SimpleJsonParser in Ruby

json-parser.rb

require 'strscan'

class SimpleJsonParser < StringScanner
  WHITESPACE = /\s+/
  SINGLE_LINE_COMMENT = /\/\/[^\n\r]*\s*/

  NULL = /null/

  NUMBER       = /\-?\d+(\.\d+)?([eE][\+\-]?\d+)?/
  NUMBER_START = /\-?\d/

  BOOLEAN       = /(true|false)/
  BOOLEAN_START = /[tf]/

  STRING       = /"(\\.|[^"\\])*"/
  STRING_START = /\"/

  ARRAY_START           = /\[/
  ARRAY_END             = /\]/
  ARRAY_VALUE_DELIMITER = /\,/

  OBJECT_START           = /\{/
  OBJECT_END             = /\}/
  OBJECT_KEY_DELIMITER   = /\:/
  OBJECT_VALUE_DELIMITER = /\,/

  def parse
    value = parse_value
    _skip_ignore
    raise 'Parse Error' unless eos?
    value
  end

  private
  def parse_value
    _skip_ignore
    value =
      case
      when match?(NULL)          then parse_null
      when match?(NUMBER_START)  then parse_number
      when match?(BOOLEAN_START) then parse_boolean
      when match?(STRING_START)  then parse_string
      when match?(ARRAY_START)   then parse_array
      when match?(OBJECT_START)  then parse_object
      else raise 'Value Error'
      end
  end

  def parse_null
    raise 'Null Error' unless scan(NULL)
    nil
  end

  def parse_number
    num_str = scan(NUMBER)
    raise 'Number Error' if num_str == nil
    num_str.to_f
  end

  def parse_boolean
    bool_str = scan(BOOLEAN)
    case
    when bool_str == 'true' then true
    when bool_str == 'false' then false
    else raise 'Boolean Error'
    end
  end

  def parse_string
    str = scan(STRING)
    raise 'String Error' if str == nil
    str[1..-2]
  end

  def parse_array
    skip(ARRAY_START)
    _skip_ignore
    return [] if scan(ARRAY_END)

    arr = []
    while true
      _skip_ignore

      value = parse_value
      arr.push(value)

      _skip_ignore
      case
      when scan(ARRAY_END) then break
      when scan(ARRAY_VALUE_DELIMITER) then next
      else raise 'Array Error'
      end
    end
    arr
  end

  def parse_object
    skip(OBJECT_START)
    _skip_ignore
    return {} if scan(OBJECT_END)

    obj = {}
    while true
      _skip_ignore

      key = parse_string
      _skip_ignore
      raise 'Object Error' unless scan(OBJECT_KEY_DELIMITER)
      _skip_ignore

      value = parse_value
      obj[key] = value

      _skip_ignore
      case
      when scan(OBJECT_END) then break
      when scan(OBJECT_VALUE_DELIMITER) then next
      else raise 'Object Error'
      end
    end
    obj
  end

  def _skip_ignore
    skip(WHITESPACE)
    skip(SINGLE_LINE_COMMENT)
  end
end

json = <<~JSON
        {
          "test": null,
          "key": "poor parser",
          "key2": 123e3,      // Hello, World
          "key3": [1, 210.5, 3, {
            "hello": "world",
            "world": null
          }]
        }
       JSON
parser = SimpleJsonParser.new(json)
puts(parser.parse)