Help fix my KMP search algorithm.

Question

Hello I am trying to write a C# version of KMP search from Algorithms in C book. Having trouble finding the flaw in my algorithm. Would someone help?

static int KMP(string p, string str) {
    int m = p.Length;
    int n = str.Length;
    int i;
    int j;

    int[] next = new int[m];
    next[0] = -1;

    for (i = 0, j = -1; i < m; i++, j++, next[i] = j) { 
                                        //Getting index out of bounds
        while (j > 0 && p[i] != p[j]) j = next[j];
    }

    for (i = 0, j = 0; i < n && j < m; i++, j++) {
        while (j >= 0 && p[j] != str[i]) j = next[j];
        if (j == m) return i - m;
    }

    return -1;
}

Answer 1

The simple answer is in the first loop i++ is settling before next[i] = j so on the last character of the search string its trying to set next[m+1] to j - which causes an index out of bounds exception. Try changing the order:

for (i = 0, j = -1; i < m;  next[i] = j, i++, j++)

More fundamentally, try breaking the implementation into testable parts. For example, you can extract a testable method for the first loop as it is building the computed table for the search word. Start with:

public int[] BuildTable(string word)
{
    // todo
}

and some NUnit tests based on the wiki description

[Test]
public void Should_get_computed_table_0_0_0_0_1_2_given_ABCDABD()
{
    const string input = "ABCDABD";
    var result = BuildTable(input);
    result.Length.ShouldBeEqualTo(input.Length);
    result[0].ShouldBeEqualTo(-1);
    result[1].ShouldBeEqualTo(0);
    result[2].ShouldBeEqualTo(0);
    result[3].ShouldBeEqualTo(0);
    result[4].ShouldBeEqualTo(0);
    result[5].ShouldBeEqualTo(1);
    result[6].ShouldBeEqualTo(2);
}

[Test]
public void Should_get_computed_table_0_1_2_3_4_5_given_AAAAAAA()
{
    const string input = "AAAAAAA";
    var result = BuildTable(input);
    result.Length.ShouldBeEqualTo(input.Length);
    result[0].ShouldBeEqualTo(-1);
    result[1].ShouldBeEqualTo(0);
    result[2].ShouldBeEqualTo(1);
    result[3].ShouldBeEqualTo(2);
    result[4].ShouldBeEqualTo(3);
    result[5].ShouldBeEqualTo(4);
    result[6].ShouldBeEqualTo(5);
}

Next write one or more tests for the KMP method.

[Test]
public void Should_get_15_given_text_ABC_ABCDAB_ABCDABCDABDE_and_word_ABCDABD()
{
    const string text = "ABC ABCDAB ABCDABCDABDE";
    const string word = "ABCDABD";
    int location = KMP(word, text);
    location.ShouldBeEqualTo(15);
}

Then implement using the structure used on the wiki description of the algorithm and it should come together for you.

public int KMP(string word, string textToBeSearched)
{
    var table = BuildTable(word);
    // rest of algorithm
}