Initial commit

c++/Others/String matching in linear-time.cpp

@@ -0,0 +1,114 @@
+String matching in linear-time
+
+#include <iostream>
+#include <fstream>
+#include <string>
+#include <vector>
+
+typedef std::vector<int> int_vec;
+
+class string_search
+{
+  int_vec shifts;
+  void compute_shifts(const std::string &pattern);
+public:
+  int find_first(const std::string &text, const std::string &pattern);
+  int_vec find_all(const std::string &text, const std::string &pattern);
+
+};
+
+// create the shift-lookup-table
+void string_search::compute_shifts(const std::string &pattern)
+{
+  int next_shift = 0;
+  shifts.clear();
+  shifts.push_back(0); // shift to the first character
+
+  // start with the second character, since the shift to the first is always 0
+  for(int i = 1; i < pattern.length(); i++)
+  {
+    while(next_shift > 0 && pattern[next_shift] != pattern[i])
+      next_shift = shifts[next_shift];
+
+    if(pattern[next_shift] == pattern[i])
+      next_shift++;
+
+    shifts.push_back(next_shift);
+  }
+}
+
+// search the string and return when the first occurrence is found
+int
+string_search::find_first(const std::string &text, const std::string &pattern)
+{
+  int next_shift = 0;
+  compute_shifts(pattern);
+  for(int i = 0; i < text.length(); i++)
+  {
+    while(next_shift > 0 && pattern[next_shift] != text[i])
+      next_shift = shifts[next_shift - 1];
+
+    if(pattern[next_shift] == text[i])
+      next_shift++;
+
+    if(next_shift == pattern.length())
+      return i - (pattern.length() - 1); // found the first so return
+  }
+  return -1;
+}
+
+// search the string and put every occurence in a vector
+int_vec
+string_search::find_all(const std::string &text, const std::string &pattern)
+{
+  int next_shift = 0;
+  int_vec positions;
+  compute_shifts(pattern);
+  for(int i = 0; i < text.length(); i++)
+  {
+    while(next_shift > 0 && pattern[next_shift] != text[i])
+      next_shift = shifts[next_shift - 1];
+
+    if(pattern[next_shift] == text[i])
+      next_shift++;
+
+    if(next_shift == pattern.length())
+    {
+      positions.push_back(i - (pattern.length() - 1)); // found one, put in list
+      next_shift = shifts[next_shift - 1]; // restart pattern with last shift
+    }
+  }
+  return positions;
+}
+
+int main(int argc, char **argv)
+{
+  if(argc <= 2){
+    cout << "Usage: " << argv[0] << " filename searchpattern" << endl;
+    return 0;
+  }
+  std::string pattern = argv[2];
+
+  // read the file. Since the file is read like this all white-characters
+  // are eaten, so a search including white-characters will fail...
+  fstream fs;
+  std::string text, temp;
+  fs.open(argv[1], ios::in);
+  while(!fs.eof()){
+    fs >> temp;
+    text += temp;
+  }
+  fs.close();
+
+  // search the file
+  string_search search;
+  int_vec pos_list = search.find_all(text, pattern);
+
+  // print out result
+  std::vector<int>::iterator it;
+  cout << "Found " << pos_list.size() << " occurrences" << endl;
+  for(it = pos_list.begin(); it != pos_list.end(); it++){
+    temp = text.substr(*it, pattern.length());
+    cout << "Pos=" << *it << " == " << temp << endl;
+  }
+}