Allow empty parens in a string to be safely parsed

This change allows an empty set of parens, `()`, to be in a string and to be parsed as a token. Prior to this change, and empty set of parens would be parsed as a list with no contents which could not be successfully parsed, but instead results in a error as described in projectblacklight#101.

This change fixes issue projectblacklight#101.

lib/parsing_nesting/grammar.rb

@@ -30,6 +30,11 @@ class Grammar < Parslet::Parser
  paren_list
  end
 
+ rule :empty_paren do
+ (str('()'))
+ end
+
+
  # Note well: It was tricky to parse the thing we want where you can
  # have a flat list with boolean operators, but where 'OR' takes precedence.
  # eg "A AND B OR C AND C" or "A OR B AND C OR D". Tricky to parse at all,
@@ -59,7 +64,7 @@ class Grammar < Parslet::Parser
  end
 
  rule :token do
- match['^ ")('].repeat(1).as(:token)
+ (match['^ ")('] | empty_paren ).repeat(1).as(:token)
  end
  rule :phrase do
  match('"') >> match['^"'].repeat(1).as(:phrase) >> match('"')

spec/parsing_nesting/build_tree_spec.rb

@@ -198,6 +198,29 @@ def should_be_not_expression(graph)
  end
  end
 
+ it "should parse term list with empty parens () " do
+ should_be_list parse("foo () bar") do |list|
+ expect(list.length).to eq(3)
+ expect(list[1].value).to eq('()')
+ end
+ end
+
+ it "should parse term list with leading or trailing empty parens () " do
+ should_be_list parse("() foo ()") do |list|
+ expect(list.length).to eq(3)
+ expect(list[0].value).to eq('()')
+ expect(list[2].value).to eq('()')
+ end
+ end
+
+ it "should parse term list with nested parens ()" do
+ should_be_list parse("(()) foo") do |list|
+ expect(list.length).to eq(2)
+ expect(list[0].value).to eq('()')
+ end
+ end
+
+
  it "should build for a crazy complicated one" do
  should_be_list parse("mark +twain AND huck OR fun OR ((jim AND river) AND (red -dogs))") do |list|
  should_be_term list[0], "mark"

spec/parsing_nesting/consuming_spec.rb

@@ -29,7 +29,8 @@
  "NOT (four five)",
  "(one two three) OR (four five) AND six",
  '"foo+bar (baz"',
- "(foo bar one AND two) AND (three four ten OR twelve)"
+ "(foo bar one AND two) AND (three four ten OR twelve)",
+ "one () two"
  ].each do |query|
  it "should consume<<#{query}>>" do
  expect { @parser.parse(query) }.not_to raise_error