Question

Anyone know how I would find & replace text in a string? Basically I have two strings:

string firstS = "/9j/4AAQSkZJRgABAQEAYABgAAD/2wBDABQODxIPDRQSERIXFhQYHzMhHxwcHz8tLyUzSkFOTUlBSEZSXHZkUldvWEZIZoxob3p9hIWET2ORm4+AmnaBhH//2wBDARYXFx8bHzwhITx/VEhUf39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f3//";

string secondS = "abcdefg2wBDABQODxIPDRQSERIXFh/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/abcdefg";

I want to search firstS to see if it contains any sequence of characters that’s in secondS and then replace it. It also needs to be replaced with the number of replaced characters in squared brackets:

[NUMBER-OF-CHARACTERS-REPLACED]

For example, because firstS and secondS both contain “2wBDABQODxIPDRQSERIXFh” and “/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/f39/” they would need to be replaced. So then firstS becomes:

string firstS = "/9j/4AAQSkZJRgABAQEAYABgAAD/[22]QYHzMhHxwcHz8tLyUzSkFOTUlBSEZSXHZkUldvWEZIZoxob3p9hIWET2ORm4+AmnaBhH//2wBDARYXFx8bHzwhITx/VEhUf39[61]f3//";

Hope that makes sense. I think I could do this with Regex, but I don’t like the inefficiency of it. Does anyone know of another, faster way?

Answer 1

Does anyone know of another, faster way?

Yes, this problem actually has a proper name. It is called the Longest Common Substring, and it has a reasonably fast solution.

Here is an implementation on ideone. It finds and replaces all common substrings of ten characters or longer.

// This comes straight from Wikipedia article linked above:
private static string FindLcs(string s, string t) {
    var L = new int[s.Length, t.Length];
    var z = 0;
    var ret = new StringBuilder();
    for (var i = 0 ; i != s.Length ; i++) {
        for (var j = 0 ; j != t.Length ; j++) {
            if (s[i] == t[j]) {
                if (i == 0 || j == 0) {
                    L[i,j] = 1;
                } else {
                    L[i,j] = L[i-1,j-1] + 1;
                }
                if (L[i,j] > z) {
                    z = L[i,j];
                    ret = new StringBuilder();
                }
                if (L[i,j] == z) {
                    ret.Append(s.Substring( i-z+1, z));
                }
            } else {
                L[i,j]=0;
            }
        }
    }
    return ret.ToString();
}
// With the LCS in hand, building the answer is easy
public static string CutLcs(string s, string t) {
    for (;;) {
        var lcs = FindLcs(s, t);
        if (lcs.Length < 10) break;
        s = s.Replace(lcs, string.Format("[{0}]", lcs.Length));
    }
    return s;
}