atomic_reference_cache_backend.rb

module ThreadSafe
  # A Ruby port of the Doug Lea's jsr166e.ConcurrentHashMapV8 class version 1.59 available in public domain.
  # Original source code available here: http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/ConcurrentHashMapV8.java?revision=1.59
  #
  # The Ruby port skips out the +TreeBin+ (red-black trees for use in bins
  # whose size exceeds a threshold).
  #
  # A hash table supporting full concurrency of retrievals and
  # high expected concurrency for updates. However, even though all
  # operations are thread-safe, retrieval operations do _not_ entail locking,
  # and there is _not_ any support for locking the entire table
  # in a way that prevents all access.
  #
  # Retrieval operations generally do not block, so may overlap with
  # update operations. Retrievals reflect the results of the most
  # recently _completed_ update operations holding upon their
  # onset. (More formally, an update operation for a given key bears a
  # _happens-before_ relation with any (non +nil+) retrieval for
  # that key reporting the updated value.)  For aggregate operations
  # such as +clear()+, concurrent retrievals may reflect insertion or removal
  # of only some entries.  Similarly, the +each_pair+ iterator yields elements
  # reflecting the state of the hash table at some point at or since
  # the start of the +each_pair+. Bear in mind that the results of
  # aggregate status methods including +size()+ and +empty?+} are typically
  # useful only when a map is not undergoing concurrent updates in other
  # threads. Otherwise the results of these methods reflect transient
  # states that may be adequate for monitoring or estimation purposes, but not
  # for program control.
  #
  # The table is dynamically expanded when there are too many
  # collisions (i.e., keys that have distinct hash codes but fall into
  # the same slot modulo the table size), with the expected average
  # effect of maintaining roughly two bins per mapping (corresponding
  # to a 0.75 load factor threshold for resizing). There may be much
  # variance around this average as mappings are added and removed, but
  # overall, this maintains a commonly accepted time/space tradeoff for
  # hash tables.  However, resizing this or any other kind of hash
  # table may be a relatively slow operation. When possible, it is a
  # good idea to provide a size estimate as an optional :initial_capacity
  # initializer argument. An additional optional :load_factor constructor
  # argument provides a further means of customizing initial table capacity
  # by specifying the table density to be used in calculating the amount of
  # space to allocate for the given number of elements. Note that using
  # many keys with exactly the same +hash+ is a sure way to slow down
  # performance of any hash table.
  #
  # == Design overview
  #
  # The primary design goal of this hash table is to maintain
  # concurrent readability (typically method +[]+, but also
  # iteration and related methods) while minimizing update
  # contention. Secondary goals are to keep space consumption about
  # the same or better than plain +Hash+, and to support high
  # initial insertion rates on an empty table by many threads.
  #
  # Each key-value mapping is held in a +Node+. The validation-based
  # approach explained below leads to a lot of code sprawl because
  # retry-control precludes factoring into smaller methods.
  #
  # The table is lazily initialized to a power-of-two size upon the
  # first insertion.  Each bin in the table normally contains a
  # list of +Node+s (most often, the list has only zero or one +Node+).
  # Table accesses require volatile/atomic reads, writes, and
  # CASes. The lists of nodes within bins are always accurately traversable
  # under volatile reads, so long as lookups check hash code
  # and non-nullness of value before checking key equality.
  #
  # We use the top two bits of +Node+ hash fields for control
  # purposes -- they are available anyway because of addressing
  # constraints.  As explained further below, these top bits are
  # used as follows:
  #  00 - Normal
  #  01 - Locked
  #  11 - Locked and may have a thread waiting for lock
  #  10 - +Node+ is a forwarding node
  #
  # The lower 28 bits of each +Node+'s hash field contain a
  # the key's hash code, except for forwarding nodes, for which
  # the lower bits are zero (and so always have hash field == +MOVED+).
  #
  # Insertion (via +[]=+ or its variants) of the first node in an
  # empty bin is performed by just CASing it to the bin.  This is
  # by far the most common case for put operations under most
  # key/hash distributions.  Other update operations (insert,
  # delete, and replace) require locks.  We do not want to waste
  # the space required to associate a distinct lock object with
  # each bin, so instead use the first node of a bin list itself as
  # a lock. Blocking support for these locks relies +Util::CheapLockable.
  # However, we also need a +try_lock+ construction, so we overlay
  # these by using bits of the +Node+ hash field for lock control (see above),
  # and so normally use builtin monitors only for blocking and signalling using
  # +cheap_wait+/+cheap_broadcast+ constructions. See +Node#try_await_lock+.
  #
  # Using the first node of a list as a lock does not by itself
  # suffice though: When a node is locked, any update must first
  # validate that it is still the first node after locking it, and
  # retry if not. Because new nodes are always appended to lists,
  # once a node is first in a bin, it remains first until deleted
  # or the bin becomes invalidated (upon resizing).  However,
  # operations that only conditionally update may inspect nodes
  # until the point of update. This is a converse of sorts to the
  # lazy locking technique described by Herlihy & Shavit.
  #
  # The main disadvantage of per-bin locks is that other update
  # operations on other nodes in a bin list protected by the same
  # lock can stall, for example when user +eql?+ or mapping
  # functions take a long time.  However, statistically, under
  # random hash codes, this is not a common problem.  Ideally, the
  # frequency of nodes in bins follows a Poisson distribution
  # (http://en.wikipedia.org/wiki/Poisson_distribution) with a
  # parameter of about 0.5 on average, given the resizing threshold
  # of 0.75, although with a large variance because of resizing
  # granularity. Ignoring variance, the expected occurrences of
  # list size k are (exp(-0.5) * pow(0.5, k) / factorial(k)). The
  # first values are:
  #
  #  0:    0.60653066
  #  1:    0.30326533
  #  2:    0.07581633
  #  3:    0.01263606
  #  4:    0.00157952
  #  5:    0.00015795
  #  6:    0.00001316
  #  7:    0.00000094
  #  8:    0.00000006
  #  more: less than 1 in ten million
  #
  # Lock contention probability for two threads accessing distinct
  # elements is roughly 1 / (8 * #elements) under random hashes.
  #
  # The table is resized when occupancy exceeds a percentage
  # threshold (nominally, 0.75, but see below).  Only a single
  # thread performs the resize (using field +size_control+, to arrange
  # exclusion), but the table otherwise remains usable for reads
  # and updates. Resizing proceeds by transferring bins, one by
  # one, from the table to the next table.  Because we are using
  # power-of-two expansion, the elements from each bin must either
  # stay at same index, or move with a power of two offset. We
  # eliminate unnecessary node creation by catching cases where old
  # nodes can be reused because their next fields won't change.  On
  # average, only about one-sixth of them need cloning when a table
  # doubles. The nodes they replace will be garbage collectable as
  # soon as they are no longer referenced by any reader thread that
  # may be in the midst of concurrently traversing table.  Upon
  # transfer, the old table bin contains only a special forwarding
  # node (with hash field +MOVED+) that contains the next table as
  # its key. On encountering a forwarding node, access and update
  # operations restart, using the new table.
  #
  # Each bin transfer requires its bin lock. However, unlike other
  # cases, a transfer can skip a bin if it fails to acquire its
  # lock, and revisit it later. Method +rebuild+ maintains a buffer of
  # TRANSFER_BUFFER_SIZE bins that have been skipped because of failure
  # to acquire a lock, and blocks only if none are available
  # (i.e., only very rarely). The transfer operation must also ensure
  # that all accessible bins in both the old and new table are usable by
  # any traversal. When there are no lock acquisition failures, this is
  # arranged simply by proceeding from the last bin (+table.size - 1+) up
  # towards the first.  Upon seeing a forwarding node, traversals arrange
  # to move to the new table without revisiting nodes. However, when any
  # node is skipped during a transfer, all earlier table bins may have
  # become visible, so are initialized with a reverse-forwarding node back
  # to the old table until the new ones are established. (This
  # sometimes requires transiently locking a forwarding node, which
  # is possible under the above encoding.) These more expensive
  # mechanics trigger only when necessary.
  #
  # The traversal scheme also applies to partial traversals of
  # ranges of bins (via an alternate Traverser constructor)
  # to support partitioned aggregate operations.  Also, read-only
  # operations give up if ever forwarded to a null table, which
  # provides support for shutdown-style clearing, which is also not
  # currently implemented.
  #
  # Lazy table initialization minimizes footprint until first use.
  #
  # The element count is maintained using a +ThreadSafe::Util::Adder+,
  # which avoids contention on updates but can encounter cache thrashing
  # if read too frequently during concurrent access. To avoid reading so
  # often, resizing is attempted either when a bin lock is
  # contended, or upon adding to a bin already holding two or more
  # nodes (checked before adding in the +x_if_absent+ methods, after
  # adding in others). Under uniform hash distributions, the
  # probability of this occurring at threshold is around 13%,
  # meaning that only about 1 in 8 puts check threshold (and after
  # resizing, many fewer do so). But this approximation has high
  # variance for small table sizes, so we check on any collision
  # for sizes <= 64. The bulk putAll operation further reduces
  # contention by only committing count updates upon these size
  # checks.
  class AtomicReferenceCacheBackend
    class Table < Util::PowerOfTwoTuple
      def cas_new_node(i, hash, key, value)
        cas(i, nil, Node.new(hash, key, value))
      end

      def try_to_cas_in_computed(i, hash, key)
        succeeded = false
        new_value = nil
        new_node  = Node.new(locked_hash = hash | LOCKED, key, NULL)
        if cas(i, nil, new_node)
          begin
            if NULL == (new_value = yield(NULL))
              was_null = true
            else
              new_node.value = new_value
            end
            succeeded = true
          ensure
            volatile_set(i, nil) if !succeeded || was_null
            new_node.unlock_via_hash(locked_hash, hash)
          end
        end
        return succeeded, new_value
      end

      def try_lock_via_hash(i, node, node_hash)
        node.try_lock_via_hash(node_hash) do
          yield if volatile_get(i) == node
        end
      end

      def delete_node_at(i, node, predecessor_node)
        if predecessor_node
          predecessor_node.next = node.next
        else
          volatile_set(i, node.next)
        end
      end
    end

    # Key-value entry. Nodes with a hash field of +MOVED+ are special,
    # and do not contain user keys or values.  Otherwise, keys are never +nil+,
    # and +NULL+ +value+ fields indicate that a node is in the process
    # of being deleted or created. For purposes of read-only access, a key may be read
    # before a value, but can only be used after checking value to be +!= NULL+.
    class Node
      extend Util::Volatile
      attr_volatile :hash, :value, :next

      include Util::CheapLockable

      bit_shift = Util::FIXNUM_BIT_SIZE - 2 # need 2 bits for ourselves
      # Encodings for special uses of Node hash fields. See above for explanation.
      MOVED     = ('10' << ('0' * bit_shift)).to_i(2) # hash field for forwarding nodes
      LOCKED    = ('01' << ('0' * bit_shift)).to_i(2) # set/tested only as a bit
      WAITING   = ('11' << ('0' * bit_shift)).to_i(2) # both bits set/tested together
      HASH_BITS = ('00' << ('1' * bit_shift)).to_i(2) # usable bits of normal node hash

      SPIN_LOCK_ATTEMPTS = Util::CPU_COUNT > 1 ? Util::CPU_COUNT * 2 : 0

      attr_reader :key

      def initialize(hash, key, value, next_node = nil)
        super()
        @key = key
        self.lazy_set_hash(hash)
        self.lazy_set_value(value)
        self.next = next_node
      end

      # Spins a while if +LOCKED+ bit set and this node is the first
      # of its bin, and then sets +WAITING+ bits on hash field and
      # blocks (once) if they are still set.  It is OK for this
      # method to return even if lock is not available upon exit,
      # which enables these simple single-wait mechanics.
      #
      # The corresponding signalling operation is performed within
      # callers: Upon detecting that +WAITING+ has been set when
      # unlocking lock (via a failed CAS from non-waiting +LOCKED+
      # state), unlockers acquire the +cheap_synchronize+ lock and
      # perform a +cheap_broadcast+.
      def try_await_lock(table, i)
        if table && i >= 0 && i < table.size # bounds check, TODO: why are we bounds checking?
          spins = SPIN_LOCK_ATTEMPTS
          randomizer = base_randomizer = Util::XorShiftRandom.get
          while equal?(table.volatile_get(i)) && self.class.locked_hash?(my_hash = hash)
            if spins >= 0
              if (randomizer = (randomizer >> 1)).even? # spin at random
                if (spins -= 1) == 0
                  Thread.pass # yield before blocking
                else
                  randomizer = base_randomizer = Util::XorShiftRandom.xorshift(base_randomizer) if randomizer.zero?
                end
              end
            elsif cas_hash(my_hash, my_hash | WAITING)
              force_aquire_lock(table, i)
              break
            end
          end
        end
      end

      def key?(key)
        @key.eql?(key)
      end

      def matches?(key, hash)
        pure_hash == hash && key?(key)
      end

      def pure_hash
        hash & HASH_BITS
      end

      def try_lock_via_hash(node_hash = hash)
        if cas_hash(node_hash, locked_hash = node_hash | LOCKED)
          begin
            yield
          ensure
            unlock_via_hash(locked_hash, node_hash)
          end
        end
      end

      def locked?
        self.class.locked_hash?(hash)
      end

      def unlock_via_hash(locked_hash, node_hash)
        unless cas_hash(locked_hash, node_hash)
          self.hash = node_hash
          cheap_synchronize { cheap_broadcast }
        end
      end

      private
      def force_aquire_lock(table, i)
        cheap_synchronize do
          if equal?(table.volatile_get(i)) && (hash & WAITING) == WAITING
            cheap_wait
          else
            cheap_broadcast # possibly won race vs signaller
          end
        end
      end

      class << self
        def locked_hash?(hash)
          (hash & LOCKED) != 0
        end
      end
    end

    # shorthands
    MOVED     = Node::MOVED
    LOCKED    = Node::LOCKED
    WAITING   = Node::WAITING
    HASH_BITS = Node::HASH_BITS

    NOW_RESIZING     = -1
    DEFAULT_CAPACITY = 16
    MAX_CAPACITY     = Util::MAX_INT

    # The buffer size for skipped bins during transfers. The
    # value is arbitrary but should be large enough to avoid
    # most locking stalls during resizes.
    TRANSFER_BUFFER_SIZE = 32

    extend Util::Volatile
    attr_volatile :table, # The array of bins. Lazily initialized upon first insertion. Size is always a power of two.

                  # Table initialization and resizing control.  When negative, the
                  # table is being initialized or resized. Otherwise, when table is
                  # null, holds the initial table size to use upon creation, or 0
                  # for default. After initialization, holds the next element count
                  # value upon which to resize the table.
                  :size_control

    def initialize(options = nil)
      super()
      @counter = Util::Adder.new
      initial_capacity  = options && options[:initial_capacity] || DEFAULT_CAPACITY
      self.size_control = (capacity = table_size_for(initial_capacity)) > MAX_CAPACITY ? MAX_CAPACITY : capacity
    end

    def get_or_default(key, else_value = nil)
      hash          = key_hash(key)
      current_table = table
      while current_table
        node = current_table.volatile_get_by_hash(hash)
        current_table =
          while node
            if (node_hash = node.hash) == MOVED
              break node.key
            elsif (node_hash & HASH_BITS) == hash && node.key?(key) && NULL != (value = node.value)
              return value
            end
            node = node.next
          end
      end
      else_value
    end

    def [](key)
      get_or_default(key)
    end

    def key?(key)
      get_or_default(key, NULL) != NULL
    end

    def []=(key, value)
      get_and_set(key, value)
      value
    end

    def compute_if_absent(key)
      hash          = key_hash(key)
      current_table = table || initialize_table
      while true
        if !(node = current_table.volatile_get(i = current_table.hash_to_index(hash)))
          succeeded, new_value = current_table.try_to_cas_in_computed(i, hash, key) { yield }
          if succeeded
            increment_size
            return new_value
          end
        elsif (node_hash = node.hash) == MOVED
          current_table = node.key
        elsif NULL != (current_value = find_value_in_node_list(node, key, hash, node_hash & HASH_BITS))
          return current_value
        elsif Node.locked_hash?(node_hash)
          try_await_lock(current_table, i, node)
        else
          succeeded, value = attempt_internal_compute_if_absent(key, hash, current_table, i, node, node_hash) { yield }
          return value if succeeded
        end
      end
    end

    def compute_if_present(key)
      new_value = nil
      internal_replace(key) do |old_value|
        if (new_value = yield(NULL == old_value ? nil : old_value)).nil?
          NULL
        else
          new_value
        end
      end
      new_value
    end

    def compute(key)
      internal_compute(key) do |old_value|
        if (new_value = yield(NULL == old_value ? nil : old_value)).nil?
          NULL
        else
          new_value
        end
      end
    end

    def merge_pair(key, value)
      internal_compute(key) do |old_value|
        if NULL == old_value || !(value = yield(old_value)).nil?
          value
        else
          NULL
        end
      end
    end

    def replace_pair(key, old_value, new_value)
      NULL != internal_replace(key, old_value) { new_value }
    end

    def replace_if_exists(key, new_value)
      if (result = internal_replace(key) { new_value }) && NULL != result
        result
      end
    end

    def get_and_set(key, value) # internalPut in the original CHMV8
      hash          = key_hash(key)
      current_table = table || initialize_table
      while true
        if !(node = current_table.volatile_get(i = current_table.hash_to_index(hash)))
          if current_table.cas_new_node(i, hash, key, value)
            increment_size
            break
          end
        elsif (node_hash = node.hash) == MOVED
          current_table = node.key
        elsif Node.locked_hash?(node_hash)
          try_await_lock(current_table, i, node)
        else
          succeeded, old_value = attempt_get_and_set(key, value, hash, current_table, i, node, node_hash)
          break old_value if succeeded
        end
      end
    end

    def delete(key)
      replace_if_exists(key, NULL)
    end

    def delete_pair(key, value)
      result = internal_replace(key, value) { NULL }
      if result && NULL != result
        !!result
      else
        false
      end
    end

    def each_pair
      return self unless current_table = table
      current_table_size = base_size = current_table.size
      i = base_index = 0
      while base_index < base_size
        if node = current_table.volatile_get(i)
          if node.hash == MOVED
            current_table      = node.key
            current_table_size = current_table.size
          else
            begin
              if NULL != (value = node.value) # skip deleted or special nodes
                yield node.key, value
              end
            end while node = node.next
          end
        end

        if (i_with_base = i + base_size) < current_table_size
          i = i_with_base # visit upper slots if present
        else
          i = base_index += 1
        end
      end
      self
    end

    def size
      (sum = @counter.sum) < 0 ? 0 : sum # ignore transient negative values
    end

    def empty?
      size == 0
    end

    # Implementation for clear. Steps through each bin, removing all nodes.
    def clear
      return self unless current_table = table
      current_table_size = current_table.size
      deleted_count = i = 0
      while i < current_table_size
        if !(node = current_table.volatile_get(i))
          i += 1
        elsif (node_hash = node.hash) == MOVED
          current_table      = node.key
          current_table_size = current_table.size
        elsif Node.locked_hash?(node_hash)
          decrement_size(deleted_count) # opportunistically update count
          deleted_count = 0
          node.try_await_lock(current_table, i)
        else
          current_table.try_lock_via_hash(i, node, node_hash) do
            begin
              deleted_count += 1 if NULL != node.value # recheck under lock
              node.value = nil
            end while node = node.next
            current_table.volatile_set(i, nil)
            i += 1
          end
        end
      end
      decrement_size(deleted_count)
      self
    end

    private
    # Internal versions of the insertion methods, each a
    # little more complicated than the last. All have
    # the same basic structure:
    #  1. If table uninitialized, create
    #  2. If bin empty, try to CAS new node
    #  3. If bin stale, use new table
    #  4. Lock and validate; if valid, scan and add or update
    #
    # The others interweave other checks and/or alternative actions:
    #  * Plain +get_and_set+ checks for and performs resize after insertion.
    #  * compute_if_absent prescans for mapping without lock (and fails to add
    #    if present), which also makes pre-emptive resize checks worthwhile.
    #
    # Someday when details settle down a bit more, it might be worth
    # some factoring to reduce sprawl.
    def internal_replace(key, expected_old_value = NULL, &block)
      hash          = key_hash(key)
      current_table = table
      while current_table
        if !(node = current_table.volatile_get(i = current_table.hash_to_index(hash)))
          break
        elsif (node_hash = node.hash) == MOVED
          current_table = node.key
        elsif (node_hash & HASH_BITS) != hash && !node.next # precheck
          break # rules out possible existence
        elsif Node.locked_hash?(node_hash)
          try_await_lock(current_table, i, node)
        else
          succeeded, old_value = attempt_internal_replace(key, expected_old_value, hash, current_table, i, node, node_hash, &block)
          return old_value if succeeded
        end
      end
      NULL
    end

    def attempt_internal_replace(key, expected_old_value, hash, current_table, i, node, node_hash)
      current_table.try_lock_via_hash(i, node, node_hash) do
        predecessor_node = nil
        old_value        = NULL
        begin
          if node.matches?(key, hash) && NULL != (current_value = node.value)
            if NULL == expected_old_value || expected_old_value == current_value # NULL == expected_old_value means whatever value
              old_value = current_value
              if NULL == (node.value = yield(old_value))
                current_table.delete_node_at(i, node, predecessor_node)
                decrement_size
              end
            end
            break
          end

          predecessor_node = node
        end while node = node.next

        return true, old_value
      end
    end

    def find_value_in_node_list(node, key, hash, pure_hash)
      do_check_for_resize = false
      while true
        if pure_hash == hash && node.key?(key) && NULL != (value = node.value)
          return value
        elsif node = node.next
          do_check_for_resize = true # at least 2 nodes -> check for resize
          pure_hash = node.pure_hash
        else
          return NULL
        end
      end
    ensure
      check_for_resize if do_check_for_resize
    end

    def internal_compute(key, &block)
      hash          = key_hash(key)
      current_table = table || initialize_table
      while true
        if !(node = current_table.volatile_get(i = current_table.hash_to_index(hash)))
          succeeded, new_value = current_table.try_to_cas_in_computed(i, hash, key, &block)
          if succeeded
            if NULL == new_value
              break nil
            else
              increment_size
              break new_value
            end
          end
        elsif (node_hash = node.hash) == MOVED
          current_table = node.key
        elsif Node.locked_hash?(node_hash)
          try_await_lock(current_table, i, node)
        else
          succeeded, new_value = attempt_compute(key, hash, current_table, i, node, node_hash, &block)
          break new_value if succeeded
        end
      end
    end

    def attempt_internal_compute_if_absent(key, hash, current_table, i, node, node_hash)
      added = false
      current_table.try_lock_via_hash(i, node, node_hash) do
        while true
          if node.matches?(key, hash) && NULL != (value = node.value)
            return true, value
          end
          last = node
          unless node = node.next
            last.next = Node.new(hash, key, value = yield)
            added = true
            increment_size
            return true, value
          end
        end
      end
    ensure
      check_for_resize if added
    end

    def attempt_compute(key, hash, current_table, i, node, node_hash)
      added = false
      current_table.try_lock_via_hash(i, node, node_hash) do
        predecessor_node = nil
        while true
          if node.matches?(key, hash) && NULL != (value = node.value)
            if NULL == (node.value = value = yield(value))
              current_table.delete_node_at(i, node, predecessor_node)
              decrement_size
              value = nil
            end
            return true, value
          end
          predecessor_node = node
          unless node = node.next
            if NULL == (value = yield(NULL))
              value = nil
            else
              predecessor_node.next = Node.new(hash, key, value)
              added = true
              increment_size
            end
            return true, value
          end
        end
      end
    ensure
      check_for_resize if added
    end

    def attempt_get_and_set(key, value, hash, current_table, i, node, node_hash)
      node_nesting = nil
      current_table.try_lock_via_hash(i, node, node_hash) do
        node_nesting    = 1
        old_value       = nil
        found_old_value = false
        while node
          if node.matches?(key, hash) && NULL != (old_value = node.value)
            found_old_value = true
            node.value = value
            break
          end
          last = node
          unless node = node.next
            last.next = Node.new(hash, key, value)
            break
          end
          node_nesting += 1
        end

        return true, old_value if found_old_value
        increment_size
        true
      end
    ensure
      check_for_resize if node_nesting && (node_nesting > 1 || current_table.size <= 64)
    end

    def initialize_copy(other)
      super
      @counter = Util::Adder.new
      self.table = nil
      self.size_control = (other_table = other.table) ? other_table.size : DEFAULT_CAPACITY
      self
    end

    def try_await_lock(current_table, i, node)
      check_for_resize # try resizing if can't get lock
      node.try_await_lock(current_table, i)
    end

    def key_hash(key)
      key.hash & HASH_BITS
    end

    # Returns a power of two table size for the given desired capacity.
    def table_size_for(entry_count)
      size = 2
      size <<= 1 while size < entry_count
      size
    end

    # Initializes table, using the size recorded in +size_control+.
    def initialize_table
      until current_table ||= table
        if (size_ctrl = size_control) == NOW_RESIZING
          Thread.pass # lost initialization race; just spin
        else
          try_in_resize_lock(current_table, size_ctrl) do
            initial_size = size_ctrl > 0 ? size_ctrl : DEFAULT_CAPACITY
            current_table = self.table = Table.new(initial_size)
            initial_size - (initial_size >> 2) # 75% load factor
          end
        end
      end
      current_table
    end

    # If table is too small and not already resizing, creates next
    # table and transfers bins.  Rechecks occupancy after a transfer
    # to see if another resize is already needed because resizings
    # are lagging additions.
    def check_for_resize
      while (current_table = table) && MAX_CAPACITY > (table_size = current_table.size) && NOW_RESIZING != (size_ctrl = size_control) && size_ctrl < @counter.sum
        try_in_resize_lock(current_table, size_ctrl) do
          self.table = rebuild(current_table)
          (table_size << 1) - (table_size >> 1) # 75% load factor
        end
      end
    end

    def try_in_resize_lock(current_table, size_ctrl)
      if cas_size_control(size_ctrl, NOW_RESIZING)
        begin
          if current_table == table # recheck under lock
            size_ctrl = yield # get new size_control
          end
        ensure
          self.size_control = size_ctrl
        end
      end
    end

    # Moves and/or copies the nodes in each bin to new table. See above for explanation.
    def rebuild(table)
      old_table_size = table.size
      new_table      = table.next_in_size_table
      # puts "#{old_table_size} -> #{new_table.size}"
      forwarder      = Node.new(MOVED, new_table, NULL)
      rev_forwarder  = nil
      locked_indexes = nil # holds bins to revisit; nil until needed
      locked_arr_idx = 0
      bin            = old_table_size - 1
      i              = bin
      while true
        if !(node = table.volatile_get(i))
          # no lock needed (or available) if bin >= 0, because we're not popping values from locked_indexes until we've run through the whole table
          redo unless (bin >= 0 ? table.cas(i, nil, forwarder) : lock_and_clean_up_reverse_forwarders(table, old_table_size, new_table, i, forwarder))
        elsif Node.locked_hash?(node_hash = node.hash)
          locked_indexes ||= Array.new
          if bin < 0 && locked_arr_idx > 0
            locked_arr_idx -= 1
            i, locked_indexes[locked_arr_idx] = locked_indexes[locked_arr_idx], i # swap with another bin
            redo
          end
          if bin < 0 || locked_indexes.size >= TRANSFER_BUFFER_SIZE
            node.try_await_lock(table, i) # no other options -- block
            redo
          end
          rev_forwarder ||= Node.new(MOVED, table, NULL)
          redo unless table.volatile_get(i) == node && node.locked? # recheck before adding to list
          locked_indexes << i
          new_table.volatile_set(i, rev_forwarder)
          new_table.volatile_set(i + old_table_size, rev_forwarder)
        else
          redo unless split_old_bin(table, new_table, i, node, node_hash, forwarder)
        end

        if bin > 0
          i = (bin -= 1)
        elsif locked_indexes && !locked_indexes.empty?
          bin = -1
          i = locked_indexes.pop
          locked_arr_idx = locked_indexes.size - 1
        else
          return new_table
        end
      end
    end

    def lock_and_clean_up_reverse_forwarders(old_table, old_table_size, new_table, i, forwarder)
      # transiently use a locked forwarding node
      locked_forwarder = Node.new(moved_locked_hash = MOVED | LOCKED, new_table, NULL)
      if old_table.cas(i, nil, locked_forwarder)
        new_table.volatile_set(i, nil) # kill the potential reverse forwarders
        new_table.volatile_set(i + old_table_size, nil) # kill the potential reverse forwarders
        old_table.volatile_set(i, forwarder)
        locked_forwarder.unlock_via_hash(moved_locked_hash, MOVED)
        true
      end
    end

    # Splits a normal bin with list headed by e into lo and hi parts; installs in given table.
    def split_old_bin(table, new_table, i, node, node_hash, forwarder)
      table.try_lock_via_hash(i, node, node_hash) do
        split_bin(new_table, i, node, node_hash)
        table.volatile_set(i, forwarder)
      end
    end

    def split_bin(new_table, i, node, node_hash)
      bit          = new_table.size >> 1 # bit to split on
      run_bit      = node_hash & bit
      last_run     = nil
      low          = nil
      high         = nil
      current_node = node
      # this optimises for the lowest amount of volatile writes and objects created
      while current_node = current_node.next
        unless (b = current_node.hash & bit) == run_bit
          run_bit  = b
          last_run = current_node
        end
      end
      if run_bit == 0
        low = last_run
      else
        high = last_run
      end
      current_node = node
      until current_node == last_run
        pure_hash = current_node.pure_hash
        if (pure_hash & bit) == 0
          low = Node.new(pure_hash, current_node.key, current_node.value, low)
        else
          high = Node.new(pure_hash, current_node.key, current_node.value, high)
        end
        current_node = current_node.next
      end
      new_table.volatile_set(i, low)
      new_table.volatile_set(i + bit, high)
    end

    def increment_size
      @counter.increment
    end

    def decrement_size(by = 1)
      @counter.add(-by)
    end
  end
end