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copied!<p>The problem you are trying to solve is actually how to decrement discreet sections of a sequence of characters, each with it's own counting system, where each section is separated by a change between Alpha and Numeric. The rest of the problem is easy once you identify this.</p> <p>The skipping of unwanted characters is simply a matter of repeating the decrement if you get an unwanted character in the result.</p> <p>One difficultly is the ambiguous definition of the sequences. e.g. what to do when you get down to say A00, what is next? "A" or "-". For the sake of argument I am assuming a practical implementation based loosely on Excel cell names (i.e. each section operates independently of the others). </p> <p>The code below does 95% of what you wanted, however there is a bug in the exclusions code. e.g. "ABB" becomes "AAY". I feel the exclusions need to be applied at a higher level (e.g. repeat decrement until no character is in the exclusions list), but I don't have time to finish it now. Also it is resulting in a blank string when it counts down to nothing, rather than the "-" you wanted, but that is trivial to add at the end of the process.</p> <h2>Part 1 (divide the problem into sections):</h2> <pre><code>public static string DecreaseName( string name, string exclusions ) { if (string.IsNullOrEmpty(name)) { return name; } // Split the problem into sections (reverse order) List<StringBuilder> sections = new List<StringBuilder>(); StringBuilder result = new StringBuilder(name.Length); bool isNumeric = char.IsNumber(name[0]); StringBuilder sb = new StringBuilder(); sections.Add(sb); foreach (char c in name) { // If we change between alpha and number, start new string. if (char.IsNumber(c) != isNumeric) { isNumeric = char.IsNumber(c); sb = new StringBuilder(); sections.Insert(0, sb); } sb.Append(c); } // Now process each section bool cascadeToNext = true; foreach (StringBuilder section in sections) { if (cascadeToNext) { result.Insert(0, DecrementString(section, exclusions, out cascadeToNext)); } else { result.Insert(0, section); } } return result.ToString().Replace(" ", ""); } </code></pre> <h2>Part2 (decrement a given string):</h2> <pre><code>private static string DecrementString(StringBuilder section, string exclusions, out bool cascadeToNext) { bool exclusionsExist = false; do { exclusionsExist = false; cascadeToNext = true; // Process characters in reverse for (int i = section.Length - 1; i >= 0 && cascadeToNext; i--) { char c = section[i]; switch (c) { case 'A': c = (i > 0) ? 'Z' : ' '; cascadeToNext = (i > 0); break; case 'a': c = (i > 0) ? 'z' : ' '; cascadeToNext = (i > 0); break; case '0': c = (i > 0) ? '9' : ' '; cascadeToNext = (i > 0); break; case ' ': cascadeToNext = false; break; default: c = (char)(((int)c) - 1); if (i == 0 && c == '0') { c = ' '; } cascadeToNext = false; break; } section[i] = c; if (exclusions.Contains(c.ToString())) { exclusionsExist = true; } } } while (exclusionsExist); return section.ToString(); } </code></pre> <p>The dividing can of course be done more efficiently, just passing start and end indexes to the DecrementString, but this is easier to write & follow and not much slower in practical terms.</p>

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