ruby-concurrency/thread_safe

module ThreadSafe
  module Util
    # A Ruby port of the Doug Lea's jsr166e.LondAdder class version 1.8
    # available in public domain.
    #
    # Original source code available here:
    # http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/LongAdder.java?revision=1.8
    #
    # One or more variables that together maintain an initially zero
    # sum. When updates (method +add+) are contended across threads,
    # the set of variables may grow dynamically to reduce contention.
    # Method +sum+ returns the current total combined across the
    # variables maintaining the sum.
    #
    # This class is usually preferable to single +Atomic+ reference when
    # multiple threads update a common sum that is used for purposes such
    # as collecting statistics, not for fine-grained synchronization
    # control.  Under low update contention, the two classes have similar
    # characteristics. But under high contention, expected throughput of
    # this class is significantly higher, at the expense of higher space
    # consumption.
    class Adder < Striped64
      # Adds the given value.
      def add(x)
        if (current_cells = cells) || !cas_base_computed {|current_base| current_base + x}
          was_uncontended = true
          hash            = hash_code
          unless current_cells && (cell = current_cells.volatile_get_by_hash(hash)) && (was_uncontended = cell.cas_computed {|current_value| current_value + x})
            retry_update(x, hash, was_uncontended) {|current_value| current_value + x}
          end
        end
      end

      def increment
        add(1)
      end

      def decrement
        add(-1)
      end

      # Returns the current sum.  The returned value is _NOT_ an
      # atomic snapshot: Invocation in the absence of concurrent
      # updates returns an accurate result, but concurrent updates that
      # occur while the sum is being calculated might not be
      # incorporated.
      def sum
        x = base
        if current_cells = cells
          current_cells.each do |cell|
            x += cell.value if cell
          end
        end
        x
      end

      def reset
        internal_reset(0)
      end
    end
  end
end