CODEWITHSUNDEEP
c#.netvb.netperformancestringbuilder
Alaa Alweish
Alaa Alweish

Reputation: 9084

Fastest search method in StringBuilder

I have a StringBuilder named stb_Swap_Tabu used to store Course's Names, I am using the following method to find a course:

stb_Swap_Tabu.ToString.Contains("CourseName")

in my case, Performance is the most important issue. Is there any faster way?

Upvotes: 16

Views: 41016

Answers (2)

вʀaᴎᴅᴏƞ вєнᴎєƞ
вʀaᴎᴅᴏƞ вєнᴎєƞ

Reputation: 1678

I know this is an old question but it came up in my search results when I was trying to create a solution for my own project. I thought I needed to search a StringBuilder.ToString's method results but then I realized I could just call methods on the StringBuilder itself. My situation may not be the same as yours, but I though I'd share:

Private Function valueFormatter(ByVal value As String) As String
    ' This will correct any formatting to make the value valid for a CSV format
    '
    ' 1) Any value that as a , in it then it must be wrapped in a " i.e. Hello,World -> "Hello,World"
    ' 2) In order to escape a " in the value add a " i.e. Hello"World -> Hello""World
    ' 3) if the value has a " in it then it must also be wrapped in a " i.e. "Hello World" -> ""Hello World"" -> """Hello World"""
    ' 
    ' VB NOTATAION 
    ' " -> """"
    ' "" -> """"""

    If value.Contains(",") Or value.Contains("""") Then
        Dim sb As New StringBuilder(value)
        If value.Contains("""") Then sb.Replace("""", """""")
        sb.Insert(0, """").Append("""")
        Return sb.ToString
    Else
        Return value
    End If
End Function

Upvotes: 0

Jon Hanna
Jon Hanna

Reputation: 113392

StringBuilder wasn't really intended for all string purposes. If you really need to search one, you have to write your own method.

There are several string-searching algorithms suited to different cases.

The following is a simple implementation of the Knuth–Morris–Pratt algorithm that only cares about ordinal matches (no case-folding, no culture-related collation, just a plain codepoint to codepoint match). It has some initial Θ(m) overhead where m is the length of the word sought, and then finds in Θ(n) where n is the distance to the word sought, or the length of the whole string-builder if it isn't there. This beats the simple char-by-char compare which is Θ((n-m+1) m) (Where O() notation describes upper-bounds, Θ() describes both upper and lower bounds).

All this said, it does sound like creating a list might be a better approach to the task in hand.

public static class StringBuilderSearching
{
  public static bool Contains(this StringBuilder haystack, string needle)
  {
    return haystack.IndexOf(needle) != -1;
  }
  public static int IndexOf(this StringBuilder haystack, string needle)
  {
    if(haystack == null || needle == null)
      throw new ArgumentNullException();
    if(needle.Length == 0)
      return 0;//empty strings are everywhere!
    if(needle.Length == 1)//can't beat just spinning through for it
    {
      char c = needle[0];
      for(int idx = 0; idx != haystack.Length; ++idx)
        if(haystack[idx] == c)
          return idx;
      return -1;
    }
    int m = 0;
    int i = 0;
    int[] T = KMPTable(needle);
    while(m + i < haystack.Length)
    {
      if(needle[i] == haystack[m + i])
      {
        if(i == needle.Length - 1)
          return m == needle.Length ? -1 : m;//match -1 = failure to find conventional in .NET
        ++i;
      }
      else
      {
        m = m + i - T[i];
        i = T[i] > -1 ? T[i] : 0;
      }
    }
    return -1;
  }      
  private static int[] KMPTable(string sought)
  {
    int[] table = new int[sought.Length];
    int pos = 2;
    int cnd = 0;
    table[0] = -1;
    table[1] = 0;
    while(pos < table.Length)
      if(sought[pos - 1] == sought[cnd])
        table[pos++] = ++cnd;
      else if(cnd > 0)
        cnd = table[cnd];
      else
        table[pos++] = 0;
    return table;
  }
}

Upvotes: 31

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