Count-Min Sketch Data Structure with Implementation

The Count-Min Sketch is a probabilistic data structure and is defined as a simple technique to summarize large amounts of frequency data. Count-min sketch algorithm talks about keeping track of the count of things. i.e, How many times an element is present in the set.

What is Count-Min Sketch?

Count-min sketch approach was proposed by Graham Cormode and S. Muthukrishnan. in the paper approximating data with the count-min sketch published in 2011/12. Count-min sketch is used to count the frequency of the events on the streaming data. Like the Bloom filter, Count-min sketch algorithm also works with hash codes. It uses multiple hash functions to map these frequencies on to the matrix (Consider sketch here a two dimensional array or matrix).

Need for Count-Min Sketch

Since Count-Min Sketch is used to find the frequency of an element, one might think if there is actually a need for such data structure! The answer is Yes. Let us see with the help of an example.

Let us try to solve this frequency estimation problem using a MultiSet Data Structure

Trying MultiSet as an alternative to Count-min sketch 

Let’s try to implement this data structure using MultiSet with the below source code and try to find out the issues with this approach. 

Output: 

Understanding the problem of using MultiSet instead of Count-Min Sketch

Now let’s look at the time and space consumed with this type of approach. 

Let’s have a look at the memory (space) consumed: 

We can easily understand that as data grows, the above approach is consuming a lot of memory and time to process the data.

This can be optimised if we use the Count-Min Sketch.

How does Count-Min Sketch work? 

Let’s look at the below example step by step. 

Creating a Count-Min Sketch using Matrix

• Consider the below 2D array with 4 rows and 16 columns, also the number of rows is equal to the number of hash functions. That means we are taking four hash functions for our example. Initialize/mark each cell in the matrix with zero. 

Note: The more accurate result you want, the more hash function to be used. 

Now let’s add some elements (Input) to the Count-Min Sketch.

To do so we have to pass that element with all four hash functions which will result as follows. 

• Input 1: 192.170.0.1

Passing the input through Hash Functions:

• hashFunction1(192.170.0.1): 1
• hashFunction2(192.170.0.1): 6
• hashFunction3(192.170.0.1): 3
• hashFunction4(192.170.0.1): 1

Now visit the indexes retrieved above by all four hash functions and mark them as 1. 

• Input 2: 75.245.10.1  

Passing the input through Hash Functions:

• hashFunction1(75.245.10.1): 1
• hashFunction2(75.245.10.1): 2
• hashFunction3(75.245.10.1): 4
• hashFunction4(75.245.10.1): 6

Now visit the indexes retrieved above by all four hash functions and mark them as 1. 

Now, take these indexes and visit the matrix, if the given index has already been marked as 1. This is called collision, i.e., the index of that row was already marked by some previous inputs.

In this case, just increment the index value by 1.

In our case, since we have already marked index 1 of row 1 i.e., hashFunction1() as 1 by previous input, so this time it will be incremented by 1, and now this cell entry will be 2, but for the rest of the index of rest rows, it will be 0, since there was no collision. 

• Input 3: 10.125.22.20  

Passing the input through Hash Functions:

• hashFunction1(10.125.22.20): 3
• hashFunction2(10.125.22.20): 4
• hashFunction3(10.125.22.20): 1
• hashFunction4(10.125.22.20): 6

Lets, represent it on matrix, do remember to increment the count by 1 if already some entry exist. 

 

• Input 4: 192.170.0.1  

Passing the input through Hash Functions:

• hashFunction1(192.170.0.1): 1
• hashFunction2(192.170.0.1): 6
• hashFunction3(192.170.0.1): 3
• hashFunction4(192.170.0.1): 1

Lets, represent it on matrix, do remember to increment the count by 1 if already some entry exist. 

Testing Count-Min Sketch data structure against Test cases:

Now let’s test some element and check how many time are they present. 

• Input 1: 192.170.0.1

Pass above input to all four hash functions, and take the index numbers generated by hash functions. 

• hashFunction1(192.170.0.1): 1
• hashFunction2(192.170.0.1): 6
• hashFunction3(192.170.0.1): 3
• hashFunction4(192.170.0.1): 1

Now visit to each index and take note down the entry present on that index. 

So the final entry on each index was 3, 2, 2, 2.

Now take the minimum count among these entries and that is the result. So min(3, 2, 2, 2) is 2, that means the above test input is processed 2 times in the above list. 

Hence Output (Frequency of 192.170.0.1) = 2.

• Input 2: 10.125.22.20

Pass above input to all four hash functions, and take the index numbers generated by hash functions. 

• hashFunction1(10.125.22.20): 3
• hashFunction2(10.125.22.20): 4
• hashFunction3(10.125.22.20): 1
• hashFunction4(10.125.22.20): 6

Now visit to each index and take note down the entry present on that index. 

So the final entry on each index was 1, 1, 1, 2.

Now take the minimum count among these entries and that is the result. So min(1, 1, 1, 2) is 1, that means the above test input is processed only once in the above list.

Hence Output (Frequency of 10.125.22.20) = 1.

Implementation of Count-min sketch using Guava library in Java: 

We can implement the Count-min sketch using Java library provided by Guava. Below is the step by step implementation: 

• Use below maven dependency.

• The detailed Java code is as follows: 

Above example takes three arguments in the constructor which are 

- 0.001 = the epsilon i.e., error rate
- 0.99 = the delta i.e., confidence or accuracy rate
- 1 = the seed

Output: 

Time and Space Complexity of Count-Min Sketch Data Structure 

Now let’s look at the time and space consumed with this type of approach (wrt to above Java-Guava Implementation)

Let’s have a look at the memory (space) consumed: 

Applications of Count-min sketch: 

• Compressed Sensing
• Networking
• NLP
• Stream Processing
• Frequency tracking
• Extension: Heavy-hitters
• Extension: Range-query

Issue with Count-min sketch and its solution: 

What if one or more elements got the same hash values and then they all incremented. So, in that case, the value would have been increased because of the hash collision. Thus sometimes (in very rare cases) Count-min sketch overcounts the frequencies because of the hash functions.

So the more hash function we take there will be less collision. The fewer hash functions we take there will be a high probability of collision. Hence it always recommended taking more number of hash functions. 

Conclusion:

We have observed that the Count-min sketch is a good choice in a situation where we have to process a large data set with low memory consumption. We also saw that the more accurate result we want the number of hash functions(rows/width) has to be increased.

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Article Tags :

• Advanced Data Structure
• DSA

Practice Tags :

• Advanced Data Structure