我已经在C#.NET中实现了Rabin-Karp算法,遵循这个伪代码:
问题是,模式与原始文本不匹配。我彻底完成了代码,但我无法在代码中识别出问题。有人能告诉我代码中的错误吗?
static void Main(string[] args)
{
string text = "ratcatpat catbats";
string pattern = "cat";
int d = text.Select(e => e).Distinct().Count();
RabinCarp(text, pattern, d, 17);
Console.ReadKey();
}
static void RabinCarp(string text, string pattern, int sizeOfAlphabet, int moduloValue)
{
int rollingHashOf_P = 0;
int rollingHashOf_T = 0;
int lengthOfText = text.Length;
int lengthOfPattern = pattern.Length;
int h = (int)(Math.Pow(sizeOfAlphabet, lengthOfPattern - 1) % moduloValue);
for (int i = 0; i < lengthOfPattern; i++)
{
rollingHashOf_P = (sizeOfAlphabet * rollingHashOf_P + (int)pattern[i]) % moduloValue;
rollingHashOf_T = (sizeOfAlphabet * rollingHashOf_T + (int)text[i]) % moduloValue;
}
int diffNM = lengthOfText - lengthOfPattern;
for (int i = 0; i <= diffNM; i++)
{
if (Math.Abs(rollingHashOf_P) == Math.Abs(rollingHashOf_T))
{
if (text.Substring(i, lengthOfPattern).Contains(pattern))
{
string message = "pattern identified";
Console.WriteLine(message);
}
}
if (i < diffNM)
{
rollingHashOf_T = Math.Abs(sizeOfAlphabet * (rollingHashOf_T - (int)text[i] * h) + (int)text[i + lengthOfPattern]) % moduloValue;
}
}
}
答案 0 :(得分:2)
我不熟悉Rabin-Karp算法,但我确信你应该提升 rollingHashOf_P
以及rollingHashOf_T
:
if (i < diffNM)
{
rollingHashOf_T = Math.Abs(sizeOfAlphabet * (rollingHashOf_T - (int)text[i] * h) + (int)text[i + lengthOfPattern]) % moduloValue;
rollingHashOf_P = Math.Abs(sizeOfAlphabet * (rollingHashOf_P - (int)pattern[i] * h) + (int)pattern[i + lengthOfPattern]) % moduloValue;
}
击> <击> 撞击>
OP在下面的评论中共享了这个伪代码:
很明显,以上是错误的。将其与帖子中的代码进行比较,虽然显示错误可能在推进rollingHashOf_T
之后,正如它所说的那样:
rollingHashOf_T = Math.Abs(sizeOfAlphabet * (rollingHashOf_T -
(int)text[i] * h) + (int)text[i + lengthOfPattern]) % moduloValue;
虽然伪代码表明它应该是:
rollingHashOf_T = Math.Abs(sizeOfAlphabet * (rollingHashOf_T -
(int)text[i + 1] * h) + (int)text[i + lengthOfPattern + 1]) % moduloValue;