const
  trailingZeros100: array[100, int8] = [2'i8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0,
    0, 0, 0, 0, 0, 0]

  digits100: array[200, char] = ['0', '0', '0', '1', '0', '2', '0', '3', '0', '4', '0', '5',
    '0', '6', '0', '7', '0', '8', '0', '9', '1', '0', '1', '1', '1', '2', '1', '3', '1', '4',
    '1', '5', '1', '6', '1', '7', '1', '8', '1', '9', '2', '0', '2', '1', '2', '2', '2', '3',
    '2', '4', '2', '5', '2', '6', '2', '7', '2', '8', '2', '9', '3', '0', '3', '1', '3', '2',
    '3', '3', '3', '4', '3', '5', '3', '6', '3', '7', '3', '8', '3', '9', '4', '0', '4', '1',
    '4', '2', '4', '3', '4', '4', '4', '5', '4', '6', '4', '7', '4', '8', '4', '9', '5', '0',
    '5', '1', '5', '2', '5', '3', '5', '4', '5', '5', '5', '6', '5', '7', '5', '8', '5', '9',
    '6', '0', '6', '1', '6', '2', '6', '3', '6', '4', '6', '5', '6', '6', '6', '7', '6', '8',
    '6', '9', '7', '0', '7', '1', '7', '2', '7', '3', '7', '4', '7', '5', '7', '6', '7', '7',
    '7', '8', '7', '9', '8', '0', '8', '1', '8', '2', '8', '3', '8', '4', '8', '5', '8', '6',
    '8', '7', '8', '8', '8', '9', '9', '0', '9', '1', '9', '2', '9', '3', '9', '4', '9', '5',
    '9', '6', '9', '7', '9', '8', '9', '9']

# Inspired by https://engineering.fb.com/2013/03/15/developer-tools/three-optimization-tips-for-c
# Generates:
# .. code-block:: nim
#   var res = ""
#   for i in 0 .. 99:
#     if i < 10:
#       res.add "0" & $i
#     else:
#       res.add $i
#   doAssert res == digits100

proc utoa2Digits*(buf: var openArray[char]; pos: int; digits: uint32) {.inline.} =
  assert(digits <= 99)
  buf[pos] = digits100[2 * digits]
  buf[pos+1] = digits100[2 * digits + 1]
  #copyMem(buf, unsafeAddr(digits100[2 * digits]), 2 * sizeof((char)))

proc trailingZeros2Digits*(digits: uint32): int32 {.inline.} =
  assert(digits <= 99)
  return trailingZeros100[digits]

func digits10*(num: uint64): int {.noinline.} =
  if num < 10'u64:
    result = 1
  elif num < 100'u64:
    result = 2
  elif num < 1_000'u64:
    result = 3
  elif num < 10_000'u64:
    result = 4
  elif num < 100_000'u64:
    result = 5
  elif num < 1_000_000'u64:
    result = 6
  elif num < 10_000_000'u64:
    result = 7
  elif num < 100_000_000'u64:
    result = 8
  elif num < 1_000_000_000'u64:
    result = 9
  elif num < 10_000_000_000'u64:
    result = 10
  elif num < 100_000_000_000'u64:
    result = 11
  elif num < 1_000_000_000_000'u64:
    result = 12
  else:
    result = 12 + digits10(num div 1_000_000_000_000'u64)

template numToString*(result: var string, origin: uint64, length: int) =
  var num = origin
  var next = length - 1
  const nbatch = 100

  while num >= nbatch:
    let originNum = num
    num = num div nbatch
    let index = (originNum - num * nbatch) shl 1
    result[next] = digits100[index + 1]
    result[next - 1] = digits100[index]
    dec(next, 2)

  # process last 1-2 digits
  if num < 10:
    result[next] = chr(ord('0') + num)
  else:
    let index = num * 2
    result[next] = digits100[index + 1]
    result[next - 1] = digits100[index]