module Liquid
  # Container for liquid nodes which conveniently wraps decision making logic
  #
  # Example:
  #
  #   c = Condition.new('1', '==', '1')
  #   c.evaluate #=> true
  #
  class Condition #:nodoc:
    @@operators = {
      '==' => lambda { |cond, left, right|  cond.send(:equal_variables, left, right) },
      '!=' => lambda { |cond, left, right| !cond.send(:equal_variables, left, right) },
      '<>' => lambda { |cond, left, right| !cond.send(:equal_variables, left, right) },
      '<'  => :<,
      '>'  => :>,
      '>=' => :>=,
      '<=' => :<=,
      'contains' => lambda { |cond, left, right| left.include?(right) },
    }

    def self.operators
      @@operators
    end

    attr_reader :attachment
    attr_accessor :left, :operator, :right

    def initialize(left = nil, operator = nil, right = nil)
      @left, @operator, @right = left, operator, right
      @child_relation  = nil
      @child_condition = nil
    end

    def evaluate(context = Context.new)
      result = interpret_condition(left, right, operator, context)

      case @child_relation
      when :or
        result || @child_condition.evaluate(context)
      when :and
        result && @child_condition.evaluate(context)
      else
        result
      end
    end

    def or(condition)
      @child_relation, @child_condition = :or, condition
    end

    def and(condition)
      @child_relation, @child_condition = :and, condition
    end

    def attach(attachment)
      @attachment = attachment
    end

    def else?
      false
    end

    def inspect
      "#<Condition #{[@left, @operator, @right].compact.join(' ')}>"
    end

    private

    def equal_variables(left, right)
      if left.is_a?(Symbol)
        if right.respond_to?(left)
          return right.send(left.to_s)
        else
          return nil
        end
      end

      if right.is_a?(Symbol)
        if left.respond_to?(right)
          return left.send(right.to_s)
        else
          return nil
        end
      end

      left == right
    end

    def interpret_condition(left, right, op, context)
      # If the operator is empty this means that the decision statement is just
      # a single variable. We can just poll this variable from the context and
      # return this as the result.
      return context[left] if op == nil

      left, right = context[left], context[right]

      operation = self.class.operators[op] || raise(ArgumentError.new("Unknown operator #{op}"))

      if operation.respond_to?(:call)
        operation.call(self, left, right)
      elsif left.respond_to?(operation) and right.respond_to?(operation)
        left.send(operation, right)
      else
        nil
      end
    end
  end


  class ElseCondition < Condition
    def else?
      true
    end

    def evaluate(context)
      true
    end
  end

end