解压zip_VBA解压缩ZIP文件03——解压准备工作

要解压缩ZIP文件，所以肯定需要读写文件的功能，为了方便，把VBA中对文件的读写功能进行一个简单的封装，方便使用。

ZIP文件压缩后，保存数据使用的最小单位是bit，注意不是Byte，计算机的1Byte=8bit，正常在VBA中操作的最小单位是Byte，为了方便读取bit位的数据，写几个简单的函数。

01 CFile文件读写

主要是使用类模块对文件操作Open、Put、Get等关键字的简单封装，这样使用起来就更加的方便。

Public Enum OpenAccess    O_RDONLY    O_WRONLY    O_RDWREnd EnumPublic Enum SeekPos    OriginF    CurrentF    EndFEnd Enum'注意大文件long类型会溢出Private lFileLen As LongPrivate num_file As Integer'写入文件Function WriteFile(b() As Byte) As Long    Put #num_file, , bEnd Function'读取len(b)个byteFunction Read(b() As Byte) As Long    Dim ilen As Long    ilen = UBound(b) - LBound(b) + 1    Dim iseek As Long    iseek = VBA.Seek(num_file)    If iseek + ilen > lFileLen Then        ilen = lFileLen - iseek + 1    End If    Get #num_file, , b    Read = ilenEnd Function'读取一个2Byte的整数Function ReadInteger() As Integer    Dim i As Integer    Get #num_file, , i    ReadInteger = iEnd Function'读取1个4Byte的整数Function ReadLong() As Long    Dim i As Long    Get #num_file, , i    ReadLong = iEnd Function'在offset处开始读取Function ReadAt(b() As Byte, offset As Long) As Long    SeekFile offset, 0    ReadAt = Read(b)End Function'设置读取的位置Function SeekFile(offset As Long, whence As SeekPos) As Long    Dim iseek As Long    iseek = VBA.Seek(num_file)    'vba Seek是下标1开始    If whence = SeekPos.OriginF Then        iseek = 1 + offset    ElseIf whence = SeekPos.CurrentF Then        iseek = iseek + offset    Else        iseek = 1 + lFileLen - offset    End If    Seek #num_file, iseek    SeekFile = iseekEnd Function'以字节方式读取文本Function OpenFile(Filename As String, Optional m As OpenAccess = OpenAccess.O_RDWR) As Long    '避免多次调用OpenFile的时候，前面的文件未关闭    If num_file Then Close #num_file    num_file = VBA.FreeFile    Select Case m        Case OpenAccess.O_RDONLY        Open Filename For Binary Access Read As #num_file        Case OpenAccess.O_WRONLY        Open Filename For Binary Access Write As #num_file        Case OpenAccess.O_RDWR        Open Filename For Binary Access Read Write As #num_file        Case Else    End Select    lFileLen = VBA.FileLen(Filename)End FunctionFunction CloseFile()    Close #num_fileEnd FunctionPrivate Sub Class_Terminate()    CloseFileEnd Sub

02 bit位操作

计算机中1Byte=8bit，bit的排列顺序和数学中的个位、十位、百位……是一样的：

解压ZIP的过程中，需要不停的从压缩数据的Byte数组中读取需要的bit，实现几个简单的函数：

'取某一位的BitFunction GetBitFromByte(b As Byte, ZeroBaseIndex As Long) As Long    GetBitFromByte = VBA.CLng(b) And (2 ^ ZeroBaseIndex)    If GetBitFromByte > 0 Then        GetBitFromByte = 1    Else        GetBitFromByte = 0    End IfEnd Function

'从Byte数组中取某一位的BitFunction GetBit(b() As Byte, ZeroBaseIndex As Long) As Long    '数组b中，开始的下标    Dim bindex As Long    bindex = ZeroBaseIndex \ 8    GetBit = VBA.CLng(b(bindex)) And (2 ^ (ZeroBaseIndex Mod 8))    If GetBit > 0 Then        GetBit = 1    Else        GetBit = 0    End IfEnd Function

'从Byte数组中取多位bit' 0000 0000     0000 0000' 7654 3210     fedc ba98Function GetBits(b() As Byte, IndexFromZeroBase As Long, iBits As Long) As Long    Dim i As Long    Dim tmp As Long    For i = 0 To iBits - 1        tmp = GetBit(b, IndexFromZeroBase + i)        tmp = BitMoveLeft(tmp, i)        GetBits = GetBits Or tmp    NextEnd Function'左移Function BitMoveLeft(ByRef l As Long, num As Long) As Long    Dim i As Long    For i = 1 To num        '会溢出 0x7FFF FFFF        '判断第31位是否=1        '不管等不等于1都把第31为置换为0，负数待处理        l = l And &H3FFFFFFF        l = l * 2    Next    BitMoveLeft = lEnd Function'右移Function BitMoveRight(ByRef l As Long, num As Long) As Long    Dim i As Long    For i = 1 To num        l = l \ 2    Next    BitMoveRight = lEnd Function

'从Byte数组中取多位bit'取出后的bit存储顺序倒置Function GetBitsRev(b() As Byte, IndexFromZeroBase As Long, iBits As Long) As Long    Dim i As Long    Dim tmp As Long    For i = 0 To iBits - 1        tmp = GetBit(b, IndexFromZeroBase + i)        tmp = BitMoveLeft(tmp, iBits - i - 1)        GetBitsRev = GetBitsRev Or tmp    NextEnd Function

• 00 ZIP是什么

• 01 实现的功能

• 02 压缩过程