From: Ruby Quiz <james@...>
Date: 2008-02-01T22:55:39+09:00
Subject: [QUIZ] Parsing JSON (#155)

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There has been a lot of talk recently about parsing with Ruby.  We're seeing
some parser generator libraries pop up that make the task that much easier and
they've been stirring up interest.

In honor of that, this week's Ruby Quiz is to write a parser for JSON.

JSON turns out to turns out to be a great little example for writing parsers for
two reasons.  First, it's pretty easy stuff.  You can hand-roll a JSON parser in
under 100 lines of Ruby.  The second advantage is that the data format is
wonderfully documented:

	http://json.org/

Since JSON is just a data format and Ruby supports all of the data types, I vote
we just use Ruby itself as the abstract syntax tree produced by the parse.

Feel free to show off your favorite parser generator, if you don't want to roll
your own.  Anything goes.

Here are a few tests to get you started:

	require "test/unit"
	
	class TestJSONParser < Test::Unit::TestCase
	  def setup
	    @parser = JSONParser.new
	  end
	  
	  def test_keyword_parsing
	    assert_equal(true,  @parser.parse("true"))
	    assert_equal(false, @parser.parse("false"))
	    assert_equal(nil,   @parser.parse("null"))
	  end
	  
	  def test_number_parsing
	    assert_equal(42,      @parser.parse("42"))
	    assert_equal(-13,     @parser.parse("-13"))
	    assert_equal(3.1415,  @parser.parse("3.1415"))
	    assert_equal(-0.01,   @parser.parse("-0.01"))
	
	    assert_equal(0.2e1,   @parser.parse("0.2e1"))
	    assert_equal(0.2e+1,  @parser.parse("0.2e+1"))
	    assert_equal(0.2e-1,  @parser.parse("0.2e-1"))
	    assert_equal(0.2E1,   @parser.parse("0.2e1"))
	  end
	  
	  def test_string_parsing
	    assert_equal(String.new,          @parser.parse(%Q{""}))
	    assert_equal("JSON",              @parser.parse(%Q{"JSON"}))
	    
	    assert_equal( %Q{nested "quotes"},
	                  @parser.parse('"nested \"quotes\""') )
	    assert_equal("\n",                @parser.parse(%Q{"\\n"}))
	    assert_equal( "a",
	                  @parser.parse(%Q{"\\u#{"%04X" % ?a}"}) )
	  end
	  
	  def test_array_parsing
	    assert_equal(Array.new, @parser.parse(%Q{[]}))
	    assert_equal( ["JSON", 3.1415, true],
	                  @parser.parse(%Q{["JSON", 3.1415, true]}) )
	    assert_equal([1, [2, [3]]], @parser.parse(%Q{[1, [2, [3]]]}))
	  end
	  
	  def test_object_parsing
	    assert_equal(Hash.new, @parser.parse(%Q{{}}))
	    assert_equal( {"JSON" => 3.1415, "data" => true},
	                  @parser.parse(%Q{{"JSON": 3.1415, "data": true}}) )
	    assert_equal( { "Array"  => [1, 2, 3],
	                    "Object" => {"nested" => "objects"} },
	                  @parser.parse(<<-END_OBJECT) )
	    {"Array": [1, 2, 3], "Object": {"nested": "objects"}}
	    END_OBJECT
	  end
	  
	  def test_parse_errors
	    assert_raise(RuntimeError) { @parser.parse("{") }
	    assert_raise(RuntimeError) { @parser.parse(%q{{"key": true false}}) }
	
	    assert_raise(RuntimeError) { @parser.parse("[") }
	    assert_raise(RuntimeError) { @parser.parse("[1,,2]") }
	
	    assert_raise(RuntimeError) { @parser.parse(%Q{"}) }
	    assert_raise(RuntimeError) { @parser.parse(%Q{"\\i"}) }
	
	    assert_raise(RuntimeError) { @parser.parse("$1,000") }
	    assert_raise(RuntimeError) { @parser.parse("1_000") }
	    assert_raise(RuntimeError) { @parser.parse("1K") }
	
	    assert_raise(RuntimeError) { @parser.parse("unknown") }
	  end
	end