How many permutations of one string are there in the other string (hash and counter comparison)

CountPermutations.cs

// Given a smaller string "pattern" and a bigger string "text",
// design an algorithm to find all permutations of the shorter
// string within the longer one. Return the location of each permutation.

CountPermutations("cbabadcbbabbcbabaabccbabc", "abbc").Dump();
// Output: [0, 6, 9, 11, 12, 20, 21]

CountPermutations("encyclopedia", "dep").Dump();
// Output: [7]

static IEnumerable<int> CountPermutations(string text, string pattern)
{
    if (text.Length < pattern.Length)
        yield break;
    
    long patternHash = 0;
    var patternCounter = new Dictionary<char, int>(text.Length);
    long windowHash = 0;
    var windowCounter = new Dictionary<char, int>(text.Length);
    
    for (int i = 0; i < pattern.Length; i++)
    {
        patternHash += pattern[i].GetHashCode();
        if (!patternCounter.TryAdd(pattern[i], 1))
            patternCounter[pattern[i]]++;

        windowHash += text[i].GetHashCode();
        if (!windowCounter.TryAdd(text[i], 1))
            windowCounter[text[i]]++;
    }        

    if (AreDictionariesEqual(patternCounter, windowCounter))
        yield return 0;
        
    for (int i = pattern.Length; i < text.Length; i++)
    {
        var charOut = text[i - pattern.Length];
        windowHash -= charOut.GetHashCode();
        var outCount = --windowCounter[charOut];
        if (outCount is 0)
            windowCounter.Remove(charOut);

        var charIn = text[i];
        windowHash += charIn.GetHashCode();
        if (!windowCounter.TryAdd(charIn, 1))
            windowCounter[charIn]++;

        if (patternHash == windowHash && AreDictionariesEqual(patternCounter, windowCounter))
            yield return i - pattern.Length + 1;
    }
}

static bool AreDictionariesEqual(Dictionary<char, int> first, Dictionary<char, int> second)
{
    if (first.Count != second.Count)
        return false;
        
    foreach (var (chr, firstCount) in first)
        if (!second.TryGetValue(chr, out var secondCount) || firstCount != secondCount)
            return false;
            
    return true;
}