159 lines
3.0 KiB
Ruby
159 lines
3.0 KiB
Ruby
module Oga
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##
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# Thread-safe LRU cache using a Hash as the underlying storage engine.
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# Whenever the size of the cache exceeds the given limit the oldest keys are
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# removed (base on insert order).
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#
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# This class uses its own list of keys (as returned by {LRU#keys}) instead of
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# relying on `Hash#keys` as the latter allocates a new Array upon every call.
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#
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# This class doesn't use MonitorMixin due to the extra overhead it adds
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# compared to using a Mutex directly.
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#
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# Example usage:
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#
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# cache = LRU.new(3)
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#
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# cache[:a] = 10
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# cache[:b] = 20
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# cache[:c] = 30
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# cache[:d] = 40
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#
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# cache.keys # => [:b, :c, :d]
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#
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class LRU
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##
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# @param [Fixnum] maximum
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#
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def initialize(maximum = 1024)
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@maximum = maximum
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@cache = {}
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@keys = []
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@mutex = Mutex.new
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@owner = Thread.current
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end
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##
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# @param [Fixnum] value
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#
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def maximum=(value)
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synchronize do
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@maximum = value
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resize
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end
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end
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##
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# @return [Fixnum]
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#
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def maximum
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return synchronize { @maximum }
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end
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##
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# Returns the value of the key.
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#
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# @param [Mixed] key
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# @return [Mixed]
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#
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def [](key)
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return synchronize { @cache[key] }
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end
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##
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# Sets the key and its value. Old keys are discarded if the LRU size exceeds
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# the limit.
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#
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# @param [Mixed] key
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# @param [Mixed] value
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#
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def []=(key, value)
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synchronize do
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@cache[key] = value
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@keys.delete(key) if @keys.include?(key)
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@keys << key
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resize
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end
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end
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##
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# Returns a key if it exists, otherwise yields the supplied block and uses
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# its return value as the key value.
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#
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# @param [Mixed] key
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# @return [Mixed]
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#
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def get_or_set(key)
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return synchronize { self[key] ||= yield }
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end
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##
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# @return [Array]
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#
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def keys
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return synchronize { @keys }
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end
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##
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# @param [Mixed] key
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# @return [TrueClass|FalseClass]
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#
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def key?(key)
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return synchronize { @cache.key?(key) }
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end
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##
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# Removes all keys from the cache.
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#
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def clear
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synchronize do
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@keys.clear
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@cache.clear
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end
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end
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##
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# @return [Fixnum]
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#
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def size
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return synchronize { @cache.size }
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end
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alias_method :length, :size
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private
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##
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# Yields the supplied block in a synchronized manner (if needed). This
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# method is heavily based on `MonitorMixin#mon_enter`.
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#
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def synchronize
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if @owner != Thread.current
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@mutex.synchronize do
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@owner = Thread.current
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yield
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end
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else
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yield
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end
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end
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##
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# Removes old keys until the size of the hash no longer exceeds the maximum
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# size.
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#
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def resize
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return unless size > @maximum
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to_remove = @keys.shift(size - @maximum)
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to_remove.each { |key| @cache.delete(key) }
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end
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end # LRU
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end # Oga
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