88 lines
2.1 KiB
C++
88 lines
2.1 KiB
C++
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/* C++ Program for Bad Character Heuristic of Boyer
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Moore String Matching Algorithm */
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#include <bits/stdc++.h>
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using namespace std;
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# define NO_OF_CHARS 256
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// The preprocessing function for Boyer Moore's
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// bad character heuristic
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void badCharHeuristic( string str, int size,
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int badchar[NO_OF_CHARS])
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{
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int i;
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// Initialize all occurrences as -1
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for (i = 0; i < NO_OF_CHARS; i++)
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badchar[i] = -1;
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// Fill the actual value of last occurrence
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// of a character
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for (i = 0; i < size; i++)
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badchar[(int) str[i]] = i;
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}
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/* A pattern searching function that uses Bad
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Character Heuristic of Boyer Moore Algorithm */
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void search( string txt, string pat)
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{
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int m = pat.size();
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int n = txt.size();
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int badchar[NO_OF_CHARS];
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/* Fill the bad character array by calling
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the preprocessing function badCharHeuristic()
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for given pattern */
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badCharHeuristic(pat, m, badchar);
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int s = 0; // s is shift of the pattern with
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// respect to text
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while(s <= (n - m))
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{
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int j = m - 1;
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/* Keep reducing index j of pattern while
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characters of pattern and text are
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matching at this shift s */
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while(j >= 0 && pat[j] == txt[s + j])
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j--;
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/* If the pattern is present at current
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shift, then index j will become -1 after
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the above loop */
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if (j < 0)
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{
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cout << "pattern occurs at shift = " << s << endl;
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/* Shift the pattern so that the next
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character in text aligns with the last
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occurrence of it in pattern.
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The condition s+m < n is necessary for
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the case when pattern occurs at the end
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of text */
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s += (s + m < n)? m-badchar[txt[s + m]] : 1;
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}
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else
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/* Shift the pattern so that the bad character
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in text aligns with the last occurrence of
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it in pattern. The max function is used to
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make sure that we get a positive shift.
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We may get a negative shift if the last
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occurrence of bad character in pattern
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is on the right side of the current
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character. */
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s += max(1, j - badchar[txt[s + j]]);
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}
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}
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/* Driver code */
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int main()
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{
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string txt= "ABAAABCD";
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string pat = "ABC";
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search(txt, pat);
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return 0;
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}
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