What is a Unicode safe replica of String.IndexOf(string input) that can handle Surrogate Pairs?

Question

I am trying to figure out an equivalent to C# string.IndexOf(string) that can handle surrogate pairs in Unicode characters.

I am able to get the index when only comparing single characters, like in the code below:

    public static int UnicodeIndexOf(this string input, string find)
    {
        return input.ToTextElements().ToList().IndexOf(find);
    }

    public static IEnumerable<string> ToTextElements(this string input)
    {
        var e = StringInfo.GetTextElementEnumerator(input);
        while (e.MoveNext())
        {
            yield return e.GetTextElement();
        }
    }

But if I try to actually use a string as the find variable then it won't work because each text element only contains a single character to compare against.

Are there any suggestions as to how to go about writing this?

Thanks for any and all help.

EDIT:

Below is an example of why this is necessary:

CODE

 Console.WriteLine("HolyCow𪘁BUBBYY𪘁YY𪘁Y".IndexOf("BUBB"));
 Console.WriteLine("HolyCow@BUBBYY@YY@Y".IndexOf("BUBB"));

OUTPUT

9
8

Notice where I replace the 𪘁 character with @ the values change.

Answer 1

You basically want to find index of one string array in another string array. We can adapt code from this question for that:

public static class Extensions {
    public static int UnicodeIndexOf(this string input, string find, StringComparison comparison = StringComparison.CurrentCulture) {
        return IndexOf(
           // split input by code points
           input.ToTextElements().ToArray(),
           // split searched value by code points
           find.ToTextElements().ToArray(), 
           comparison);
    }
    // code from another answer
    private static int IndexOf(string[] haystack, string[] needle, StringComparison comparision) {
        var len = needle.Length;
        var limit = haystack.Length - len;
        for (var i = 0; i <= limit; i++) {
            var k = 0;
            for (; k < len; k++) {
                if (!String.Equals(needle[k], haystack[i + k], comparision)) break;
            }

            if (k == len) return i;
        }

        return -1;
    }

    public static IEnumerable<string> ToTextElements(this string input) {
        var e = StringInfo.GetTextElementEnumerator(input);
        while (e.MoveNext()) {
            yield return e.GetTextElement();
        }
    }
}