require "json"

struct = { "a" => 1, "b" => 2, "c" => [1, 2, 3], "d" => [{ "e" => 3 }, nil, false, true, [], {}] }
source = JSON.dump(struct)

tokens = []
index = 0

until source.empty?
  tokens <<
    case source.strip
    when /\A[\{\}\[\],:]/ then $&.to_sym
    when /\A(\d+)/        then $1.to_i
    when /\A"([^"]*?)"/   then $1
    when /\A(true|false)/ then $1 == "true"
    when /\Anull/         then nil
    else                  raise
    end

  source = $'
end

def parse_array(tokens)
  result = []

  loop do
    item, tokens = parse_item(tokens)
    result << item

    case tokens
    in [:"]", *rest] then return result, rest
    in [:",", *rest] then tokens = rest
    end
  end
end

def parse_object(tokens)
  result = {}

  loop do
    tokens => [String => key, :":", *tokens]
    value, tokens = parse_item(tokens)
    result[key] = value

    case tokens
    in [:"}", *rest] then return result, rest
    in [:",", *rest] then tokens = rest
    end
  end
end

def parse_item(tokens)
  case tokens
  in [:"[", :"]", *rest] then [[], rest]
  in [:"[", *rest]       then parse_array(rest)
  in [:"{", :"}", *rest] then [{}, rest]
  in [:"{", *rest]       then parse_object(rest)
  in [FalseClass | Integer | NilClass | String | TrueClass => value, *rest]
    [value, rest]
  end
end

def parse(tokens)
  parse_item(tokens) => [value, []]
  value
end

p struct
p parse(tokens)