package azblob

import (

	"os"

	"reflect"

	"syscall"

	"unsafe"

)

type mmf []byte

func newMMF(file *os.File, writable bool, offset int64, length int) (mmf, error) {

	prot, access := uint32(syscall.PAGE_READONLY), uint32(syscall.FILE_MAP_READ) // Assume read-only

	if writable {

		prot, access = uint32(syscall.PAGE_READWRITE), uint32(syscall.FILE_MAP_WRITE)

	}

	hMMF, errno := syscall.CreateFileMapping(syscall.Handle(file.Fd()), nil, prot, uint32(int64(length)>>32), uint32(int64(length)&0xffffffff), nil)

	if hMMF == 0 {

		return nil, os.NewSyscallError("CreateFileMapping", errno)

	}

	defer syscall.CloseHandle(hMMF)

	addr, errno := syscall.MapViewOfFile(hMMF, access, uint32(offset>>32), uint32(offset&0xffffffff), uintptr(length))

	m := mmf{}

	h := (*reflect.SliceHeader)(unsafe.Pointer(&m))

	h.Data = addr

	h.Len = length

	h.Cap = h.Len

	return m, nil

}

func (m *mmf) unmap() {

	addr := uintptr(unsafe.Pointer(&(([]byte)(*m)[0])))

	*m = mmf{}

	err := syscall.UnmapViewOfFile(addr)

	if err != nil {

		panic("if we are unable to unmap the memory-mapped file, there is serious concern for memory corruption")

	}

}