widestr.nim

type
  Utf16Char* = distinct int16
  WideCString* = ref array[0.. 1_000_000, Utf16Char]

proc len*(w: WideCString): int =
  ## returns the length of a widestring. This traverses the whole string to
  ## find the binary zero end marker!
  while int16(w[result]) != 0'i16: inc result

const
  UNI_REPLACEMENT_CHAR = Utf16Char(0xFFFD'i16)
  UNI_MAX_BMP = 0x0000FFFF
  UNI_MAX_UTF16 = 0x0010FFFF
  UNI_MAX_UTF32 = 0x7FFFFFFF
  UNI_MAX_LEGAL_UTF32 = 0x0010FFFF

  halfShift = 10
  halfBase = 0x0010000
  halfMask = 0x3FF

  UNI_SUR_HIGH_START = 0xD800
  UNI_SUR_HIGH_END = 0xDBFF
  UNI_SUR_LOW_START = 0xDC00
  UNI_SUR_LOW_END = 0xDFFF

template ones(n: expr): expr = ((1 shl n)-1)

template fastRuneAt(s: cstring, i: int, result: expr, doInc = true) =
  ## Returns the unicode character ``s[i]`` in `result`. If ``doInc == true``
  ## `i` is incremented by the number of bytes that have been processed.
  bind ones

  if ord(s[i]) <=% 127:
    result = ord(s[i])
    when doInc: inc(i)
  elif ord(s[i]) shr 5 == 0b110:
    #assert(ord(s[i+1]) shr 6 == 0b10)
    result = (ord(s[i]) and (ones(5))) shl 6 or (ord(s[i+1]) and ones(6))
    when doInc: inc(i, 2)
  elif ord(s[i]) shr 4 == 0b1110:
    #assert(ord(s[i+1]) shr 6 == 0b10)
    #assert(ord(s[i+2]) shr 6 == 0b10)
    result = (ord(s[i]) and ones(4)) shl 12 or
             (ord(s[i+1]) and ones(6)) shl 6 or
             (ord(s[i+2]) and ones(6))
    when doInc: inc(i, 3)
  elif ord(s[i]) shr 3 == 0b11110:
    #assert(ord(s[i+1]) shr 6 == 0b10)
    #assert(ord(s[i+2]) shr 6 == 0b10)
    #assert(ord(s[i+3]) shr 6 == 0b10)
    result = (ord(s[i]) and ones(3)) shl 18 or
             (ord(s[i+1]) and ones(6)) shl 12 or
             (ord(s[i+2]) and ones(6)) shl 6 or
             (ord(s[i+3]) and ones(6))
    when doInc: inc(i, 4)
  else:
    result = 0xFFFD
    when doInc: inc(i)

iterator runes(s: cstring): int =
  var
    i = 0
    result: int
  while s[i] != '\0':
    fastRuneAt(s, i, result, true)
    yield result

proc newWideCString*(source: cstring, L: int): WideCString =
  unsafeNew(result, L * 4 + 2)
  #result = cast[wideCString](alloc(L * 4 + 2))
  var d = 0
  for ch in runes(source):
    if ch <=% UNI_MAX_BMP:
      if ch >=% UNI_SUR_HIGH_START and ch <=% UNI_SUR_LOW_END:
        result[d] = UNI_REPLACEMENT_CHAR
      else:
        result[d] = Utf16Char(toU16(ch))
    elif ch >% UNI_MAX_UTF16:
      result[d] = UNI_REPLACEMENT_CHAR
    else:
      let ch = ch -% halfBase
      result[d] = Utf16Char(toU16((ch shr halfShift) +% UNI_SUR_HIGH_START))
      inc d
      result[d] = Utf16Char(toU16((ch and halfMask) +% UNI_SUR_LOW_START))
    inc d
  result[d] = Utf16Char(0'i16)

proc newWideCString*(s: cstring): WideCString =
  if s.isNil: return nil

  when not declared(c_strlen):
    proc c_strlen(a: cstring): int {.
      header: "<string.h>", noSideEffect, importc: "strlen".}

  let L = c_strlen(s)
  result = newWideCString(s, L)

proc newWideCString*(s: string): WideCString =
  result = newWideCString(s, s.len)

proc `$`*(w: WideCString, estimate: int, replacement: int = 0xFFFD): string =
  result = newStringOfCap(estimate + estimate shr 2)

  var i = 0
  while w[i].int16 != 0'i16:
    var ch = int(cast[uint16](w[i]))
    inc i
    if ch >= UNI_SUR_HIGH_START and ch <= UNI_SUR_HIGH_END:
      # If the 16 bits following the high surrogate are in the source buffer...
      let ch2 = int(cast[uint16](w[i]))

      # If it's a low surrogate, convert to UTF32:
      if ch2 >= UNI_SUR_LOW_START and ch2 <= UNI_SUR_LOW_END:
        ch = (((ch and halfMask) shl halfShift) + (ch2 and halfMask)) + halfBase
        inc i
      else:
        #invalid UTF-16
        ch = replacement
    elif ch >= UNI_SUR_LOW_START and ch <= UNI_SUR_LOW_END:
      #invalid UTF-16
      ch = replacement

    if ch < 0x80:
      result.add chr(ch)
    elif ch < 0x800:
      result.add chr((ch shr 6) or 0xc0)
      result.add chr((ch and 0x3f) or 0x80)
    elif ch < 0x10000:
      result.add chr((ch shr 12) or 0xe0)
      result.add chr(((ch shr 6) and 0x3f) or 0x80)
      result.add chr((ch and 0x3f) or 0x80)
    elif ch <= 0x10FFFF:
      result.add chr((ch shr 18) or 0xf0)
      result.add chr(((ch shr 12) and 0x3f) or 0x80)
      result.add chr(((ch shr 6) and 0x3f) or 0x80)
      result.add chr((ch and 0x3f) or 0x80)
    else:
      # replacement char(in case user give very large number):
      result.add chr(0xFFFD shr 12 or 0b1110_0000)
      result.add chr(0xFFFD shr 6 and ones(6) or 0b10_0000_00)
      result.add chr(0xFFFD and ones(6) or 0b10_0000_00)

proc `$`*(s: WideCString): string =
  result = s $ 80