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module ActiveSupport
module CachingTools #:nodoc:
# Provide shortcuts to simply the creation of nested default hashes. This
# pattern is useful, common practice, and unsightly when done manually.
module HashCaching
# Dynamically create a nested hash structure used to cache calls to +method_name+
# The cache method is named +#{method_name}_cache+ unless :as => :alternate_name
# is given.
#
# The hash structure is created using nested Hash.new. For example:
#
# def slow_method(a, b) a ** b end
#
# can be cached using hash_cache :slow_method, which will define the method
# slow_method_cache. We can then find the result of a ** b using:
#
# slow_method_cache[a][b]
#
# The hash structure returned by slow_method_cache would look like this:
#
# Hash.new do |as, a|
# as[a] = Hash.new do |bs, b|
# bs[b] = slow_method(a, b)
# end
# end
#
# The generated code is actually compressed onto a single line to maintain
# sensible backtrace signatures.
#
def hash_cache(method_name, options = {})
selector = options[:as] || "#{method_name}_cache"
method = self.instance_method(method_name)
args = []
code = "def #{selector}(); @#{selector} ||= "
(1..method.arity).each do |n|
args << "v#{n}"
code << "Hash.new {|h#{n}, v#{n}| h#{n}[v#{n}] = "
end
# Add the method call with arguments, followed by closing braces and end.
code << "#{method_name}(#{args * ', '}) #{'}' * method.arity} end"
# Extract the line number information from the caller. Exceptions arising
# in the generated code should point to the +hash_cache :...+ line.
if caller[0] && /^(.*):(\d+)$/ =~ caller[0]
file, line_number = $1, $2.to_i
else # We can't give good trackback info; fallback to this line:
file, line_number = __FILE__, __LINE__
end
# We use eval rather than building proc's because it allows us to avoid
# linking the Hash's to this method's binding. Experience has shown that
# doing so can cause obtuse memory leaks.
class_eval code, file, line_number
end
end
end
end
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