Source code in the purebasic programming language

; using PureBasic 5.50 (x64)
EnableExplicit

Macro IsValid(expression)
  If expression
    PrintN("Valid")
  Else
    PrintN("Invalid")
  EndIf
EndMacro

Procedure.i DecodeBase58(Address$, Array result.a(1)) 
  Protected i, j, p
  Protected charSet$ = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
  Protected c$
  
  For i = 1 To Len(Address$)
    c$ = Mid(Address$, i, 1)
    p = FindString(charSet$, c$) - 1
    If p = -1 : ProcedureReturn #False : EndIf; Address contains invalid Base58 character
    For j = 24 To 1 Step -1
      p + 58 * result(j)
      result(j) = p % 256
      p  / 256
    Next j
    If p <> 0 : ProcedureReturn #False : EndIf ; Address is too long
  Next i
  ProcedureReturn #True
EndProcedure

Procedure HexToBytes(hex$, Array result.a(1))
  Protected i
  For i = 1 To Len(hex$) - 1 Step 2
    result(i/2) = Val("$" + Mid(hex$, i, 2))
  Next
EndProcedure

Procedure.i IsBitcoinAddressValid(Address$)
  Protected format$, digest$
  Protected i, isValid
  Protected Dim result.a(24)
  Protected Dim result2.a(31)
  Protected result$, result2$
   ; Address length must be between 26 and 35 - see 'https://en.bitcoin.it/wiki/Address'
  If Len(Address$) < 26 Or Len(Address$) > 35 : ProcedureReturn #False : EndIf
  ; and begin with either 1 or 3 which is the format number
  format$ = Left(Address$, 1)
  If format$ <> "1" And format$ <> "3" : ProcedureReturn #False : EndIf
  isValid = DecodeBase58(Address$, result())  
  If Not isValid : ProcedureReturn #False : EndIf
  UseSHA2Fingerprint(); Using functions from PB's built-in Cipher library
  digest$ = Fingerprint(@result(), 21, #PB_Cipher_SHA2, 256); apply SHA2-256 to first 21 bytes
  HexToBytes(digest$, result2()); change hex string to ascii array 
  digest$ = Fingerprint(@result2(), 32,  #PB_Cipher_SHA2, 256); apply SHA2-256 again to all 32 bytes
  HexToBytes(digest$, result2())
  result$ = PeekS(@result() + 21, 4, #PB_Ascii); last 4 bytes
  result2$ = PeekS(@result2(), 4, #PB_Ascii); first 4 bytes
  If result$ <> result2$ : ProcedureReturn #False : EndIf
  ProcedureReturn #True  
EndProcedure

If OpenConsole()
  Define address$ = "1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62i"
  Define address2$ = "1BGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62i"
  Define address3$ = "1AGNa15ZQXAZUgFiqJ2i7Z2DPU2J6hW62I"
  Print(address$ + " -> ") 
  IsValid(IsBitcoinAddressValid(address$))
  Print(address2$ + " -> ") 
  IsValid(IsBitcoinAddressValid(address2$))
  Print(address3$ + " -> ") 
  IsValid(IsBitcoinAddressValid(address3$))
  PrintN("")
  PrintN("Press any key to close the console")
  Repeat: Delay(10) : Until Inkey() <> ""
  CloseConsole()
EndIf