I am directly copying a paragraph from Programming Ruby 1.9:
Hash keys must respond to the message hash
by returning a hash code, and the hash code for a given key must not change. The keys used in hashes must also be comparable using eql?
. If eql?
returns true
for two keys, then those keys must also have the same hash code. This means that certain classes (such as Array
and Hash
) can't conveniently be used as keys, because their hash values can change based on their contents.
So you might generate your hash as something like ["#{source_ip} #{source_port}", "#{dest_ip} #{dest_port}", protocol.to_s].sort.join.hash
such that the result will be identical when the source and destination are switched.
For example:
source_ip = "1.2.3.4"
source_port = 1234
dest_ip = "5.6.7.8"
dest_port = 5678
protocol = "http"
def make_hash(s_ip, s_port, d_ip, d_port, proto)
["#{s_ip} #{s_port}", "#{d_ip} #{d_port}", proto.to_s].sort.join.hash
end
puts make_hash(source_ip, source_port, dest_ip, dest_port, protocol)
puts make_hash(dest_ip, dest_port, source_ip, source_port, protocol)
This will output the same hash even though the arguments are in a different order between the two calls. Correctly encapsulating this functionality into a class is left as an exercise to the reader.