module AST
  # Processor is a class which helps transforming one AST into another.
  # In a nutshell, the {#process} method accepts a {Node} and dispatches
  # it to a handler corresponding to its type, and returns a (possibly)
  # updated variant of the node.
  #
  # Processor has a set of associated design patterns. They are best
  # explained with a concrete example. Let's define a simple arithmetic
  # language and an AST format for it:
  #
  # Terminals (AST nodes which do not have other AST nodes inside):
  #
  #   * `(integer <int-literal>)`,
  #
  # Nonterminals (AST nodes with other nodes as children):
  #
  #   * `(add <node> <node>)`,
  #   * `(multiply <node> <node>)`,
  #   * `(divide <node> <node>)`,
  #   * `(negate <node>)`,
  #   * `(store <node> <string-literal>)`: stores value of `<node>` into a variable named `<string-literal>`,
  #   * `(load <string-literal>)`: loads value of a variable named `<string-literal>`,
  #   * `(each <node> ...): computes each of the `<node>`s and prints the result.
  #
  # All AST nodes have the same Ruby class, and therefore they don't
  # know how to traverse themselves. (A solution which dynamically checks the
  # type of children is possible, but is slow and error-prone.) So, a subclass
  # of Processor which knows how to traverse the entire tree should be defined.
  # Such subclass has a handler for each nonterminal node which recursively
  # processes children nodes:
  #
  #     require 'ast'
  #
  #     class ArithmeticsProcessor < AST::Processor
  #       # This method traverses any binary operators such as (add) or (multiply).
  #       def process_binary_op(node)
  #         # Children aren't decomposed automatically; it is suggested to use Ruby
  #         # multiple assignment expansion, as it is very convenient here.
  #         left_expr, right_expr = *node
  #
  #         # AST::Node#updated won't change node type if nil is passed as a first
  #         # argument, which allows to reuse the same handler for multiple node types
  #         # using `alias' (below).
  #         node.updated(nil, [
  #           process(left_expr),
  #           process(right_expr)
  #         ])
  #       end
  #       alias on_add      process_binary_op
  #       alias on_multiply process_binary_op
  #       alias on_divide   process_binary_op
  #
  #       def on_negate(node)
  #         # It is also possible to use #process_all for more compact code
  #         # if every child is a Node.
  #         node.updated(nil, process_all(node))
  #       end
  #
  #       def on_store(node)
  #         expr, variable_name = *node
  #
  #         # Note that variable_name is not a Node and thus isn't passed to #process.
  #         node.updated(nil, [
  #           process(expr),
  #           variable_name
  #         ])
  #       end
  #
  #       # (load) is effectively a terminal node, and so it does not need
  #       # an explicit handler, as the following is the default behavior.
  #       def on_load(node)
  #         nil
  #       end
  #
  #       def on_each(node)
  #         node.updated(nil, process_all(node))
  #       end
  #     end
  #
  # Let's test our ArithmeticsProcessor:
  #
  #     include AST::Sexp
  #     expr = s(:add, s(:integer, 2), s(:integer, 2))
  #
  #     p ArithmeticsProcessor.new.process(expr) == expr # => true
  #
  # As expected, it does not change anything at all. This isn't actually
  # very useful, so let's now define a Calculator, which will compute the
  # expression values:
  #
  #     # This Processor folds nonterminal nodes and returns an (integer)
  #     # terminal node.
  #     class ArithmeticsCalculator < ArithmeticsProcessor
  #       def compute_op(node)
  #         # First, node children are processed and then unpacked to local
  #         # variables.
  #         nodes = process_all(node)
  #
  #         if nodes.all? { |node| node.type == :integer }
  #           # If each of those nodes represents a literal, we can fold this
  #           # node!
  #           values = nodes.map { |node| node.children.first }
  #           AST::Node.new(:integer, [
  #             yield(values)
  #           ])
  #         else
  #           # Otherwise, we can just leave the current node in the tree and
  #           # only update it with processed children nodes, which can be
  #           # partially folded.
  #           node.updated(nil, nodes)
  #         end
  #       end
  #
  #       def on_add(node)
  #         compute_op(node) { |left, right| left + right }
  #       end
  #
  #       def on_multiply(node)
  #         compute_op(node) { |left, right| left * right }
  #       end
  #     end
  #
  # Let's check:
  #
  #     p ArithmeticsCalculator.new.process(expr) # => (integer 4)
  #
  # Excellent, the calculator works! Now, a careful reader could notice that
  # the ArithmeticsCalculator does not know how to divide numbers. What if we
  # pass an expression with division to it?
  #
  #     expr_with_division = \
  #       s(:add,
  #         s(:integer, 1),
  #         s(:divide,
  #           s(:add, s(:integer, 8), s(:integer, 4)),
  #           s(:integer, 3))) # 1 + (8 + 4) / 3
  #
  #     folded_expr_with_division = ArithmeticsCalculator.new.process(expr_with_division)
  #     p folded_expr_with_division
  #     # => (add
  #     #      (integer 1)
  #     #      (divide
  #     #        (integer 12)
  #     #        (integer 3)))
  #
  # As you can see, the expression was folded _partially_: the inner `(add)` node which
  # could be computed was folded to `(integer 12)`, the `(divide)` node is left as-is
  # because there is no computing handler for it, and the root `(add)` node was also left
  # as it is because some of its children were not literals.
  #
  # Note that this partial folding is only possible because the _data_ format, i.e.
  # the format in which the computed values of the nodes are represented, is the same as
  # the AST itself.
  #
  # Let's extend our ArithmeticsCalculator class further.
  #
  #     class ArithmeticsCalculator
  #       def on_divide(node)
  #         compute_op(node) { |left, right| left / right }
  #       end
  #
  #       def on_negate(node)
  #         # Note how #compute_op works regardless of the operator arity.
  #         compute_op(node) { |value| -value }
  #       end
  #     end
  #
  # Now, let's apply our renewed ArithmeticsCalculator to a partial result of previous
  # evaluation:
  #
  #     p ArithmeticsCalculator.new.process(expr_with_division) # => (integer 5)
  #
  # Five! Excellent. This is also pretty much how CRuby 1.8 executed its programs.
  #
  # Now, let's do some automated bug searching. Division by zero is an error, right?
  # So if we could detect that someone has divided by zero before the program is even
  # run, that could save some debugging time.
  #
  #     class DivisionByZeroVerifier < ArithmeticsProcessor
  #       class VerificationFailure < Exception; end
  #
  #       def on_divide(node)
  #         # You need to process the children to handle nested divisions
  #         # such as:
  #         # (divide
  #         #   (integer 1)
  #         #   (divide (integer 1) (integer 0))
  #         left, right = process_all(node)
  #
  #         if right.type == :integer &&
  #            right.children.first == 0
  #           raise VerificationFailure, "Ouch! This code divides by zero."
  #         end
  #       end
  #
  #       def divides_by_zero?(ast)
  #         process(ast)
  #         false
  #       rescue VerificationFailure
  #         true
  #       end
  #     end
  #
  #     nice_expr = \
  #       s(:divide,
  #         s(:add, s(:integer, 10), s(:integer, 2)),
  #         s(:integer, 4))
  #
  #     p DivisionByZeroVerifier.new.divides_by_zero?(nice_expr)
  #     # => false. Good.
  #
  #     bad_expr = \
  #       s(:add, s(:integer, 10),
  #         s(:divide, s(:integer, 1), s(:integer, 0)))
  #
  #     p DivisionByZeroVerifier.new.divides_by_zero?(bad_expr)
  #     # => true. WHOOPS. DO NOT RUN THIS.
  #
  # Of course, this won't detect more complex cases... unless you use some partial
  # evaluation before! The possibilites are endless. Have fun.
  class Processor
    # Dispatches `node`. If a node has type `:foo`, then a handler named
    # `on_foo` is invoked with one argument, the `node`; if there isn't
    # such a handler, {#handler_missing} is invoked with the same argument.
    #
    # If the handler returns `nil`, `node` is returned; otherwise, the return
    # value of the handler is passed along.
    #
    # @param  [AST::Node, nil] node
    # @return [AST::Node, nil]
    def process(node)
      return if node.nil?

      node = node.to_ast

      # Invoke a specific handler
      on_handler = :"on_#{node.type}"
      if respond_to? on_handler
        new_node = send on_handler, node
      else
        new_node = handler_missing(node)
      end

      node = new_node if new_node

      node
    end

    # {#process}es each node from `nodes` and returns an array of results.
    #
    # @param  [Array<AST::Node>] nodes
    # @return [Array<AST::Node>]
    def process_all(nodes)
      nodes.to_a.map do |node|
        process node
      end
    end

    # Default handler. Does nothing.
    #
    # @param  [AST::Node] node
    # @return [AST::Node, nil]
    def handler_missing(node)
    end
  end
end
