type
  sharedStringObj = object
    len: int
    sptr: pointer

  sharedString* = ptr sharedStringObj

var
  aCount = 0
  aDataCount = 0
  checkEnabled = false

proc checkOnExit() {.noconv.} =
  doAssert aCount == 0, "All sharedString instances not freed"
  doAssert aDataCount == 0, "All sharedString data instances not freed"

proc freeSharedStringData(ss: var sharedString) =
  if not ss.isNil:
    if not ss.sptr.isNil and ss.len != 0:
      aDataCount -= 1
      ss.sptr.deallocShared()
      ss.len = 0
      ss.sptr = nil

proc freeSharedString*(ss: var sharedString) =
  if not ss.isNil:
    ss.freeSharedStringData()
    aCount -= 1
    ss.deallocShared()
    ss = nil

proc setSharedStringData(ss: var sharedString, s: cstring) =
  if not ss.isNil:
    if not ss.sptr.isNil and ss.len != 0:
      ss.freeSharedStringData()

    if s.len != 0:
      ss.len = s.len
      aDataCount += 1
      ss.sptr = allocShared0(s.len)

      var
        ssc = cast[cstring](ss.sptr)
      for i in 0 .. s.len-1:
        ssc[i] = s[i]

proc newSharedString*(s: cstring = nil): sharedString =
  if not checkEnabled:
    addQuitProc(checkOnExit)
    checkEnabled = true
  aCount += 1
  result = cast[sharedString](allocShared0(sizeof(sharedStringObj)))
  result.setSharedStringData(s)

proc `$`*(ss: sharedString): string =
  result = $(cast[cstring](ss.sptr))

proc `&`*(ss: sharedString, c: char|string|cstring|sharedString): string =
  $ss & $c

proc `&=`*(ss: var sharedString, c: char|string|cstring|sharedString) =
  ss.setSharedStringData($ss & $c)

proc set*(ss: var sharedString, c: char|string|cstring|sharedString) =
  ss.setSharedStringData($c)

proc clear*(ss: var sharedString) =
  ss.freeSharedStringData()

proc `[]`*(ss: var sharedString, i: int): char =
  ($ss)[i]

proc test() =
  var
    a = newSharedString()
    b = newSharedString("abc")
    c = newSharedString("def".cstring)

  defer:
    a.freeSharedString()
    b.freeSharedString()
    c.freeSharedString()

  echo a
  echo b
  echo c

  echo b & 'c'
  echo b & "d"
  echo b & "e".cstring
  echo b & c

  b &= 'c'
  echo b

  c.set("ghi")
  echo c

  c.clear()

  echo b[0]
  try:
    echo b[5]
  except IndexError:
    echo "IndexError"

when isMainModule:
  test()