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Merged
mperry merged 4 commits into from
Jan 26, 2015
Merged

Add union to TreeMap #83

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e9c7a1c
Add setAll() method to TreeMap, to combine two TreeMap instances.
dobesv Jan 15, 2015
79fbe5e
Merge branch 'master' of https://github.com/functionaljava/functional…
dobesv Jan 26, 2015
164e0b7
Rename TreeMap.setAll() to TreeMap.union(), and make it left-biased
dobesv Jan 16, 2015
8133493
Add a union() method that takes a list of pairs in addition to the on…
dobesv Jan 26, 2015
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33 changes: 33 additions & 0 deletions core/src/main/java/fj/data/TreeMap.java
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Original file line number Diff line number Diff line change
Expand Up @@ -305,4 +305,37 @@ public <W> TreeMap<K, W> map(final F<V, W> f) {
return new TreeMap<K, W>(tree.map(TreeMap.<K, Option<W>>ord(o), g));
}

/**
* The expression <code>t1.union(t2)</code> takes the left-biased union of <code>t1</code>
* and <code>t2</code>. It prefers <code>t1</code> when duplicate keys are encountered.
*
* @param t2 The other tree we wish to combine with this one
* @return The combined TreeMap
*/
public TreeMap<K, V> union(TreeMap<K, V> t2) {
// TODO This could be implemented more efficiently using "hedge union"
TreeMap<K, V> result = t2;
for(P2<K,V> p : this) {
result = result.set(p._1(), p._2());
}
return result;
}

@mperry mperry Jan 15, 2015

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Lets call this union. This is the defition of union in Haskell, https://downloads.haskell.org/~ghc/6.12.2/docs/html/libraries/containers-0.3.0.0/Data-Map.html#v%3Aunion:

union :: Ord k => Map k a -> Map k a -> Map k a 
O(n+m). The expression (union t1 t2) takes the left-biased union of t1 and t2. It prefers t1 when duplicate keys are encountered, i.e. (union == unionWith const). The implementation uses the efficient hedge-union algorithm. Hedge-union is more efficient on (bigset `union` smallset).

union (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (7, "C")]) == fromList [(3, "b"), (5, "a"), (7, "C")]

Please review and let me know when this is updated. Please update the javadoc, return type (it should return TreeMap<K, V>) and method body.

There is an opportunity to implement some of the Haskell combine methods: unionWith, unionWithKey, unions and unionsWith.

There are also lots of other methods we could add from the Haskell doc above.

@dobesv dobesv Jan 15, 2015

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Should it left biased in that case?
On Jan 15, 2015 3:16 AM, "Mark Perry" notifications@github.com wrote:

In core/src/main/java/fj/data/TreeMap.java
#83 (diff)
:

  • /**
  • * Extend this TreeMap to include all mapping from another TreeMap. Where a
  • * mapping exists in both this TreeMap and the other one, the value from the
  • * other one will replace the value in this one.
  • * @param other
  • * @return
  • */
  • public Object setAll(TreeMap<K, V> other) {
  •   TreeMap<K, V> result = this;

  •   for(P2<K,V> p : other) {

  •       result = result.set(p._1(), p._2());

  •   }

  •   return result;
  • }

Lets call this union. This is the defition of union in Haskell,
https://downloads.haskell.org/~ghc/6.12.2/docs/html/libraries/containers-0.3.0.0/Data-Map.html#v%3Aunion
https://downloads.haskell.org/%7Eghc/6.12.2/docs/html/libraries/containers-0.3.0.0/Data-Map.html#v%3Aunion
:

union :: Ord k => Map k a -> Map k a -> Map k a
O(n+m). The expression (union t1 t2) takes the left-biased union of t1 and t2. It prefers t1 when duplicate keys are encountered, i.e. (union == unionWith const). The implementation uses the efficient hedge-union algorithm. Hedge-union is more efficient on (bigset union smallset).

union (fromList [(5, "a"), (3, "b")]) (fromList [(5, "A"), (7, "C")]) == fromList [(3, "b"), (5, "a"), (7, "C")]

Please review and let me know when this is updated. Please update the
javadoc, return type (it should return TreeMap) and method body.

There is an opportunity to implement some of the Haskell combine methods:
unionWith, unionWithKey, unions and unionsWith.

There are also lots of other methods we could add from the Haskell doc
above.

—
Reply to this email directly or view it on GitHub
https://github.com/functionaljava/functionaljava/pull/83/files#r23003389
.

@mperry mperry Jan 15, 2015

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Yes, only add the key value pair if the key does not exist in the first map.

@mperry mperry Jan 26, 2015

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For later reference, see "Implementing Sets Efficiently in a Functional Language" by Stephen Adams as mentioned here, purescript-deprecated/purescript-maps#1.

A current link to the paper is:

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=77314EAFB757E19867DA32911F66400C?doi=10.1.1.48.1134&rep=rep1&type=pdf

Implementation in Haskell:

https://downloads.haskell.org/~ghc/7.6.2/docs/html/libraries/containers-0.5.0.0/src/Data-Set-Base.html#hedgeUnion

/**
* The expression <code>t1.union(t2)</code> takes the left-biased union of <code>t1</code>
* and <code>t2</code>. It prefers <code>t1</code> when duplicate keys are encountered.
*
* @param t2 The other list/set of pairs we wish to combine with this one
* @return The combined TreeMap
*/
public TreeMap<K, V> union(Iterable<P2<K, V>> t2) {
TreeMap<K, V> result = this;
for(P2<K,V> p : t2) {
if(!this.contains(p._1())) {
result = result.set(p._1(), p._2());
}
}
return result;
}

}