block:
  type
    Number = ref object
      height: int
      case isPair: bool
      of true:
          left, right: Number

      of false:
          value: int

    
type
  Number = object
    values: seq[int]
    lefts: seq[seq[bool]]

func parse(text: string): Number =
  var left: seq[bool]
  for c in text:
    case c
    of '[':
      left.add true
    of '0' .. '9':
      result.values.add ord(c) - ord('0')
      result.lefts.add left
    of ',':
      left[^1] = false
    of ']':
      discard left.pop
    else:
      raise ValueError.newException &"invalid character: \'{c}\'"

func `$`(n: Number): string =
  var left: seq[bool]
  for i in 0 .. n.values.high:
    if left.len < n.lefts[i].len:
      for _ in left.len ..< n.lefts[i].len:
        result.add '['
    elif left.len == n.lefts[i].len:
      assert left[^1] != n.lefts[i][^1]
      var count = 0
      for j in 1 .. left.len:
        if left[^j] != n.lefts[i][^j]:
          inc count
        else:
          break
      assert count >= 1
      result.add ']'.repeat(count - 1)
      result.add ','
      result.add '['.repeat(count - 1)
    else:
      for _ in n.lefts[i].len ..< left.len:
        result.add ']'
      result.add ','
    result.add $n.values[i]
    left = n.lefts[i]
  for _ in 0 ..< left.len:
    result.add ']'

for line in """[1,2]
[[1,2],3]
[9,[8,7]]
[[1,9],[8,5]]
[[[[1,2],[3,4]],[[5,6],[7,8]]],9]
[[[9,[3,8]],[[0,9],6]],[[[3,7],[4,9]],3]]
[[[[1,3],[5,3]],[[1,3],[8,7]]],[[[4,9],[6,9]],[[8,2],[7,3]]]]""".splitLines:
  echo line, " -> ", parse(line)
  echo parse(line).lefts

[1,2] -> [1,2]
@[@[true], @[false]]
[[1,2],3] -> [[1,2],3]
@[@[true, true], @[true, false], @[false]]
[9,[8,7]] -> [9[8,7]]
@[@[true], @[false, true], @[false, false]]
[[1,9],[8,5]] -> [[1,9],[8,5]]
@[@[true, true], @[true, false], @[false, true], @[false, false]]
[[[[1,2],[3,4]],[[5,6],[7,8]]],9] -> [[[[1,2],[3,4]],[[5,6],[7,8]]],9]
@[@[true, true, true, true], @[true, true, true, false], @[true, true, false, true], @[true, true, false, false], @[true, false, true, true], @[true, false, true, false], @[true, false, false, true], @[true, false, false, false], @[false]]
[[[9,[3,8]],[[0,9],6]],[[[3,7],[4,9]],3]] -> [[[9[3,8]],[[0,9],6[3,7],[4,9]],3]]
@[@[true, true, true], @[true, true, false, true], @[true, true, false, false], @[true, false, true, true], @[true, false, true, false], @[true, false, false], @[false, true, true, true], @[false, true, true, false], @[false, true, false, true], @[false, true, false, false], @[false, false]]
[[[[1,3],[5,3]],[[1,3],[8,7]]],[[[4,9],[6,9]],[[8,2],[7,3]]]] -> [[[[1,3],[5,3]],[[1,3],[8,7]]],[[[4,9],[6,9]],[[8,2],[7,3]]]]
@[@[true, true, true, true], @[true, true, true, false], @[true, true, false, true], @[true, true, false, false], @[true, false, true, true], @[true, false, true, false], @[true, false, false, true], @[true, false, false, false], @[false, true, true, true], @[false, true, true, false], @[false, true, false, true], @[false, true, false, false], @[false, false, true, true], @[false, false, true, false], @[false, false, false, true], @[false, false, false, false]]