The Quick Search algorithm uses only the bad-character shift table (see chapter Boyer-Moore algorithm). After an attempt where the window is positioned on the text factor y[j .. j+m-1], the length of the shift is at least equal to one. So, the character y[j+m] is necessarily involved in the next attempt, and thus can be used for the bad-character shift of the current attempt.
The bad-character shift of the present algorithm is slightly modified to take into account the last character of x as follows: for c in Sigma, qsBc[c]=min{i : 0 < i leq m and x[m-i]=c} if c occurs in x, m+1 otherwise (thanks to Darko Brljak).
The preprocessing phase is in O(m+sigma) time and O(sigma) space complexity.
During the searching phase the comparisons between pattern and text characters during each attempt can be done in any order. The searching phase has a quadratic worst case time complexity but it has a good practical behaviour.
void preQsBc(char *x, int m, int qsBc[]) {
int i;
for (i = 0; i < ASIZE; ++i)
qsBc[i] = m + 1;
for (i = 0; i < m; ++i)
qsBc[x[i]] = m - i;
}
void QS(char *x, int m, char *y, int n) {
int j, qsBc[ASIZE];
/* Preprocessing */
preQsBc(x, m, qsBc);
/* Searching */
j = 0;
while (j <= n - m) {
if (memcmp(x, y + j, m) == 0)
OUTPUT(j);
j += qsBc[y[j + m]]; /* shift */
}
}
Preprocessing phase
Quick Search algorithm qsBc table
qsBc table used by Quick Search algorithm
Searching phase