToBase36(input As String) As String
If String.IsNullOrEmpty(input) Then Return String.Empty
' Step 1: Convert string to UTF-8 bytes
Dim bytes As Byte() = Encoding.UTF8.GetBytes(input)
' Step 2: Reverse bytes to convert from Big-Endian (String order) to Little-Endian (.NET BigInteger order)
Array.Reverse(bytes)
' Step 3: Append a 0x00 byte at the end of the little-endian array to guarantee it's interpreted as positive
Dim positiveBytes(bytes.Length) As Byte
Array.Copy(bytes, positiveBytes, bytes.Length)
Dim bigInt As New BigInteger(positiveBytes)
' Step 4: Extract Base-36 digits (least-significant to most-significant)
Const Digits As String = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
Dim result As New StringBuilder()
While bigInt > 0
Dim remainder As BigInteger = 0
bigInt = BigInteger.DivRem(bigInt, 36, remainder)
result.Append(Digits(CInt(remainder)))
End While
' Step 5: Reverse the accumulated digits to get the final Big-Endian Base-36 string
Return ReverseString(result.ToString())
End Function
''' <summary>
''' Decodes a Base-36 string back into its original text representation.
''' </summary>
Function DecodeFromBase36(encoded As String) As String
If String.IsNullOrEmpty(encoded) Then Return String.Empty
Dim bigInt As BigInteger = 0
Const Digits As String = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
' Step 1: Convert Base-36 string back to BigInteger
For Each ch As Char In encoded.ToUpper()
Dim value As Integer = Digits.IndexOf(ch)
If value = -1 Then Throw New ArgumentException("Invalid Base-36 character.")
bigInt = (bigInt * 36) + value
Next
' Step 2: Get the little-endian byte array from BigInteger
Dim bytes As Byte() = bigInt.ToByteArray()
' Trim any trailing zero bytes padded by BigInteger before reversing
Dim actualLength As Integer = bytes.Length
While actualLength > 0 AndAlso bytes(actualLength - 1) = 0
actualLength -= 1
End While
' Step 3: Reverse the bytes back to Big-Endian (original string byte order)
Dim orderedBytes(actualLength - 1) As Byte
Array.Copy(bytes, orderedBytes, actualLength)
Array.Reverse(orderedBytes)
' Step 4: Convert bytes back to the original UTF-8 string
Return Encoding.UTF8.GetString(orderedBytes)
End Function
Private Function ReverseString(s As String) As String
Dim arr As Char() = s.ToCharArray()
Array.Reverse(arr)
Return New String(arr)
End Function
Sub Main()
Dim text As String = "Hello Universe!"
Dim encoded As String = EncodeToBase36(text)
Dim decoded As String = DecodeFromBase36(encoded)
Console.WriteLine("Original: " & text)
Console.WriteLine("Base-36 encoded: " & encoded)
Console.WriteLine("Decoded: " & decoded)
End Sub
End Module
'
' run:
'
' Original: Hello Universe!
' Base-36 encoded: LP4N024HJ1YVBVD84Y8TYQP
' Decoded: Hello Universe!
'
