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using System;
namespace DataStructures.Probabilistic
{
public class CountMinSketch<T> where T : notnull
{
private readonly int[][] sketch;
private readonly int numHashes;
/// <summary>
/// Initializes a new instance of the <see cref="CountMinSketch{T}"/> class based off dimensions
/// passed by the user.
/// </summary>
/// <param name="width">The width of the sketch.</param>
/// <param name="numHashes">The number of hashes to use in the sketch.</param>
public CountMinSketch(int width, int numHashes)
{
sketch = new int[numHashes][];
for (var i = 0; i < numHashes; i++)
{
sketch[i] = new int[width];
}
this.numHashes = numHashes;
}
/// <summary>
/// Initializes a new instance of the <see cref="CountMinSketch{T}"/> class based off the optimizing error rate
/// and error probability formula width = e/errorRate numHashes = ln(1.0/errorProp).
/// </summary>
/// <param name="errorRate">The amount of acceptable over counting for the sketch.</param>
/// <param name="errorProb">The probability that an item will be over counted.</param>
public CountMinSketch(double errorRate, double errorProb)
{
var width = (int)Math.Ceiling(Math.E / errorRate);
numHashes = (int)Math.Ceiling(Math.Log(1.0 / errorProb));
sketch = new int[numHashes][];
for (var i = 0; i < numHashes; i++)
{
sketch[i] = new int[width];
}
}
/// <summary>
/// Inserts the provided item into the sketch.
/// </summary>
/// <param name="item">Item to insert.</param>
public void Insert(T item)
{
var initialHash = item.GetHashCode();
for (int i = 0; i < numHashes; i++)
{
var slot = GetSlot(i, initialHash);
sketch[i][slot]++;
}
}
/// <summary>
/// Queries the count of the given item that have been inserted into the sketch.
/// </summary>
/// <param name="item">item to insert into the sketch.</param>
/// <returns>the number of times the provided item has been inserted into the sketch.</returns>
public int Query(T item)
{
var initialHash = item.GetHashCode();
var min = int.MaxValue;
for (int i = 0; i < numHashes; i++)
{
var slot = GetSlot(i, initialHash);
min = Math.Min(sketch[i][slot], min);
}
return min;
}
private int GetSlot(int i, int initialHash) => Math.Abs((i + 1) * initialHash) % sketch[0].Length;
}
}
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