15
几周前，我对Stackoverflow提出了一个问题，即创建一个有效的算法来搜索大块文本中的模式。现在我正在使用String函数indexOf来执行搜索。一个建议是使用拉宾卡普作为替代。我按照以下方法编写了一个小测试程序来测试Rabin-Karp的实现。Java indexOf函数比Rabin-Karp更高效吗？搜索文本的效率
public static void main(String[] args) {
String test = "Mary had a little lamb whose fleece was white as snow";
String p = "was";
long start = Calendar.getInstance().getTimeInMillis();
for (int x = 0; x < 200000; x++)
test.indexOf(p);
long end = Calendar.getInstance().getTimeInMillis();
end = end -start;
System.out.println("Standard Java Time->"+end);
RabinKarp searcher = new RabinKarp("was");
start = Calendar.getInstance().getTimeInMillis();
for (int x = 0; x < 200000; x++)
searcher.search(test);
end = Calendar.getInstance().getTimeInMillis();
end = end -start;
System.out.println("Rabin Karp time->"+end);
}
这里是拉宾，卡普的实现，我使用：
import java.math.BigInteger;
import java.util.Random;
public class RabinKarp {
private String pat; // the pattern // needed only for Las Vegas
private long patHash; // pattern hash value
private int M; // pattern length
private long Q; // a large prime, small enough to avoid long overflow
private int R; // radix
private long RM; // R^(M-1) % Q
static private long dochash = -1L;
public RabinKarp(int R, char[] pattern) {
throw new RuntimeException("Operation not supported yet");
}
public RabinKarp(String pat) {
this.pat = pat; // save pattern (needed only for Las Vegas)
R = 256;
M = pat.length();
Q = longRandomPrime();
// precompute R^(M-1) % Q for use in removing leading digit
RM = 1;
for (int i = 1; i <= M - 1; i++)
RM = (R * RM) % Q;
patHash = hash(pat, M);
}
// Compute hash for key[0..M-1].
private long hash(String key, int M) {
long h = 0;
for (int j = 0; j < M; j++)
h = (R * h + key.charAt(j)) % Q;
return h;
}
// Las Vegas version: does pat[] match txt[i..i-M+1] ?
private boolean check(String txt, int i) {
for (int j = 0; j < M; j++)
if (pat.charAt(j) != txt.charAt(i + j))
return false;
return true;
}
// check for exact match
public int search(String txt) {
int N = txt.length();
if (N < M)
return -1;
long txtHash;
if (dochash == -1L) {
txtHash = hash(txt, M);
dochash = txtHash;
} else
txtHash = dochash;
// check for match at offset 0
if ((patHash == txtHash) && check(txt, 0))
return 0;
// check for hash match; if hash match, check for exact match
for (int i = M; i < N; i++) {
// Remove leading digit, add trailing digit, check for match.
txtHash = (txtHash + Q - RM * txt.charAt(i - M) % Q) % Q;
txtHash = (txtHash * R + txt.charAt(i)) % Q;
// match
int offset = i - M + 1;
if ((patHash == txtHash) && check(txt, offset))
return offset;
}
// no match
return -1; // was N
}
// a random 31-bit prime
private static long longRandomPrime() {
BigInteger prime = new BigInteger(31, new Random());
return prime.longValue();
}
// test client
}
拉宾，卡普的实施工作，它返回正确的我正在寻找字符串的偏移量。然而，令我惊讶的是，当我运行测试程序时发生的时间统计。他们是：
Standard Java Time->39
Rabin Karp time->409
这真是令人惊讶。不仅Rabin-Karp（至少在这里实现）不比标准的java indexOf字符串函数更快，而且速度更慢一个数量级。我不知道什么是错的（如果有的话）。任何人都有这个想法？
感谢，
埃利奥特
btw，BigInteger（31，new Random（））'不会返回素数（至少有时） – 2012-03-16 16:46:07
首先，您正在寻找一个非常短的字符串... – 2012-03-16 16:46:10
我没有彻底查看该代码也不知道Rabin-Karb算法，但“在大量文本中”暗示该算法针对大文本进行了优化，对于较小的文本可能会较慢。你是否尝试过大幅度的文字呢？ – Thomas 2012-03-16 16:46:16