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com.tangosol.util
Class SafeSortedMap

java.lang.Object
java.util.AbstractMap
com.tangosol.util.SafeSortedMap

All Implemented Interfaces:

java.util.Map, java.util.SortedMap

public class SafeSortedMap

extends java.util.AbstractMap

implements java.util.SortedMap

SafeSortedMap is an implementation of SortedMap based on a skip-list that is structurally thread-safe. SafeSortedMap uses lockless (CAS-based) algorithms so all operations are performed without synchronization. In the presence of concurrent updates, SafeSortedMap iterators will reflect some state of the map, but will not throw ConcurrentModificationException or other unexpected exceptions.

Note that the ordering maintained by a sorted map (whether or not an explicit comparator is provided) must be consistent with equals if this sorted map is to correctly implement the Map interface. (See Comparable or Comparator for a precise definition of consistent with equals.) This is so because the Map interface is defined in terms of the equals operation, but a map performs all key comparisons using its compareTo (or compare) method, so two keys that are deemed equal by this method are, from the standpoint of the sorted map, equal. The behavior of a sorted map is well-defined even if its ordering is inconsistent with equals; it just fails to obey the general contract of the Map interface.

The structure of the SafeSortedMap is maintained in a 2-dimensional lattice:

  Top
   |
  I3 -------------------------------> S5 ------------->
   |                                   |
  I2 -------------> S2 -------------> S5 ------------->
   |                 |                 |
  I1 -------> S1 -> S2 -------> S4 -> S5 -------> S7 ->
   |           |     |           |     |           |
  E  -> E0 -> E1 -> E2 -> E3 -> E4 -> E5 -> E6 -> E7 ->
 

Searching for a key involves traversing, starting from the top index node, as far to the right at each index level, then down. For example, the search path for the element E4 in the above example would be:

1. compare E4 with S5 on index level 3; move down to level 2
2. compare E4 with S2 on index level 2; move right
3. compare E4 with S5 on index level 2; move down to level 1
4. compare E4 with S4 on index level 1; move right
5. move down

The search path for the element E7 in the above example would be:

1. compare E7 with S5 on index level 3; move right
2. move down to level 2
3. move down to level 1
4. compare E7 with S7 on index level 1; move right
5. move down

Removing a key involves inserting a temporary marker-entry while the entry pointers are fixed up. The marker node allows uncontended read threads to traverse the safely. Additionally, removing a key causes any index nodes for that key to be removed as well.

In the above example, the steps to remove entry E3 would be:

1. find LT search-path of Top->I3->S2->S2->E2
2. create new marker node D3
3. set D3 -> E4
4. set E3 -> D3
5. set E2 -> E3

The steps to remove E4 (indexed to level 1) would be:

1. find LT search-path of Top->I3->S2->S2->E3
2. create new marker node D4
3. set D4 -> E5
4. set E4 -> D4
5. set E3 -> E5
6. move up to level 1
7. set D4' -> S5
8. set S4 -> D4'
9. set S2 -> S5

Inserting a key happens in 'bottom-up' fashion, with the entry node being inserted first, (possibly) followed by index nodes of increasing level. Inserts are fully synchronized on the map. When inserting a new entry, an index level randomly computed such that, the probability that a given insertion will be indexed to level n is approximately given by: (1/k)ⁿ where k is the average span of each index level. In the above example, the steps to insert entry E3.5 at index-level 1 would be:

1. find LT search-path of Top->I3->S2->S2->E2
2. create new entry node E3.5
3. set E3.5 -> E4
4. set E2 -> E3.5
5. move up to level 1
6. create new entry node S3.5
7. set S3.5 -> S4
8. set S2 -> S3.5

Since:

Coherence 3.5

Author:

rhl 2009.03.31

See Also:

Bill Pugh's original paper

Nested Class Summary

protected static class	SafeSortedMap.BaseEntryNode BaseEntryNode is a synthetic EntryNode that serves as the "head" of the base entry list.
protected static class	SafeSortedMap.EntryNode EntryNode represents a key-value mapping in this map.
protected class	SafeSortedMap.IndexNode IndexNode represents the start node in the map's lattice representation at a given index-level.
protected static class	SafeSortedMap.SkipNode SkipNode is an entry or index node in the lattice for a SafeSortedMap's representation.
class	SafeSortedMap.Split Split encapsulates the headMap and tailMap of the SafeSortedMap at a given split-key and could be used as an optimization facility when both head and tail portions of the SortedMap are needed.
protected class	SafeSortedMap.ViewMap ViewMap provides a SortedMap view over a subset of the SafeSortedMap.

Nested classes/interfaces inherited from class java.util.AbstractMap

java.util.AbstractMap.SimpleEntry, java.util.AbstractMap.SimpleImmutableEntry

Nested classes/interfaces inherited from interface java.util.Map

java.util.Map.Entry

Field Summary

protected static java.lang.Object	BASE_VALUE Placeholder for a non-existent (deleted) value.
protected static int	DEFAULT_MAX_LEVEL The default limit on the index-level.
protected static int	DEFAULT_SPAN The default span value.
protected float[]	m_aflProbabilityThresholds The array of pre-computed probability thresholds for each level.
protected static java.util.concurrent.atomic.AtomicReferenceFieldUpdater	m_atomicUpdaterTopNode AtomicUpdater for the casTopNode operation.
protected java.util.Comparator	m_comparator The comparator used to sort this map.
protected SafeSortedMap.ViewMap	m_mapView The default ViewMap (over the entire map's range).
protected int	m_nLevelMax The maximum number of index levels to use for this map.
protected SafeSortedMap.IndexNode	m_nodeTop The top index node.
protected int	m_nSpan The span of the map.
protected java.util.Random	m_random The random-number generator for the map.
protected static java.lang.Object	NO_VALUE Placeholder for a non-existent (deleted) value.
protected static int	SEARCH_EQ Search mode for equal-to.
protected static int	SEARCH_GT Search mode for strictly greater-than.
protected static int	SEARCH_GTEQ Search mode for greater-than or equal.
protected static int	SEARCH_LT Search mode for strictly less-than.
protected static int	SEARCH_LTEQ Search mode for less-than or equal.

Constructor Summary

SafeSortedMap()
Construct a new SafeSortedMap using the natural ordering of the Comparable keys in this map.

SafeSortedMap(java.util.Comparator comparator)
Construct a new SafeSortedMap with the specified Comparator.

SafeSortedMap(java.util.Comparator comparator, int nSpan, int nLevelMax)
Construct a new SafeSortedMap with the specified parameters.

Method Summary

protected int	calculateRandomLevel() Randomly generate a level value 0 <= L <= MAX_LEVEL, in such a way that the probability p(l) of generating level l is equal to p(l)=(1/span)^(l+1) and p(0)=1-(sum of p(l) over l=1..MAX_LEVEL).
protected boolean	casTopNode(SafeSortedMap.IndexNode nodeTopAssume, SafeSortedMap.IndexNode nodeTopNew) Atomically set specified IndexNode to the top node iff the current top node is the expected top node.
void	clear()
java.util.Comparator	comparator()
protected static float[]	computeProbabilityThresholds(int nLevelMax, int nSpan) Compute and return an array, indexed by level, of the probability thresholds used to determine what level an entry should be indexed to.
boolean	containsKey(java.lang.Object oKey)
java.lang.String	dump() Return a human-readable description of this map's internal state.
java.util.Set	entrySet()
protected SafeSortedMap.EntryNode	findNearest(SafeSortedMap.IndexNode nodeTop, java.lang.Object oKey, int nMode, boolean fFixLinks) Return the SkipNode closest to the specified key, or null.
protected SafeSortedMap.EntryNode	findNearestIndexedEntry(SafeSortedMap.IndexNode nodeTop, java.lang.Object oKey, int nMode) Return the EntryNode nearest to the specified key according to the specified search mode, or the synthetic base node if there is none.
protected SafeSortedMap.EntryNode	findPredecessor(SafeSortedMap.IndexNode nodeTop, java.lang.Object oKey) Return an EntryNode that compares strictly less than the specified key, or the synthetic base node if there is none.
java.lang.Object	firstKey()
protected SafeSortedMap.EntryNode	firstNode() Return the first valid EntryNode, or null if there are none.
java.lang.Object	get(java.lang.Object oKey)
protected SafeSortedMap.BaseEntryNode	getBaseNode() Return the base entry node.
java.util.Map.Entry	getEntry(java.lang.Object oKey) Locate a Map.Entry in this map based on its key.
protected int	getMaxLevel() Return the maximum index-level that this map is allowed to reach.
protected float[]	getProbabilityThresholds() Return the precomputed array of probability thresholds used by this map to determine what index levels to build.
protected java.util.Random	getRandom() Return the random number generator for this map.
protected int	getSpan() Return the span of this map.
protected SafeSortedMap.IndexNode	getTopNode() Return the top index node in the map.
protected SafeSortedMap.ViewMap	getViewMap() Return a view of the entire map.
java.util.SortedMap	headMap(java.lang.Object toKey)
protected void	initialize() Initialize the index nodes.
protected void	insertIndex(SafeSortedMap.IndexNode nodeTop, SafeSortedMap.EntryNode nodeNew, int nLevelThis) Insert a index nodes for the specified newly inserted EntryNode, up to a random index-level.
java.lang.Object	lastKey()
protected SafeSortedMap.EntryNode	lastNode() Return the last valid EntryNode, or null if there are none.
java.lang.Object	put(java.lang.Object oKey, java.lang.Object oValue)
java.lang.Object	remove(java.lang.Object oKey)
protected void	setTopNode(SafeSortedMap.IndexNode nodeTop) Set the top index node in the map.
int	size()
SafeSortedMap.Split	split(java.lang.Object oKey) Return a SafeSortedMap.Split of this map at the specified key.
java.util.SortedMap	subMap(java.lang.Object fromKey, java.lang.Object toKey)
java.util.SortedMap	tailMap(java.lang.Object fromKey)

Methods inherited from class java.util.AbstractMap

clone, containsValue, equals, hashCode, isEmpty, keySet, putAll, toString, values

Methods inherited from interface java.util.SortedMap

keySet, values

Methods inherited from interface java.util.Map

containsValue, equals, hashCode, isEmpty, putAll

Field Detail

DEFAULT_SPAN

protected static final int DEFAULT_SPAN

The default span value.

See Also:

Constant Field Values

DEFAULT_MAX_LEVEL

protected static final int DEFAULT_MAX_LEVEL

The default limit on the index-level.

See Also:

Constant Field Values

SEARCH_EQ

protected static final int SEARCH_EQ

Search mode for equal-to.

See Also:

Constant Field Values

SEARCH_LT

protected static final int SEARCH_LT

Search mode for strictly less-than.

See Also:

Constant Field Values

SEARCH_GT

protected static final int SEARCH_GT

Search mode for strictly greater-than.

See Also:

Constant Field Values

SEARCH_LTEQ

protected static final int SEARCH_LTEQ

Search mode for less-than or equal.

See Also:

Constant Field Values

SEARCH_GTEQ

protected static final int SEARCH_GTEQ

Search mode for greater-than or equal.

See Also:

Constant Field Values

NO_VALUE

protected static final java.lang.Object NO_VALUE

Placeholder for a non-existent (deleted) value.

BASE_VALUE

protected static final java.lang.Object BASE_VALUE

Placeholder for a non-existent (deleted) value.

m_atomicUpdaterTopNode

protected static final java.util.concurrent.atomic.AtomicReferenceFieldUpdater m_atomicUpdaterTopNode

AtomicUpdater for the casTopNode operation.

m_comparator

protected final java.util.Comparator m_comparator

The comparator used to sort this map.

m_nLevelMax

protected final int m_nLevelMax

The maximum number of index levels to use for this map.

m_aflProbabilityThresholds

protected final float[] m_aflProbabilityThresholds

The array of pre-computed probability thresholds for each level.

m_nSpan

protected int m_nSpan

The span of the map.

m_mapView

protected final SafeSortedMap.ViewMap m_mapView

The default ViewMap (over the entire map's range).

m_nodeTop

protected volatile SafeSortedMap.IndexNode m_nodeTop

The top index node. This is declared volatile so read operations can be unsynchronized.

m_random

protected java.util.Random m_random

The random-number generator for the map.

Constructor Detail

SafeSortedMap

public SafeSortedMap()

Construct a new SafeSortedMap using the natural ordering of the Comparable keys in this map.

SafeSortedMap

public SafeSortedMap(java.util.Comparator comparator)

Construct a new SafeSortedMap with the specified Comparator.

Parameters:

comparator - the comparator used to sort this map

SafeSortedMap

public SafeSortedMap(java.util.Comparator comparator,
                     int nSpan,
                     int nLevelMax)

Construct a new SafeSortedMap with the specified parameters.

Parameters:

comparator - the comparator used to sort this map, or null for the entries' natural ordering

nSpan - the average span to use for each index-level

nLevelMax - the maximum index-level to use for this map

Method Detail

size

public int size()

Specified by:

size in interface java.util.Map

Overrides:

size in class java.util.AbstractMap

containsKey

public boolean containsKey(java.lang.Object oKey)

Specified by:

containsKey in interface java.util.Map

Overrides:

containsKey in class java.util.AbstractMap

entrySet

public java.util.Set entrySet()

Specified by:

entrySet in interface java.util.Map

Specified by:

entrySet in interface java.util.SortedMap

Specified by:

entrySet in class java.util.AbstractMap

put

public java.lang.Object put(java.lang.Object oKey,
                            java.lang.Object oValue)

Specified by:

put in interface java.util.Map

Overrides:

put in class java.util.AbstractMap

get

public java.lang.Object get(java.lang.Object oKey)

Specified by:

get in interface java.util.Map

Overrides:

get in class java.util.AbstractMap

getEntry

public java.util.Map.Entry getEntry(java.lang.Object oKey)

Locate a Map.Entry in this map based on its key.

Note: the behaviour of {#setValue} on the returned Entry is undefined in the presence of concurrent modifications

Parameters:

oKey - the key to return an Entry for

Returns:

an Entry corresponding to the specified key, or null if none exists

remove

public java.lang.Object remove(java.lang.Object oKey)

Specified by:

remove in interface java.util.Map

Overrides:

remove in class java.util.AbstractMap

clear

public void clear()

Specified by:

clear in interface java.util.Map

Overrides:

clear in class java.util.AbstractMap

comparator

public java.util.Comparator comparator()

Specified by:

comparator in interface java.util.SortedMap

subMap

public java.util.SortedMap subMap(java.lang.Object fromKey,
                                  java.lang.Object toKey)

Specified by:

subMap in interface java.util.SortedMap

headMap

public java.util.SortedMap headMap(java.lang.Object toKey)

Specified by:

headMap in interface java.util.SortedMap

tailMap

public java.util.SortedMap tailMap(java.lang.Object fromKey)

Specified by:

tailMap in interface java.util.SortedMap

firstKey

public java.lang.Object firstKey()

Specified by:

firstKey in interface java.util.SortedMap

lastKey

public java.lang.Object lastKey()

Specified by:

lastKey in interface java.util.SortedMap

getViewMap

protected SafeSortedMap.ViewMap getViewMap()

Return a view of the entire map.

Returns:

a view of the entire map

getProbabilityThresholds

protected float[] getProbabilityThresholds()

Return the precomputed array of probability thresholds used by this map to determine what index levels to build.

Returns:

the precomputed array of probability thresholds

See Also:

calculateRandomLevel()

getRandom

protected java.util.Random getRandom()

Return the random number generator for this map.

Returns:

the random number generator for this map

getMaxLevel

protected int getMaxLevel()

Return the maximum index-level that this map is allowed to reach.

Returns:

the maximum index-level that this map is allowed to reach

getSpan

protected int getSpan()

Return the span of this map.

Returns:

the span of this map

initialize

protected void initialize()

Initialize the index nodes.

getTopNode

protected SafeSortedMap.IndexNode getTopNode()

Return the top index node in the map.

Returns:

the top index node in the map

setTopNode

protected void setTopNode(SafeSortedMap.IndexNode nodeTop)

Set the top index node in the map.

Parameters:

nodeTop - the new top index node

casTopNode

protected boolean casTopNode(SafeSortedMap.IndexNode nodeTopAssume,
                             SafeSortedMap.IndexNode nodeTopNew)

Atomically set specified IndexNode to the top node iff the current top node is the expected top node.

Parameters:

nodeTopAssume - the IndexNode assumed to be the current top node

nodeTopNew - the new top node

Returns:

true iff the top node is successfully updated

getBaseNode

protected SafeSortedMap.BaseEntryNode getBaseNode()

Return the base entry node.

Returns:

the base entry node

firstNode

protected SafeSortedMap.EntryNode firstNode()

Return the first valid EntryNode, or null if there are none.

Returns:

the first EntryNode or null if empty

lastNode

protected SafeSortedMap.EntryNode lastNode()

Return the last valid EntryNode, or null if there are none.

Returns:

the last valid EntryNode or null if empty

insertIndex

protected void insertIndex(SafeSortedMap.IndexNode nodeTop,
                           SafeSortedMap.EntryNode nodeNew,
                           int nLevelThis)

Insert a index nodes for the specified newly inserted EntryNode, up to a random index-level. Return true if the key was inserted, or false otherwise (if an intervening insert was made for the same key).

Parameters:

nodeTop - the top index node

nodeNew - the newly inserted EntryNode

nLevelThis - the level at which to index the specified EntryNode

findNearest

protected SafeSortedMap.EntryNode findNearest(SafeSortedMap.IndexNode nodeTop,
                                              java.lang.Object oKey,
                                              int nMode,
                                              boolean fFixLinks)

Return the SkipNode closest to the specified key, or null.

Parameters:

nodeTop - the top index node

oKey - the key to search for

nMode - one of the SEARCH_* constants

fFixLinks - true iff deletion links must be fixed

Returns:

the SkipNode closest to the specified key, or null

findPredecessor

protected SafeSortedMap.EntryNode findPredecessor(SafeSortedMap.IndexNode nodeTop,
                                                  java.lang.Object oKey)

Return an EntryNode that compares strictly less than the specified key, or the synthetic base node if there is none.

Note: this method is merely guaranteed to return some EntryNode that is less than the specified node, not necessarily the node that is immediately less than the specified node

Parameters:

nodeTop - the top index node

oKey - the key to find a predecessor for

Returns:

an EntryNode that is strictly less than the specified key

findNearestIndexedEntry

protected SafeSortedMap.EntryNode findNearestIndexedEntry(SafeSortedMap.IndexNode nodeTop,
                                                          java.lang.Object oKey,
                                                          int nMode)

Return the EntryNode nearest to the specified key according to the specified search mode, or the synthetic base node if there is none.

Parameters:

nodeTop - the top index node

oKey - the key to find an indexed EntryNode for

nMode - SEARCH_LT or SEARCH_LTEQ)

Returns:

an indexed EntryNode that is nearest to the specified key according to the specified search mode

computeProbabilityThresholds

protected static float[] computeProbabilityThresholds(int nLevelMax,
                                                      int nSpan)

Compute and return an array, indexed by level, of the probability thresholds used to determine what level an entry should be indexed to.

Threshold values are in the range [0, 1). A randomly chosen value v in that range corresponds to a level l iff the following condition holds: threshold[l-1]<i/> &lteq; v < threshold[l-1]<i/>

Parameters:

nLevelMax - the maximum number of levels to compute thresholds for

nSpan - the span of this map

Returns:

an array containing the distribution probabilities

calculateRandomLevel

protected int calculateRandomLevel()

Randomly generate a level value 0 <= L <= MAX_LEVEL, in such a way that the probability p(l) of generating level l is equal to p(l)=(1/span)^(l+1) and p(0)=1-(sum of p(l) over l=1..MAX_LEVEL).

For example, the probabilities (with span=2, and weight of 2) of returning the following levels are: 0 (no-index) - 3/4 1 - 1/8 2 - 1/16 3 - 1/32 ...

Returns:

a random level number between 0 and MAX_LEVEL

dump

public java.lang.String dump()

Return a human-readable description of this map's internal state.

Returns:

a human-readable description of this map's internal state

split

public SafeSortedMap.Split split(java.lang.Object oKey)

Return a SafeSortedMap.Split of this map at the specified key.

Parameters:

oKey - the key at which to split the map

Returns:

a Split of this map at the specified key

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