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Commit 69d7124

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#36 Add ability to change the number of hash functions
1 parent 8b6b802 commit 69d7124

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2 files changed

+50
-15
lines changed

2 files changed

+50
-15
lines changed
Lines changed: 23 additions & 15 deletions
Original file line numberDiff line numberDiff line change
@@ -1,5 +1,6 @@
11
using System;
22
using System.Collections;
3+
using System.Collections.Generic;
34

45
namespace Advanced.Algorithms.DataStructures
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{
@@ -10,12 +11,20 @@ public class BloomFilter<T>
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{
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private readonly BitArray filter;
1213

14+
private readonly int numberOfHashFunctions;
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/// <summary>
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/// Higher the size lower the collision and
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/// failure probablity.
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/// </summary>
17-
public BloomFilter(int size)
20+
public BloomFilter(int size,intnumberOfHashFunctions=2)
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{
22+
if (size <= numberOfHashFunctions)
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{
24+
throw new ArgumentException("size cannot be less than or equal to numberOfHashFunctions.");
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}
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this.numberOfHashFunctions = numberOfHashFunctions;
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filter = new BitArray(size);
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}
2130

@@ -24,13 +33,9 @@ public BloomFilter(int size)
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/// </summary>
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public void AddKey(T key)
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{
27-
var hashCode = key.GetHashCode();
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29-
//set 8 consecutive bits (a byte)
30-
for (var i = 0; i < 8; i++)
36+
foreach (var hash in getHashes(key))
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{
32-
var index = Math.Abs(hashCode + i) % filter.Length;
33-
filter[index] = true;
38+
filter[hash % filter.Length] = true;
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}
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}
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@@ -39,21 +44,24 @@ public void AddKey(T key)
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/// </summary>
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public bool KeyExists(T key)
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{
42-
var hashCode = key.GetHashCode();
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//set 8 consecutive bits (a byte)
46-
for (var i = 0; i < 8; i++)
47+
foreach (var hash in getHashes(key))
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{
48-
var index = Math.Abs(hashCode + i) % filter.Length;
49-
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if (filter[index] == false)
49+
if (filter[hash % filter.Length] == false)
5150
{
5251
return false;
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}
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}
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return true;
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}
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58+
private IEnumerable<int> getHashes(T key)
59+
{
60+
for (var i = 1; i <= numberOfHashFunctions; i++)
61+
{
62+
var obj = new { Key = key, InitialValue = i };
63+
yield return Math.Abs(obj.GetHashCode());
64+
}
65+
}
5866
}
5967
}

‎tests/Advanced.Algorithms.Tests/DataStructures/Set/BloomFilter_Tests.cs

Lines changed: 27 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -18,5 +18,32 @@ public void BloomFilter_Smoke_Test()
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Assert.IsTrue(filter.KeyExists("cat"));
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Assert.IsFalse(filter.KeyExists("bat"));
2020
}
21+
22+
[TestMethod]
23+
public void BloomFilter_Accuracy_Test()
24+
{
25+
var bloomFilter = new BloomFilter<string>(10);
26+
27+
bloomFilter.AddKey("foo");
28+
bloomFilter.AddKey("bar");
29+
bloomFilter.AddKey("apple");
30+
bloomFilter.AddKey("orange");
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bloomFilter.AddKey("banana");
32+
33+
Assert.IsTrue(bloomFilter.KeyExists("bar"));
34+
Assert.IsFalse(bloomFilter.KeyExists("ba111r"));
35+
36+
Assert.IsTrue(bloomFilter.KeyExists("banana"));
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Assert.IsFalse(bloomFilter.KeyExists("dfs11j"));
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39+
Assert.IsTrue(bloomFilter.KeyExists("foo"));
40+
Assert.IsFalse(bloomFilter.KeyExists("1foo"));
41+
42+
Assert.IsTrue(bloomFilter.KeyExists("apple"));
43+
Assert.IsFalse(bloomFilter.KeyExists("applefoo"));
44+
45+
Assert.IsTrue(bloomFilter.KeyExists("orange"));
46+
Assert.IsFalse(bloomFilter.KeyExists("orangew"));
47+
}
2148
}
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}

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