一个优化后的压缩算法（上）

这是一个在CSDN论坛中讨论过的压缩算法代码。
与WinRAR以最快方式压缩ZIP比较，
255M的文件
Level=0时 用时24.98秒 大小95.1M
Level=255时 用时30.24秒 大小91.6M
WinRAR最快压缩ZIP 用时 25.2秒 大小58.6M
标准RAR压缩，我看了一下，实在太慢，也就没试了，估计要几分钟才会有结果。
从速度看，基本持平了，这个算法虽然最大压缩能力有限，但感觉设计得很巧妙，每次都基于动态表，使软件可以做得很小巧，资源占用也很少。非常值得收藏！
'测试窗体中的代码
Option Explicit
Private WithEvents ObjZip As ClassZip
Private BgTime As Single
Private Sub Command1_Click()
BgTime = Timer
Command1.Enabled = False
Command2.Enabled = False
With ObjZip
.InputFileName = Text1.Text
.OutputFileName = Text2.Text
.IsCompress = True
.CompressLevel = Val(Text4.Text)
.BeginProcss
End With
Label1.Caption = Round(Timer - BgTime, 2) & "秒"
Command1.Enabled = True
Command2.Enabled = True
End Sub
Private Sub Command2_Click()
BgTime = Timer
Command1.Enabled = False
Command2.Enabled = False
With ObjZip
.InputFileName = Text2.Text
.OutputFileName = Text3.Text
.IsCompress = False
.BeginProcss
End With
Label1 = Round(Timer - BgTime, 2) & "秒"
Command1.Enabled = True
Command2.Enabled = True
End Sub
Private Sub Command3_Click()
ObjZip.CancelProcss = True
End Sub
Private Sub Form_Load()
Set ObjZip = New ClassZip
Command1.Caption = "压缩"
Command2.Caption = "解压"
Command3.Caption = "中断"
End Sub
Private Sub Form_Unload(Cancel As Integer)
Set ObjZip = Nothing
End Sub
Private Sub ObjZip_FileProgress(sngPercentage As Single)
Label1 = Int(sngPercentage * 100) & "%"
End Sub
Private Sub ObjZip_ProcssError(ErrorDescription As String)
MsgBox ErrorDescription
End Sub

'ClassZip类中的声明与属性、方法、事件
Option Explicit
Public Event FileProgress(sngPercentage As Single)
Public Event ProcssError(ErrorDescription As String)
Private Type FileHeader
HeaderTag As String * 3
HeaderSize As Integer
Flag As Byte
FileLength As Long
Version As Integer
End Type
Private mintCompressLevel As Long
Private m_bEnableProcss As Boolean
Private m_bCompress As Boolean
Private m_strInputFileName As String
Private m_strOutputFileName As String
Private Const mcintWindowSize As Integer = &H1000
Private Const mcintMaxMatchLen As Integer = 18
Private Const mcintMinMatchLen As Integer = 3
Private Const mcintNull As Long = &H1000
Private Const mcstrSignature As String = "FMZ"
Private Declare Sub CopyMemory Lib "Kernel32" Alias "RtlMoveMemory" (pDest As Any, pSource As Any, ByVal dwLength As Long)
Public Sub BeginProcss()
If m_bCompress Then
Compress
Else
Decompress
End If
End Sub
Private Function LastError(ErrNo As Integer) As String
Select Case ErrNo
Case 1
LastError = "待压缩文件未设置或不存在"
Case 2
LastError = "待压缩文件长度太小"
Case 3
LastError = "待压缩文件已经过压缩"
Case 4
LastError = "待解压文件未设置或不存在"
Case 5
LastError = "待解压文件格式不对或为本软件不能认别的高版本软件所压缩"
Case 254
LastError = "用户取消了操作"
Case 255
LastError = "未知错误"
End Select
End Function
Public Property Get CompressLevel() As Integer
CompressLevel = mintCompressLevel / 16
End Property
Public Property Let CompressLevel(ByVal intValue As Integer)
mintCompressLevel = intValue * 16
If mintCompressLevel < 0 Then mintCompressLevel = 0
End Property
Public Property Get IsCompress() As Boolean
IsCompress = m_bCompress
End Property
Public Property Let IsCompress(ByVal bValue As Boolean)
m_bCompress = bValue
End Property
Public Property Let CancelProcss(ByVal bValue As Boolean)
m_bEnableProcss = Not bValue
End Property
Public Property Get InputFileName() As String
InputFileName = m_strInputFileName
End Property
Public Property Get OutputFileName() As String
OutputFileName = m_strOutputFileName
End Property
Public Property Let OutputFileName(ByVal strValue As String)
m_strOutputFileName = strValue
End Property
Public Property Let InputFileName(ByVal strValue As String)
m_strInputFileName = strValue
End Property
Private Sub Class_Terminate()
m_bEnableProcss = False
End Sub