String Matching -- Brute Force + Rabin-Karp + KMP

String Matching

这个问题已经被做烂了．．．

下面是C语言实现集合.

http://www-igm.univ-mlv.fr/~lecroq/string/

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暴力解法：

　　　　　

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"""
Programmer  :   EOF
Date        :   2015.02.28
Code file   :   nsm.py

"""

def naive_string_matcher(T, P) :
    if (T or P) is None :
        return 

    n = len(T)
    m = len(P)


    for s in range(0, n-m+1) :
        match = True
        for i in range(0, m) :
            if P[i] is not T[s+i] :
                match = False
                break

        if match is True :
            print "Pattern occurs with shift", s

#------------ for testing --------------------

string1 = "hello goodbye and hello"
string2 = "hello"
naive_string_matcher(string1, string2)

Rabin-Krap :

"""
Programmer  :   EOF
Code date   :   2015.02.28
Code file   :   rkm.py
e-mail      :   [email protected]

"""
def rabin_karp_matcher(T, P, d, q) :
    n = len(T)
    m = len(P)

    h = d**(m-1) % q
    p = 0
    t_0 = 0

    t = [0 for i in range(0, n - m + 1)]

    # preprosecessing
    for i in range(0, m) :
        p = ( d*p + ord(P[i]) ) % q
        t[0] = (d*t[0] + ord(T[i])) % q

    # matching
    for s in range(0, n-m+1) :
        match = True
        if p is t[s] :
            for i in range(0, m) :
                if P[i] is not T[s+i] :
                    match = False
                    break

            if match is True :
                print "Pattern occurs with shift", s

        if s < n-m :
            t[s+1] = (d * ( t[s] - ord(T[s]) * h ) + ord(T[s+m]) ) % q

#------------ for testing --------------------

string1 = "hello goodbye and hello"
string2 = "hello"
rabin_karp_matcher(string1, string2, 10, 13)

KMP:

"""
Programmer  :   EOF
Code date   :   2015.02.28
Code file   :   kmp.py
e-mail      :   [email protected]

Code description    :
    Here is a implementation of KMP which is a useful
algorithm for string matching.

"""

def kmp_matcher(T, P) :
    n = len(T)
    m = len(P)

    pi = compute_prefix_function(P)
    q = -1 # number of characters matched
    for i in range(0, n) : # scan the text from left to right
        while q > 0 and P[q+1] != T[i] :
            q = pi[q]       # next character doesn't match

        if P[q+1] == T[i] :
            q += 1          # next character matches
        if (q+1) == m :         # Is all of P matched ?
            print "Pattern occurs with shift ", i-m
            q = pi[q]       # look for the next match


def compute_prefix_function(P) :
    m = len(P)
    pi = [-1 for i in range(0, m)]

    k = -1 # Attention !
    for q in range(1, m) :
        while k > 0 and P[k+1] != P[q] :
            k = pi[k]
        if P[k+1] == P[q] :
            k += 1

        pi[q] = k

    return pi

#-------------for testing----------------------
#string_1 = "hello goodbye and hello"
#string_2 = "hello"
string_1 = "abcabaabcabac"
string_2 = "abaa"
kmp_matcher(string_1, string_2)

PHP神人吐槽KMP

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