Boyer-Moore and Knuth-Morris-Pratt algorithms
by Christian.Charras and Thierry.Lecroq
The C code for Boyer-Moore Algorithm
/* Bad-character shift 表 */
void preBmBc(char *x, int m, int bmBc[]) {
int i;
for (i = 0; i < ASIZE; ++i)
bmBc[i] = m;
for (i = 0; i < m - 1; ++i)
bmBc[x[i]] = m - i - 1;
}
/* suffixes[i]=k s.t. x[i-k+1,...,i]=x[m-k,...,m-1] */
void suffixes(char *x, int m, int *suff) {
int f, g, i;
suff[m - 1] = m;
g = m - 1;
for (i = m - 2; i >= 0; --i) {
if (i > g && suff[i + m - 1 - f] < i - g)
suff[i] = suff[i + m - 1 - f];
else {
if (i < g)
g = i;
f = i;
while (g >= 0 && x[g] == x[g + m - 1 - f])
--g;
suff[i] = f - g;
}
}
}
/* Good-suffix shift 表 */
void preBmGs(char *x, int m, int bmGs[]) {
int i, j, suff[XSIZE];
suffixes(x, m, suff);
for (i = 0; i < m; ++i)
bmGs[i] = m;
j = 0;
for (i = m - 1; i >= -1; --i)
if (i == -1 || suff[i] == i + 1)
for (; j < m - 1 - i; ++j)
if (bmGs[j] == m)
bmGs[j] = m - 1 - i;
for (i = 0; i <= m - 2; ++i)
bmGs[m - 1 - suff[i]] = m - 1 - i;
}
void BM(char *x, int m, char *y, int n) {
int i, j, bmGs[XSIZE], bmBc[ASIZE];
/* Preprocessing */
preBmGs(x, m, bmGs);
preBmBc(x, m, bmBc);
/* Searching */
j = 0;
while (j <= n - m) {
for (i = m - 1; i >= 0 && x[i] == y[i + j]; --i);
if (i < 0) {
OUTPUT(j);
j += bmGs[0];
}
else
j += MAX(bmGs[i], bmBc[y[i + j]] - m + 1 + i);
}
}
The C code for Knuth-Morris-Pratt Algorithm
void preKmp(char *x, int m, int kmpNext[]) {
int i, j;
i = 0;
j = kmpNext[0] = -1;
while (i < m) {
while (j > -1 && x[i] != x[j])
j = kmpNext[j];
i++;
j++;
if (x[i] == x[j])
kmpNext[i] = kmpNext[j];
else
kmpNext[i] = j;
}
}
void KMP(char *x, int m, char *y, int n) {
int i, j, kmpNext[XSIZE];
/* Preprocessing */
preKmp(x, m, kmpNext);
/* Searching */
i = j = -1;
while (j+m-i <= n) {
while (i > -1 && x[i] != y[j])
i = kmpNext[i];
i++;
j++;
if (i >= m) {
OUTPUT(j - i);
i = kmpNext[i];
}
}
}