Stupid simple JSON parser

parse-json.rb

require 'strscan'
require 'forwardable'

# Stupid JSON parser. Only handles well-formed JSON.
# Otherwise, it may go into an endless loop.
class Parser
  WSP = /\s+/
  OBJ = /[{\[]/
  NUM = /-?\d+(\.\d+)?([eE][+-]?\d+)?/
  BOL = /(?:true|false)\b/
  NUL = /null\b/

  extend Forwardable

  attr_reader :scanner
  alias_method :s, :scanner
  private :s
  def_delegators :scanner, :scan, :matched

  def initialize data
    @scanner = StringScanner.new data.to_s
  end

  def space
    scan WSP
  end

  def parse
    space
    object
  end

  def object
    case scan(OBJ)
    when '{' then hash
    when '[' then array
    end
  end

  def value
    object or string or
      (scan(NUM) || scan(BOL)) ? eval(matched) :
      scan(NUL) && nil
  end

  def hash
    current = {}
    space
    until scan /\}/
      key = string
      scan /\s*:\s*/
      current[key] = value
      scan /\s*,\s*/ or space
    end
    current
  end

  def array
    current = []
    space
    until scan /\]/
      current << value
      scan /\s*,\s*/ or space
    end
    current
  end

  def string
    if str = scan(/"/)
      begin; str << s.scan_until(/"/); end while s.pre_match[-1,1] == '\\'
      eval str
    end
  end
end

if __FILE__ == $0
  require 'test/unit'
  class ParserTest < Test::Unit::TestCase
    PARSED = Parser.new(DATA.read).parse
    def parsed() PARSED end
    def test_string
      assert_equal "Pagination library for \"Rails 3\", Sinatra, Merb, DataMapper, and more",
        parsed['head']['repository']['description']
    end
    def test_hash
      assert_equal %w[label ref repository sha user], parsed['head'].keys.sort
    end
    def test_number
      assert_equal 124.3e2, parsed['head']['repository']['size']
    end
    def test_bool
      assert_equal true, parsed['head']['repository']['fork']
      assert_equal false, parsed['head']['repository']['private']
    end
    def test_bool
      assert_nil parsed['head']['user']['company']
    end
    def test_array
      assert_equal ["4438f", {"a" => "b"}], parsed['head']['sha']
    end
  end
end

__END__
{
  "head": {
    "ref": "master",
    "repository": {
      "forks": 0,
      "integrate_branch": "rails3",
      "watchers": 1,
      "language": "Ruby",
      "description": "Pagination library for \"Rails 3\", Sinatra, Merb, DataMapper, and more",
      "has_downloads": true,
      "fork": true,
      "created_at": "2011/10/24 03:20:48 -0700",
      "homepage": "http://github.com/mislav/will_paginate/wikis",
      "size": 124.3e2,
      "private": false,
      "has_wiki": true,
      "name": "will_paginate",
      "owner": "dbackeus",
      "url": "https://github.com/dbackeus/will_paginate",
      "has_issues": false,
      "open_issues": 0,
      "pushed_at": "2011/10/25 05:44:05 -0700"
    },
    "label": "dbackeus:master",
    "sha": ["4438f", { "a" : "b" }],
    "user": {
      "name": "David Backeus",
      "company": null,
      "gravatar_id": "ebe96524f5db9e92188f0542dc9d1d1a",
      "location": "Stockholm (Sweden)",
      "type": "User",
      "login": "dbackeus"
    }
  }
}