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bufSz := 1 << 26             ; 64MB buffer
VarSetCapacity(buff,bufSz)   ; allocate buffer
file := A_ScriptFullPath     ; put your filename here
FileGetSize Sz, %file%

c := 0, offs := -bufSz
h := OpenFile(file)          ; handle to file
Loop % Sz//bufSz {           ; for each buff-full of data
   BinRead(h, buff, bufSz, offs+=bufSz)
   c := CRC32(buff,bufSz,~c) ; compute accumulated CRC
}

If (m:=mod(Sz,bufSz)) {      ; the slack
   BinRead(h, buff, m, offs+=bufSz)
   c := CRC32(buff,m,~c)
}
CloseFile(h)
                             ; c = CRC here
SetFormat Integer, Hex
MsgBox  % c+0                ; show CRC32 in hex


OpenFile(file) {             ; only for read!
   Return DllCall("CreateFile",Str,file, UInt,0x80000000, UInt,3, UInt,0, UInt,3, UInt,0, UInt,0)
}

BinRead(hFile, ByRef data, n, offset=0)  { ; offset<0: counted from the end backwards
   DllCall("SetFilePointerEx",UInt,hFile, Int64,offset, UIntP,U, Int,2*(offset<0))
   DllCall("ReadFile",UInt,hFile, Str,data, UInt,n, UIntP,r, UInt,0)
   Return r                  ; the number of bytes read
}

CloseFile(hFile) {
   DllCall("CloseHandle", UInt,hFile)
}

CRC32(ByRef Buffer, Bytes=0, Start=-1) {
   Static CRC32, CRC32LookupTable
   If (CRC32 = "") {
      MCode(CRC32,"33c06a088bc85af6c101740ad1e981f12083b8edeb02d1e94a75ec8b542404890c82403d0001000072d8c3")
      VarSetCapacity(CRC32LookupTable, 1024)
      DllCall(&CRC32, "uint",&CRC32LookupTable, "cdecl")
      MCode(CRC32,"558bec33c039450c7627568b4d080fb60c08334d108b55108b751481e1ff000000c1ea0833148e403b450c89551072db5e8b4510f7d05dc3")
   }
   If Bytes <= 0
      Bytes := StrLen(Buffer)
   Return DllCall(&CRC32, "uint",&Buffer, "uint",Bytes, "int",Start, "uint",&CRC32LookupTable, "cdecl uint")
}

MCode(ByRef code, hex) { ; allocate memory and write Machine Code there
   VarSetCapacity(code,StrLen(hex)//2)
   Loop % StrLen(hex)//2
      NumPut("0x" . SubStr(hex,2*A_Index-1,2), code, A_Index-1, "Char")
}

#NoEnv
SetBatchLines -1
;Critical On
;Process, Priority, , Realtime

StartTime := A_TickCount

aBuffer := 1024 * 32

VarSetCapacity(data,aBuffer)   ; allocate 4KB buffer (change to your taste)
file := "E:\Downloads\gos-3.1-gadgets-20081205.iso" ;A_ScriptFullPath    ; put your filename here
FileGetSize Sz, %file%

c := 0, offs := -aBuffer
Loop % Sz//aBuffer {           ; for each block of data
   BinRead(file, data, aBuffer, offs+=aBuffer)
   c := CRC32(data,aBuffer,~c) ; compute accumulated CRC
}

If (m:=mod(Sz,aBuffer)) {      ; the slack
   BinRead(file, data, m, offs+=aBuffer)
   c := CRC32(data,m,~c)
}
                            ; c = CRC here
SetFormat Integer, Hex
crc := c+0               ; show CRC32 in hex
SetFormat Integer, D

ElapsedTime := Round((A_TickCount - StartTime)/1000)

MsgBox  % "It took " ElapsedTime " seconds to scan`n`n" file "`n`nBuffer size: " aBuffer " kB`nCRC: " crc

Return

BinRead(file, ByRef data, n=0, offset=0)  {
   h := DllCall("CreateFile",Str,file, UInt,0x80000000, UInt,3, UInt,0, UInt,3, UInt,0, UInt,0)
   DllCall("SetFilePointerEx",UInt,h, Int64,offset, UIntP,U, Int,2*(offset<0))
   m := DllCall("GetFileSize",UInt,h, Int64P,r)
   If n not between 1 and %m%
      n := m
   VarSetCapacity(data, n)
   DllCall("ReadFile",UInt,h, Str,data, UInt,n, UIntP,r, UInt,0)
   DllCall("CloseHandle", UInt,h)
   Return r
}

CRC32(ByRef Buffer, Bytes=0, Start=-1) {
   Static CRC32, CRC32LookupTable
   If (CRC32 = "") {
      MCode(CRC32,"33c06a088bc85af6c101740ad1e981f12083b8edeb02d1e94a75ec8b542404890c82403d0001000072d8c3")
      VarSetCapacity(CRC32LookupTable, 1024)
      DllCall(&CRC32, "uint",&CRC32LookupTable, "cdecl")
      MCode(CRC32,"558bec33c039450c7627568b4d080fb60c08334d108b55108b751481e1ff000000c1ea0833148e403b450c89551072db5e8b4510f7d05dc3")
   }
   If Bytes <= 0
      Bytes := StrLen(Buffer)
   Return DllCall(&CRC32, "uint",&Buffer, "uint",Bytes, "int",Start, "uint",&CRC32LookupTable, "cdecl uint")
}

MCode(ByRef code, hex) { ; allocate memory and write Machine Code there
   VarSetCapacity(code,StrLen(hex)//2)
   Loop % StrLen(hex)//2
      NumPut("0x" . SubStr(hex,2*A_Index-1,2), code, A_Index-1, "Char")
}

#NoEnv
SetBatchLines -1
;Critical On
;Process, Priority, , Realtime

StartTime := A_TickCount

;aBuffer := 1024 * 4

fileToScan := "E:\Downloads\gos-3.1-gadgets-20081205.iso"
CMDdir := A_ScriptDir "\bin"

CMDin := A_ScriptDir "\bin\rehash.exe -none -crc32 -norcrsv -f """ fileToScan """"


crc := CMDret_RunReturn(CMDin, CMDdir)

ElapsedTime := Round((A_TickCount - StartTime)/1000)

MsgBox  % "It took " ElapsedTime " seconds to scan`n`n" fileToScan "`n`nBuffer size: " aBuffer " kB`nCRC: " crc              ; show CRC32 in hex

Return

; ******************************************************************
; CMDret-AHK functions
; version 1.10 beta
;
; Updated: Dec 5, 2006
; by: corrupt
; Code modifications and/or contributions made by:
; Laszlo, shimanov, toralf, Wdb 
; ******************************************************************
; Usage:
; CMDin - command to execute
; WorkingDir - full path to working directory (Optional)
; ******************************************************************
; Known Issues:
; - If using dir be sure to specify a path (example: cmd /c dir c:\)
; or specify a working directory   
; - Running 16 bit console applications may not produce output. Use
; a 32 bit application to start the 16 bit process to receive output 
; ******************************************************************
; Additional requirements:
; - none
; ******************************************************************
; Code Start
; ******************************************************************

CMDret_RunReturn(CMDin, WorkingDir=0)
{
  Global cmdretPID
  tcWrk := WorkingDir=0 ? "Int" : "Str"
  idltm := A_TickCount + 20
  CMsize = 1
  VarSetCapacity(CMDout, 1, 32)
  VarSetCapacity(sui,68, 0)
  VarSetCapacity(pi, 16, 0)
  VarSetCapacity(pa, 12, 0)
  Loop, 4 {
    DllCall("RtlFillMemory", UInt,&pa+A_Index-1, UInt,1, UChar,12 >> 8*A_Index-8)
    DllCall("RtlFillMemory", UInt,&pa+8+A_Index-1, UInt,1, UChar,1 >> 8*A_Index-8)
  }
  IF (DllCall("CreatePipe", "UInt*",hRead, "UInt*",hWrite, "UInt",&pa, "Int",0) <> 0) {
    Loop, 4
      DllCall("RtlFillMemory", UInt,&sui+A_Index-1, UInt,1, UChar,68 >> 8*A_Index-8)
    DllCall("GetStartupInfo", "UInt", &sui)
    Loop, 4 {
      DllCall("RtlFillMemory", UInt,&sui+44+A_Index-1, UInt,1, UChar,257 >> 8*A_Index-8)
      DllCall("RtlFillMemory", UInt,&sui+60+A_Index-1, UInt,1, UChar,hWrite >> 8*A_Index-8)
      DllCall("RtlFillMemory", UInt,&sui+64+A_Index-1, UInt,1, UChar,hWrite >> 8*A_Index-8)
      DllCall("RtlFillMemory", UInt,&sui+48+A_Index-1, UInt,1, UChar,0 >> 8*A_Index-8)
    }
    IF (DllCall("CreateProcess", Int,0, Str,CMDin, Int,0, Int,0, Int,1, "UInt",0, Int,0, tcWrk, WorkingDir, UInt,&sui, UInt,&pi) <> 0) {
      Loop, 4
        cmdretPID += *(&pi+8+A_Index-1) << 8*A_Index-8
      Loop {
        idltm2 := A_TickCount - idltm
        If (idltm2 < 10) {
          DllCall("Sleep", Int, 10)
          Continue
        }
        IF (DllCall("PeekNamedPipe", "uint", hRead, "uint", 0, "uint", 0, "uint", 0, "uint*", bSize, "uint", 0 ) <> 0 ) {
          Process, Exist, %cmdretPID%
          IF (ErrorLevel OR bSize > 0) {
            IF (bSize > 0) {
              VarSetCapacity(lpBuffer, bSize+1)
              IF (DllCall("ReadFile", "UInt",hRead, "Str", lpBuffer, "Int",bSize, "UInt*",bRead, "Int",0) > 0) {
                IF (bRead > 0) {
                  TRead += bRead
                  VarSetCapacity(CMcpy, (bRead+CMsize+1), 0)
                  CMcpy = a
                  DllCall("RtlMoveMemory", "UInt", &CMcpy, "UInt", &CMDout, "Int", CMsize)
                  DllCall("RtlMoveMemory", "UInt", &CMcpy+CMsize, "UInt", &lpBuffer, "Int", bRead)
                  CMsize += bRead
                  VarSetCapacity(CMDout, (CMsize + 1), 0)
                  CMDout=a   
                  DllCall("RtlMoveMemory", "UInt", &CMDout, "UInt", &CMcpy, "Int", CMsize)
                  VarSetCapacity(CMDout, -1)   ; fix required by change in autohotkey v1.0.44.14
                }
              }
            }
          }
          ELSE
            break
        }
        ELSE
          break
        idltm := A_TickCount
      }
      cmdretPID=
      DllCall("CloseHandle", UInt, hWrite)
      DllCall("CloseHandle", UInt, hRead)
    }
  }
  IF (StrLen(CMDout) < TRead) {
    VarSetCapacity(CMcpy, TRead, 32)
    TRead2 = %TRead%
    Loop {
      DllCall("RtlZeroMemory", "UInt", &CMcpy, Int, TRead)
      NULLptr := StrLen(CMDout)
      cpsize := Tread - NULLptr
      DllCall("RtlMoveMemory", "UInt", &CMcpy, "UInt", (&CMDout + NULLptr + 2), "Int", (cpsize - 1))
      DllCall("RtlZeroMemory", "UInt", (&CMDout + NULLptr), Int, cpsize)
      DllCall("RtlMoveMemory", "UInt", (&CMDout + NULLptr), "UInt", &CMcpy, "Int", cpsize)
      TRead2 --
      IF (StrLen(CMDout) > TRead2)
        break
    }
  }
  StringTrimLeft, CMDout, CMDout, 1
  Return, CMDout
}

hexData := Bin2Hex (&hBitmap, not_sure_how_to_get_hbitmap_size_at_this_level)