Write PureText Strings

This code has been declared by its author to be Public Domain code.

Read/Write PureText Strings by _PJ_

2010

Not sure exactly how practical this is, since it was part of something else I've been working towards.
Essentially, it simplifies the bytes of string data allowing standard alphanumeric characters (plus 1 or two others) to be read/written to files with 64 bytes each character.
Making use of the Bit for 128, I incorporated a very simple randomisation to 'encrypt' the data, making it harder to read by an end user direct from the output. The Read functionality identifies this value.

Limitations & Notes:
I built in some formatting to remove extraneous spaces.
Commas and full-stops (period US) / decimal points are the only accepted punctuation. It's possible to alter these, but only 3 such non-alphanumeric chars are permitted (including SPACE).
String length is limited to 65535 chars, should be enough for anyoine. This limit can be altered if really necessary, so long as Length is read/written as Int for 4-Byte values or perhaps Byte for 255-length strings rather than Short (16-Bit, 65535 limit).
Encryption is very simply substitution and reversal with the cypher given at the top of the code. (Also the byte format for the chars) - whilst it may not be a preferred form of security, it should be adequate in theory for general purposes due to the application of randomised encrypted characters.

;0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz.,

;------------------------------------------------------------------------------------------------
;EXAMPLE:
;------------------------------------------------------------------------------------------------
f=WriteFile("test.txt")

WritePureTextString(f,"Hello World",True)

CloseFile f

f=ReadFile("test.txt")

Print ReadPureTextString(f)

CloseFile f


;------------------------------------------------------------------------------------------------
;FUNCTIONS:
;------------------------------------------------------------------------------------------------


Function ReadPureTextString$(FileStream)
	If (Not(Filestream)) Then Return ""
	
	Local Length%=ReadShort(FileStream)
	If (Not (Length)) Then Return ""
	
	Local sString$=""
	Local IterChar%
	Local Char$
	Local Byte%
	For IterChar= 1 To Length
		Byte=ReadByte(Filestream)
		If (Byte>64) Then Byte=Byte-128
		Char=ConvertByteToChar(Byte)
		sString=sString+Char
	Next
	
	sString=Trim(sString)
	While Instr(sString,"  ")
		sString=Replace(sString,"  "," ")
	Wend
	
	Return sString
End Function

Function WritePureTextString(Filestream%,sString$,Encrypt=False)
	If (Not(Filestream)) Then Return
	
	Local Length=Len(sString)

	; Zero Length string	
	If (Not (Length))
		WriteShort Filestream,0
		Return
	End If
	
	Local IterChar%
	Local Char$
	Local Byte%
		
	;Convert string
	For IterChar=1 To Length
		Char=Mid(sString,IterChar,1)
		Byte=ConvertCharToByte(Char)
		Replace(sString,Chr(Char),ConvertByteToChar(Char))
	Next

	;Consecutive spaces count as 1. No leading or trailing spaces.
	sString=Trim(sString)
	While Instr(sString,"  ")
		sString=Replace(sString,"  "," ")
	Wend

	Length=Len(sString)

	; Zero Length string	
	If (Not (Length))
		WriteShort Filestream,0
		Return
	End If
	
	;Truncate long strings
	If (Length>65535)
		Length=65535
		sString=Left(sString,65535)
	End If

	;Consecutive spaces count as 1. No leading or trailing spaces.
	sString=Trim(sString)
	While Instr(sString,"  ")
		sString=Replace(sString,"  "," ")
	Wend

	Length%=Len(sString)

	;Write Length as short
	WriteShort FileStream,Length
			
	For IterChar=1 To Length
		Char=Mid(sString,IterChar,1)
		Byte=ConvertCharToByte(Char)
		If (Encrypt)
			SeedRnd MilliSecs()*RndSeed()
			If Rand(0,1) Then Byte=Byte+128
		End If
		WriteByte Filestream,Byte
	Next
End Function
	

Function ConvertCharToByte%(Char$)

	Local Ascii%=Asc(Char)

	If ((Ascii>47) And (Ascii<58))
		; NUMERAL
		Return (Ascii-48)
	Else

		If ((Ascii>64) And (Ascii<91))
			; UPPER ALPHA
			Return (Ascii-54)
	
		Else
	
			If (Ascii>96) And (Ascii<123)
				; LOWER ALPHA
			Return (Ascii-61)
		
			End If
		End If
	End If	
	
	If Char="." Then Return 62
	If Char="," Then Return 63
	
	; For everything else, there's mastercard
	Return 64
		
End Function

Function ConvertByteToChar$(Byte)

	If (Byte<10)
		;NUMERAL
		Return Chr(Byte+48)
	
	Else
		If ((Byte>9) And (Byte<37))
			; UPPER ALPHA
			Return Chr(Byte+54)
		Else
			If ((Byte>36) And (Byte<62))
				; LOWER ALPHA
				Return Chr(Byte+61)
		
			End If
		End If
	End If	
	
	If Byte=62 Then Return "."
	If Byte=63 Then Return ","	
	Return " "
End Function

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