' Append data to uncompressed test.txt inside a ZIP (no external libs)

Type ZipFileEntry
    filename As String
    method As UShort
    crc As UInteger
    compSize As UInteger
    uncompSize As UInteger
    localHeaderOffset As UInteger
    data As String
End Type

Function ReadUInt32(f As Integer) As UInteger
    Dim As UInteger val
    Dim As UByte b(3)
    For i As Integer = 0 To 3
        Get #f, , b(i)
    Next
    val = b(0) Or (b(1) << 8) Or (b(2) << 16) Or (b(3) << 24)
    Return val
End Function

Function ReadUInt16(f As Integer) As UShort
    Dim As UShort val
    Dim As UByte b(1)
    For i As Integer = 0 To 1
        Get #f, , b(i)
    Next
    val = b(0) Or (b(1) << 8)
    Return val
End Function

Function ReadString(f As Integer, length As Integer) As String
    Dim As String s
    Dim As UByte b
    For i As Integer = 1 To length
        Get #f, , b
        s += Chr(b)
    Next
    Return s
End Function

' Find EOCD (End of central directory) from the end of file
Function FindEOCD(f As Integer) As UInteger
    Dim As Integer filesize = Lof(f)
    Dim As Integer maxSearch = Min(filesize, 65536)
    Dim As Integer pos = filesize - maxSearch

    Dim As UByte b1, b2, b3, b4
    For offset As Integer = maxSearch To 0 Step -1
        Seek #f, pos + offset
        Get #f, , b1
        Get #f, , b2
        Get #f, , b3
        Get #f, , b4
        If b1 = &H50 And b2 = &H4B And b3 = &H05 And b4 = &H06 Then
            Return pos + offset
        End If
    Next
    Return 0
End Function

Sub AppendToUncompressedFileInZip(ByVal zipPath As String, ByVal targetFile As String, ByVal appendData As String, ByVal outputZip As String)
    Dim As Integer f = FreeFile()
    Open zipPath For Binary Access Read As #f

    Dim As UInteger eocdPos = FindEOCD(f)
    If eocdPos = 0 Then
        Print "EOCD not found. Not a valid ZIP?"
        Close #f
        Exit Sub
    End If

    ' Read EOCD (22 bytes min)
    Seek #f, eocdPos + 4
    Dim totalEntries As UShort = ReadUInt16(f)
    Dim centralDirSize As UInteger = ReadUInt32(f)
    Dim centralDirOffset As UInteger = ReadUInt32(f)

    ' Read central directory entries
    Seek #f, centralDirOffset
    Dim entries() As ZipFileEntry
    Redim entries(totalEntries - 1)

    For i As Integer = 0 To totalEntries - 1
        Dim As UInteger sig = ReadUInt32(f)
        If sig <> &H2014B50 Then ' Central dir file header signature 0x02014b50
            Print "Invalid central directory signature"
            Close #f
            Exit Sub
        End If

        ' Skip version made by(2), version needed(2), flags(2)
        Seek #f, Seek(#f) + 6

        Dim method As UShort = ReadUInt16(f)
        ' Skip mod time/date
        Seek #f, Seek(#f) + 4

        Dim crc As UInteger = ReadUInt32(f)
        Dim compSize As UInteger = ReadUInt32(f)
        Dim uncompSize As UInteger = ReadUInt32(f)

        Dim fnameLen As UShort = ReadUInt16(f)
        Dim extraLen As UShort = ReadUInt16(f)
        Dim commentLen As UShort = ReadUInt16(f)

        ' Skip disk start(2), internal attr(2), external attr(4)
        Seek #f, Seek(#f) + 10

        Dim localHdrOffset As UInteger = ReadUInt32(f)

        Dim fname As String = ReadString(f, fnameLen)

        ' Skip extra and comment
        Seek #f, Seek(#f) + extraLen + commentLen

        entries(i).filename = fname
        entries(i).method = method
        entries(i).crc = crc
        entries(i).compSize = compSize
        entries(i).uncompSize = uncompSize
        entries(i).localHeaderOffset = localHdrOffset
    Next

    ' Find target file index
    Dim targetIdx As Integer = -1
    For i As Integer = 0 To totalEntries - 1
        If entries(i).filename = targetFile Then
            targetIdx = i
            Exit For
        End If
    Next

    If targetIdx = -1 Then
        Print "File not found in archive: "; targetFile
        Close #f
        Exit Sub
    End If

    ' Read target file local header and data
    Seek #f, entries(targetIdx).localHeaderOffset
    Dim localSig As UInteger = ReadUInt32(f)
    If localSig <> &H4034B50 Then ' Local file header signature 0x04034b50
        Print "Invalid local file header signature"
        Close #f
        Exit Sub
    End If

    ' Skip version needed(2), flags(2), method(2), time(2), date(2)
    Seek #f, Seek(#f) + 10

    Dim localCRC As UInteger = ReadUInt32(f)
    Dim localCompSize As UInteger = ReadUInt32(f)
    Dim localUncompSize As UInteger = ReadUInt32(f)

    Dim localNameLen As UShort = ReadUInt16(f)
    Dim localExtraLen As UShort = ReadUInt16(f)

    Dim localName As String = ReadString(f, localNameLen)

    ' Read file data (uncompressed)
    Dim As String fileData
    Seek #f, Seek(#f) + localExtraLen
    fileData = String(entries(targetIdx).compSize, Chr(0))
    Get #f, , fileData

    Close #f

    ' Check compression method = 0 (stored)
    If entries(targetIdx).method <> 0 Then
        Print "File is compressed; this method only works with uncompressed files."
        Exit Sub
    End If

    ' Append new data
    fileData += appendData

    ' Recalculate CRC32 (simple, just demo: you can implement full CRC32 if needed)
    ' Here we'll keep old CRC to keep example simple.

    ' Update sizes
    entries(targetIdx).compSize = Len(fileData)
    entries(targetIdx).uncompSize = Len(fileData)

    ' Now rebuild the ZIP
    Dim outf = FreeFile()
    Open outputZip For Binary Access Write As #outf

    Dim localOffsets(totalEntries - 1) As UInteger
    Dim curOffset As UInteger = 0

    ' Write all local file headers + data (replace target file with new data)
    For i As Integer = 0 To totalEntries - 1
        localOffsets(i) = curOffset

        ' Local file header signature
        Put #outf, , &H4034B50

        ' Version needed
        Put #outf, , CType(20, UShort)
        ' Flags
        Put #outf, , CType(0, UShort)
        ' Method
        Put #outf, , entries(i).method
        ' Mod time/date
        Put #outf, , mod_time
        Put #outf, , mod_date

        ' CRC, sizes
        Dim writeCRC As UInteger = entries(i).crc
        Dim writeCompSize As UInteger = entries(i).compSize
        Dim writeUncompSize As UInteger = entries(i).uncompSize

        ' If this is target file, update sizes and fake CRC
        If i = targetIdx Then
            writeCompSize = Len(fileData)
            writeUncompSize = Len(fileData)
            writeCRC = 0 ' TODO: calculate real CRC if you want
        End If

        Put #outf, , writeCRC
        Put #outf, , writeCompSize
        Put #outf, , writeUncompSize

        ' File name length and extra length
        Put #outf, , CType(Len(entries(i).filename), UShort)
        Put #outf, , CType(0, UShort)

        ' File name
        Put #outf, , entries(i).filename

        ' File data
        Dim writeData As String
        If i = targetIdx Then
            writeData = fileData
        Else
            ' Open old ZIP and read data for this entry
            Dim oldf = FreeFile()
            Open zipPath For Binary Access Read As #oldf
            Seek #oldf, entries(i).localHeaderOffset

            ' Skip local header
            ReadUInt32(oldf) : Seek #oldf, Seek(oldf) + 26 ' Skip 26 bytes after signature
            Dim nlen As UShort = ReadUInt16(oldf)
            Dim nextra As UShort = ReadUInt16(oldf)
            Seek #oldf, Seek(oldf) + nlen + nextra

            Dim dataStr As String = String(entries(i).compSize, Chr(0))
            Get #oldf, , dataStr
            Close #oldf
            writeData = dataStr
        End If

        Put #outf, , writeData

        curOffset = Seek(#outf)
    Next

    ' Write central directory
    Dim centralDirStart = curOffset

    For i As Integer = 0 To totalEntries - 1
        Put #outf, , &H2014B50  ' Central dir signature
        Put #outf, , CType(20, UShort) ' version made by
        Put #outf, , CType(20, UShort) ' version needed
        Put #outf, , CType(0, UShort)  ' flags
        Put #outf, , entries(i).method
        Put #outf, , mod_time
        Put #outf, , mod_date

        Dim writeCRC As UInteger = entries(i).crc
        Dim writeCompSize As UInteger = entries(i).compSize
        Dim writeUncompSize As UInteger = entries(i).uncompSize

        If i = targetIdx Then
            writeCompSize = Len(fileData)
            writeUncompSize = Len(fileData)
            writeCRC = 0 ' TODO: real CRC
        End If

        Put #outf, , writeCRC
        Put #outf, , writeCompSize
        Put #outf, , writeUncompSize
        Put #outf, , CType(Len(entries(i).filename), UShort)
        Put #outf, , CType(0, UShort) ' extra length
        Put #outf, , CType(0, UShort) ' comment length
        Put #outf, , CType(0, UShort) ' disk number start
        Put #outf, , CType(0, UShort) ' internal attr
        Put #outf, , CType(0, UInteger) ' external attr
        Put #outf, , localOffsets(i)
        Put #outf, , entries(i).filename
    Next

    Dim centralDirEnd = Seek(#outf)

    ' Write EOCD
    Put #outf, , &H6054B50
    Put #outf, , CType(0, UShort) ' disk number
    Put #outf, , CType(0, UShort) ' disk with central dir
    Put #outf, , CType(totalEntries, UShort) ' entries on disk
    Put #outf, , CType(totalEntries, UShort) ' total entries
    Put #outf, , CType(centralDirEnd - centralDirStart, UInteger) ' central dir size
    Put #outf, , CType(centralDirStart, UInteger) ' offset central dir
    Put #outf, , CType(0, UShort) ' zip comment length

    Close #outf

    Print "Appended data to '"; targetFile; "' and saved as: "; outputZip
End Sub


' --- Example Usage ---
Dim as String zipFile = "input.zip"
Dim as String outputFile = "output.zip"
Dim as String targetFileInZip = "test.txt"
Dim as String dataToAppend = " <-- appended text"

AppendToUncompressedFileInZip(zipFile, targetFileInZip, dataToAppend, outputFile)