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Description: A Ruby-based parsing DSL based on parsing expression grammars.
Homepage: http://treetop.rubyforge.org
Clone URL: git://github.com/nathansobo/treetop.git 
Memoization works for the nil objects that are now returned for parse 
failures.

The hash lookup explicitly asks if a key exists rather than checking for 
nil.
pivotal (author)
Mon Dec 10 09:35:15 -0800 2007
commit  beaefb2901bd1ec9cb5ebdef2b77c330ea22b9d1
tree    7f053015978b428463fb6ca2e3ac92f0b5e7330f
parent  493653ba15d6ef1f37f5a45b3db2382fae18d91c
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trunk/lib/treetop/compiler/node_classes/parsing_rule.rb 
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@@ -32,9 +32,9 @@ module Treetop
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       end
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       def generate_cache_lookup(builder)
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-        builder.assign 'cached', "node_cache[:#{name}][index]"
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-        builder.if_ 'cached' do
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-          builder << '@index = cached.interval.end'
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+        builder.if_ "node_cache[:#{name}].has_key?(index)" do
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+          builder.assign 'cached', "node_cache[:#{name}][index]"
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+          builder << '@index = cached.interval.end if cached'
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           builder << 'return cached'
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         end
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       end

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