Finding the most frequent character in a string and repeat this for millions of strings quickly

I need to iterate through a couple million essays that are about 500 words long and for every essay & I have to return the character is repeated the most.

I have the following code but to iterate through 1 million essay (I have the same essay hard coded). It takes almost three minutes before it has a collection of all the popular characters from each essay.

How can I optimize this to be faster? If you have another method besides my GetMostFrequentCharacter(), please suggest something w/ sample.

class Program
{
    static void Main(string[] args)
    {

        var popularCharacters = new List();
        var EssayCount = 0;
        var start = DateTime.Now;
        const int noOfEssays = 1000000;

        Console.WriteLine("Looking for most popular characters in {0} articles: ", noOfEssays);

        // Gen 1,000,000 essays
        for (int i = 0; i  1)
            {                    
                Console.WriteLine("Essay {0}: Most popular chr & equal in freq: ", EssayCount);
                foreach (char c in popularCharacter)
                Console.Write(c + ", ");
            }
            else
            {
                Console.WriteLine();
                Console.Write("Essay {0}: Most frequent chr: ", EssayCount);
                Console.WriteLine(popularCharacter);
            }  
        }
        Console.WriteLine();            
    }
}

static class Utilities
{

    public static IEnumerable GetMostFrequentCharacter(this IEnumerable input)
    {
        var dict = input.ToLookup(x => x);
        if (dict.Count == 0)
            return Enumerable.Empty();
        var maxCount = dict.Max(x => x.Count());
        return dict.Where(x => x.Count() == maxCount).Select(x => x.Key);
    }
}

Some thoughts

• The method should not be generic because the methods name GetMostFrequentCharacter implies that it returns some kind of Char

• The method should be renamed to GetMostFrequentCharacters as it returns an IEnumerable.

• The variable dict should be renamed to lookup

• The check if input contains any elements should be done before the call of ToLookup()

After considering these your method looks like

public static IEnumerable GetMostFrequentCharacters(this IEnumerable input)
{
    if (!input.Any()) { return Enumerable.Empty(); }

    var lookup = input.ToLookup(x => x);
    var maxCount = lookup.Max(x => x.Count());

    return lookup.Where(x => x.Count() == maxCount).Select(x => x.Key);
}

If it is possible to limt the charset to ASCII you could gain big speed improvement by using raporttech97 idea from the comments

public static IEnumerable GetMostFrequentCharacters(this IEnumerable input)
{
    if (!input.Any()) { return Enumerable.Empty(); }

    const int maxChar = 128;
    int[] histogram = new int[maxChar];

    foreach (char c in input)
    {
        if (c > maxChar) { continue; }
        histogram[c] = histogram[c] + 1;

    }

    IList mostFrequentChars = new List();

    int max = int.MinValue;

    for (int i = 33; i  max)
        {
            max = histogram[i];
            mostFrequentChars.Clear();
            mostFrequentChars.Add((char)i);

        }
        else if (histogram[i] == max)
        {
            mostFrequentChars.Add((char)i);
        }
    }

    return mostFrequentChars;
}

This is about 4 times faster than your solution.

Timed on my computer for 1.000.000 calls

Yours: 90 seconds

This: 24 seconds

StackExchange Code Review Q#69296, answer score: 3