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94 lines
1.5 KiB
Ruby
94 lines
1.5 KiB
Ruby
# This pattern provides an interpreter to deal with an abstract language. Using
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# classes we can understand the inputs for parse them
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# Parsing entity
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class Number
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attr_reader :value
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def initialize(value)
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@value = value
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end
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def execute
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value
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end
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end
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# Operation interface
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class Operation
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attr_reader :left, :right
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def initialize(left, right)
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@left = left
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@right = right
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end
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def execute
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raise NotImplementedError
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end
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end
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# Operations
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class Plus < Operation
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def execute
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left.execute + right.execute
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end
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end
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class Minus < Operation
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def execute
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left.execute - right.execute
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end
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end
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class Multiply < Operation
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def execute
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left.execute * right.execute
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end
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end
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class Devide < Operation
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def execute
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left.execute / right.execute
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end
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end
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# Interpreter class
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class Interpreter
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def self.parse(input)
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stack = []
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input.lstrip!
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while input.length > 0
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case input
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when /\A-?\d+(\.\d+)?/
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stack << Number.new($&.to_i)
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else
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second, first = stack.pop(), stack.pop()
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case input
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when /\A\+/
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stack << Plus.new(first, second)
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when /\A\-/
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stack << Minus.new(first, second)
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when /\A\*/
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stack << Multiply.new(first, second)
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else
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raise SyntaxError
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end
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end
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input = $'
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input.lstrip!
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end
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raise SyntaxError unless stack.size == 1
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stack.first.execute
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end
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end
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# Usage
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puts Interpreter.parse('1 + 1 + 2 - 1') # => 3
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