Question

I am writing a custom string split. It will split on a dot(.) that is not preceded by an odd number of backslashes (\).

«string» -> «IEnemerable<string>»
"hello.world" -> "hello", "world"
"abc\.123" -> "abc\.123"
"aoeui\\.dhtns" -> "aoeui\\","dhtns"

I would like to know if there is a substring that will reuse the original string (for speed), or is there an existing split that can do this fast?

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This is what I have but is 2—3 times slower than input.Split('.') //where input is a string. (I know it is a (slightly more complex problem, but not that much)

    public IEnumerable<string> HandMadeSplit(string input)
    {
        var Result = new LinkedList<string>();
        var word = new StringBuilder();
        foreach (var ch in input)
        {
            if (ch == '.')
            {
                Result.AddLast(word.ToString());
                word.Length = 0;
            }
            else
            {
                word.Append(ch);
            }
        }
        Result.AddLast(word.ToString());
        return Result;
    }

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It now uses List instead of LinkedList, and record beginning and end of substring and use string.substring to create the new substrings. This does a lot and is nearly as fast as string.split but I have added my adjustments. (will add code)

Answer 1

The loop that you show is the right approach if you need performance. (Regex wouldn’t be).

Switch to an index-based for-loop. Remember the index of the start of the match. Don’t append individual chars. Instead, remember the range of characters to copy out and do that with a single Substring call per item.

Also, don’t use a LinkedList. It is slower than a List for almost all cases except random-access mutations.

You might also switch from List to a normal array that you resize with Array.Resize. This results in slightly tedious code (because you have inlined a part of the List class into your method) but it cuts out some small overheads.

Next, don’t return an IEnumerable because that forces the caller through indirection when accessing its items. Return a List or an array.