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How to compare two generic dictionaries

Zilele acestea mi-a fost pusa urmatoarea intrebare:
Intr-un unit test care este cea mai buna modalitate de a compara doua colectii?

Daca va aduceti aminte, in urma cu cateva saptamani am discutat despre CollectionAssert. Aceasta contine o metoda denumita AreEquivalent, care verifica daca doua colectii sunt echivalente. Testul o sa treaca de acest assert daca:
  • numarul de elemente este egal
  • cele doua colectii contin aceleasi elemente( indiferent de ordinea in care apar)
Doua elemente din colectie sunt egale nu daca puncteaza spre acelasi element ci daca sunt egale prin continut( se apeleaza metoda Equals, pentru a face aceasta verificare). In cazul nostru, metoda AreEquivalent o sa itereze prin intreaga colectie de element KeyValuePair pe care un dictionar le contine.
Dictionary<string,string> collection1=new Dictionary<string, string>();
Dictionary<string, string> collection2 = new Dictionary<string, string>();
...
CollectionAssert.AreEquivalent(collection1,collection2);
In cazul in care avem nevoie in cod sa comparam doua dictionare putem sa face in felul urmator:
collection1
.OrderBy(i => i)
.SequenceEqual(collection2.OrderBy(i => i));
Varianta de mai sus este O(n*log(n)). In cazul in care vreti o solutie in O(n), puteti sa incercati o implementare asemanatoare cu aceasta:
public class CollectionComparer<T> : IEqualityComparer<IEnumerable<T>>
where T : class
{
public bool Equals(IEnumerable<T> collection1, IEnumerable<T> collection2)
{
if ((collection1 == null && collection2!=null)
|| (collection2 == null && collection1!=null))
{
return false;
}

if (ReferenceEquals(collection1, collection2))
{
return true;
}

int collection1Count = collection1.Count();
if (collection1Count != collection2.Count())
{
return false;
}

return collection1Count == 0
|| !AreCollectinDifferent(collection1, collection2);
}

private static bool AreCollectinDifferent(IEnumerable<T> collection1, IEnumerable<T> collection2)
{
int collection1Count;
int collection2Count;

var firstElementCounts = GetElementCounts(collection1, out collection1Count);
var secondElementCounts = GetElementCounts(collection2, out collection2Count);

if (collection1Count != collection2Count)
{
return true;
}

foreach (KeyValuePair<T,int> keyValuePair in firstElementCounts)
{
collection1Count = keyValuePair.Value;
secondElementCounts.TryGetValue(keyValuePair.Key, out collection2Count);

if (collection1Count != collection2Count)
{
return true;
}
}

return false;
}

private static Dictionary<T, int> GetElementCounts(IEnumerable<T> enumerable,
out int nullCount)
{
var dictionary = new Dictionary<T, int>();
nullCount = 0;
int value;

foreach (T element in enumerable)
{
if (element == null)
{
nullCount++;
continue;
}


dictionary.TryGetValue(element, out value);
dictionary[element] = ++value;
}

return dictionary;
}

public int GetHashCode(IEnumerable<T> enumerable)
{
return enumerable
.OrderBy(x => x)
.Aggregate(
11,
(current, val) => current*13 + val.GetHashCode());
}
}

Enjoy!

Comments

  1. 11 si 13 ce reprezinta? :)

    Interesant ca si implementare- altfel, daca timpul m-ar fi presat as fi raspuns la intrebare: foloseste ceva gata facut, precum http://comparenetobjects.codeplex.com/ (l-am folosit in unit teste, si merge brici, exceptand niste situatii destul de particulare)

    ReplyDelete
  2. Am implementat o functie de hash. Ma asteptam ca cineva sa se ia de functie de hash. Nu e cel mai bun algoritm acolo oricum.
    + o bere Tudor pe cand te intorci :D

    ReplyDelete
  3. Imi dau seama ca e o functie de hash, eram curios de ce 13 si nu.. 15.. :)

    ReplyDelete

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