91 lines
2.2 KiB
C++
91 lines
2.2 KiB
C++
#include <bits/stdc++.h>
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void computeLPSArray(char* pat, int M, int* lps);
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// Prints occurrences of pat[] int txt[]
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void KMPSearch(char* pat, char* txt)
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{
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int M = strlen(pat);
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int N = strlen(txt);
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// create lps[] that will hold the longest prefix suffix
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// values for pattern
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int lps[M];
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// Preprocess the pattern (calculate lps[] array)
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computeLPSArray(pat, M, lps);
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int i = 0; // index for txt[]
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int j = 0; // index for pat[]
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bool f = 1;//flag to indicate pattern not found
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while (i < N) {
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if (pat[j] == txt[i]) {
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j++;
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i++;
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}
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if (j == M) {
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printf("Found pattern at index %d\n", i - j);
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f=0;
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j = lps[j - 1];
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}
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// mismatch after j matches
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else if (i < N && pat[j] != txt[i]) {
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// Do not match lps[0..lps[j-1]] characters,
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// they will match anyway
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if (j != 0)
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j = lps[j - 1];
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else
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i = i + 1;
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}
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}
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if(f)
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printf("Pattern is not found");
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}
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// Fills lps[] for given patttern pat[0..M-1]
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void computeLPSArray(char* pat, int M, int* lps)
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{
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// length of the previous longest prefix suffix
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int len = 0;
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lps[0] = 0; // lps[0] is always 0
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// the loop calculates lps[i] for i = 1 to M-1
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int i = 1;
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while (i < M) {
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if (pat[i] == pat[len]) {
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len++;
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lps[i] = len;
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i++;
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}
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else // (pat[i] != pat[len])
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{
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// This is tricky. Consider the example.
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// AAACAAAA and i = 7. The idea is similar
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// to search step.
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if (len != 0) {
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len = lps[len - 1];
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// Also, note that we do not increment
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// i here
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}
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else // if (len == 0)
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{
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lps[i] = 0;
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i++;
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}
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}
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}
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}
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// Driver program to test above function
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int main()
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{
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char txt[] = "ABABDABACDABABCABAB";
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char pat[] = "ABABCABAB";
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KMPSearch(pat, txt);
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return 0;
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}
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