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AhoCorasick.cc
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AhoCorasick.cc
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#include <vector>
#include <string>
#include <unordered_map>
#include <queue>
#include <iostream>
using namespace std;
// BEGIN
// An implementation of Aho-Corasick dictionary matching algorithm
// Taken directly from the paper
// See also: https://en.wikipedia.org/wiki/Aho-Corasick_algorithm
typedef vector<size_t> VI;
typedef vector<string> VS;
struct node {
unordered_map<char, node*> g;
node* f; // pointer to node with largest strict suffix of current node in automaton
node* output = NULL; // pointer to dictionary suffix, the next node with isWord=true by
// following f pointers
bool isWord = false;
size_t num;
node(size_t num) : num(num) {}
node() {}
};
void enter(string S, node *root, size_t &n) {
node *state = root;
size_t j = 0;
while (state->g.count(S[j]) != 0) {
state = state->g[S[j]];
++j;
}
for (; j < S.size(); ++j) {
state->g[S[j]] = new node(n++);
state = state->g[S[j]];
}
state->isWord = true;
}
void construct_f(node *root) {
queue<node*> q;
for (auto it = root->g.begin(); it != root->g.end(); ++it) {
q.push(it->second);
it->second->f = root;
}
while (!q.empty()) {
node *r = q.front(); q.pop();
for (auto it = r->g.begin(); it != r->g.end(); ++it) {
q.push(it->second);
node *state = r->f;
while (state->g.count(it->first) == 0 && state->num != 0)
state = state->f;
if (state->g.count(it->first) != 0)
it->second->f = state->g[it->first];
else
it->second->f = root;
if (it->second->f->isWord)
it->second->output = it->second->f;
else // may assign NULL pointer
it->second->output = it->second->f->output;
}
}
}
// creates Aho-Corasick automaton from a dictionary
node* ConstructAutomaton(VS &dictionary) {
size_t n = 0;
node *root = new node(n++);
for (size_t i = 0; i < dictionary.size(); ++i) {
enter(dictionary[i], root, n);
}
construct_f(root);
return root;
}
// returns a pointer to the next node after the character c is encountered
node* advance(node *state, char c) {
while (state->g.count(c) == 0 && state->num != 0) state = state->f;
if (state->g.count(c) != 0)
state = state->g[c];
return state;
}
// prints all matches of dictionary words in the given query string
void PrintMatches(node *root, string x) {
node *state = root;
for (size_t i = 0; i < x.size(); ++i) {
state = advance(state, x[i]);
node *out = state;
while (true) {
if (out->isWord) cout << i << " " << out->num << " " << endl;
if (out->output == NULL) break;
out = out->output;
}
}
}
// END
string randString(size_t length) {
string S(length, '?');
for (size_t i = 0; i < length; ++i) {
S[i] = 'a' + rand()%26;
}
return S;
}
int main() {
srand(0);
{
VS dictionary = {"a", "ab", "bab", "bc", "bca", "c", "caa"};
node *root = ConstructAutomaton(dictionary);
cout << "Expected:\n0 1\n1 2\n2 6\n2 8\n3 8\n4 1\n5 2" << endl;
cout << "Actual:" << endl;
PrintMatches(root, "abccab");
}
{
VS dictionary;
for (size_t i = 0; i < 10000; ++i) {
dictionary.push_back(randString(1000));
}
cout << "Speed Test" << endl;
cout << "Constructing Automaton..." << endl;
node *root = ConstructAutomaton(dictionary);
cout << "Complete. Querying..." << endl;
for (size_t i = 0; i < 10; ++i) {
PrintMatches(root, randString(1000000));
}
cout << "Complete" << endl;
}
// code has been successfully tested on SWERC '16 "Passwords"
}