1 files changed, 54 insertions, 3 deletions
diff --git a/02-data_structures/03-hash_tables/03-hash_substring/hash_substring.cpp b/02-data_structures/03-hash_tables/03-hash_substring/hash_substring.cpp
index 3a3680a..6c9e457 100644
--- a/02-data_structures/03-hash_tables/03-hash_substring/hash_substring.cpp
+++ b/02-data_structures/03-hash_tables/03-hash_substring/hash_substring.cpp
@@ -1,5 +1,6 @@
 #include <iostream>
 #include <string>
+#include <cmath>
 #include <vector>
 
 using std::string;
@@ -15,6 +16,39 @@ Data read_input() {
     return data;
 }
 
+unsigned long long poly_hash(const string& s, size_t sz, unsigned long x, unsigned long p)
+{
+    unsigned long long h = 0;
+    /* std::cout << "hash " << x << " " << p << std::endl; */
+    for (size_t i = sz; i-- > 0; ) {
+        h = (h * x + s[i]) % p;
+        /* std::cout << " " << s[i] << " " << h << std::endl; */
+    }
+
+    return h;
+}
+
+std::vector<unsigned long long> precompute_hashes(const string& T, const string& P, unsigned long long p, unsigned long long x)
+{
+    std::vector<unsigned long long> H(T.size() - P.size() + 1);
+
+    H[H.size() - 1] = poly_hash(&T[H.size() - 1], P.size(), x, p);
+
+    unsigned long long y = 1;
+    for (size_t i = 1; i <= P.size(); i++)
+        y = (y * x) % p;
+    /* std::cout << "y : " << y << std::endl; */
+
+    for (size_t i = H.size() - 1; i-- > 0; )
+        H[i] = ((T[i] + x * H[i + 1] - (y * T[i + P.size()] % p)) + p) % p;
+
+    /* for (size_t i = 0; i < H.size(); i++) */
+    /*     std::cout << H[i] << " "; */
+    /* std::cout << std::endl; */
+
+    return H;
+}
+
 void print_occurrences(const std::vector<int>& output) {
     for (size_t i = 0; i < output.size(); ++i)
         std::cout << output[i] << " ";
@@ -23,10 +57,27 @@ void print_occurrences(const std::vector<int>& output) {
 
 std::vector<int> get_occurrences(const Data& input) {
     const string& s = input.pattern, t = input.text;
+
+    static const unsigned long x = 6;
+    static const unsigned long p = 1000000007;
+
+    unsigned long long h = poly_hash(s, s.size(), x, p);
+
+    auto hashes = precompute_hashes(t, s, p, x);
+
     std::vector<int> ans;
-    for (size_t i = 0; i + s.size() <= t.size(); ++i)
-        if (t.substr(i, s.size()) == s)
-            ans.push_back(i);
+    for (size_t i = 0; i < hashes.size(); i++)
+        if (hashes[i] == h) {
+            bool match = true;
+            for (size_t j = 0; j < s.size(); j++) {
+                if (s[j] != t[i + j]) {
+                    match = false;
+                    break;
+                }
+            }
+            if (match)
+                ans.push_back(i);
+        }
     return ans;
 }