Pagini recente » Cod sursa (job #2950368) | Cod sursa (job #2282674) | Cod sursa (job #1415072) | Cod sursa (job #2045071) | Cod sursa (job #3287576)
// https://www.infoarena.ro/problema/strmatch
#include <array>
#include <iostream>
#include <fstream>
#include <vector>
#define ALG 2
#if ALG == 1 || ALG == 2
/**
* @brief KMP algorithm, finds max 1000 occurences of a in b.
* next[i] represents the longest prefix of a that is a suffix of a_i.
* This implementation assumes a and b are indexed from 0.
*
* Complexity: O(A+B) time, O(A+B) space
*/
auto strmatch(const std::string& a, const std::string& b)
-> std::pair<int, std::array<int, 1000>>
{
#if ALG == 2
int marked[128]; // represents the first position of the character
std::fill_n(marked, 128, -1);
marked[a[0]] = 0;
#endif
// precompute next array
std::vector<int> next(a.size());
next[0] = -1;
for (int i = 1, k = -1; i < a.size(); ++i) {
#if ALG == 2
marked[a[i]] = i;
#endif
// if the prefix cannot be extended to match the current suffix,
// go back to next[k], ensuring that the prexix of a_k is still a
// suffix of a_{i-1} (because a_k was already a suffix of a_{i-1})
while (k >= 0 && a[k + 1] != a[i])
k = next[k];
// extend the prefix
if (a[k + 1] == a[i])
++k;
next[i] = k;
}
// KMP pattern matching - same algorithm as for precomputing the next array
std::array<int, 1000> positions;
int len = 0;
for (int i = 0, q = -1; i < b.size(); ++i) {
#if ALG == 2
// S. Boyer and J. Moore 1974 improvement for large alphabets: we check
// first the last element of the pattern and if it does not match, we
// just skip by the length of the pattern; otherwise, we skip by the
// minimum amount that is consistent with the match.
if (q == -1) {
int pos = std::min(b.size() - 1, i + a.size() - 1);
if (marked[b[pos]] == -1) {
i = pos;
continue;
}
i = pos - marked[b[pos]];
}
// For smaller alphabets that can be stored in a small array of markers,
// we can do this check for each character in b, at each step of the
// loop
if (marked[b[i]] == -1) {
q = -1;
continue;
}
#endif
// if the prefix cannot be extended to match the current character
// in b, go back to next[q], ensuring that the prefix of a_q is
// still a suffix to b_{i-1} (because the next[q] step guarantees
// that a_next[q] will be a suffix of a_q, and thus b_i by
// transitivity)
while (q >= 0 && a[q + 1] != b[i])
q = next[q];
// extend the match
if (a[q + 1] == b[i])
++q;
// found a full match
if (q + 1 == a.size()) {
if (len < 1000)
positions[len] = i - a.size() + 1;
++len;
}
}
return { len, std::move(positions) };
}
#endif
int main()
{
try {
std::ios_base::sync_with_stdio(false);
std::cin.tie(nullptr);
std::ifstream in("strmatch.in");
std::ofstream out("strmatch.out");
std::string a, b;
in >> a >> b;
auto [len, positions] = strmatch(a, b);
out << len << '\n';
for (int i = 0; i < std::min(1000, len); ++i)
out << positions[i] << ' ';
}
catch (const std::exception& e) {
std::cerr << e.what();
return EXIT_FAILURE;
}
}