Between (C#)

Between.cs

public static string Between(StringBuilder result, string a, string b)
{
	if (a == b) return a;
	if (Compare(a, b) >= 0)
		throw new InvalidOperationException();

	var ai = 0;
	var bi = 0;

	while (true)
	{
		if (ai >= a.Length)
		{
			(a, ai) = ("a", 0);
			continue;
		}

		if (bi >= b.Length)
		{
			(b, bi) = ("z", 0);
			continue;
		}

		var ac = a[ai];
		var bc = b[bi];
		var d = Compare(ac, bc);

		if (d == 0)
		{
			result.Append(ac);
			ai++;
			bi++;
			continue;
		}

		if (d <= 1)
		{
			result.Append(ac);
			ai++;
			(b, bi) = ("z", 0);
			continue;
		}

		var m = (char) ((ac + bc) / 2);
		return result.Append(m).ToString();
	}
}

private static char Middle(char ac, char bc) => (char)((ac + bc) / 2);
private static int Compare(string a, string b) => string.CompareOrdinal(a, b);
private static int Compare(char a, char b) => b - a;