feat: added z_algorithm in strings

strings/z_function.cpp

@@ -0,0 +1,79 @@
+/**
+ * @file
+ * @brief The [Z function](https://cp-algorithms.com/string/z-function.html) for finding occurences of a pattern 
+ * within a piece of text with time and space complexity O(n + m)
+ * @details 
+ * DESCRIPTION
+ * 1. The Z-function for a string is an array of length n where the 
+ * i-th element is equal to the greatest number of characters starting 
+ * from the position i that coincide with the first characters of s.
+ * 2. Eg string : ababb then z[2]=2 as s[2]=s[0] and s[3]=s[1] and s[4]!=s[2]
+ * @author [Ritika Gupta](https://github.com/RitikaGupta8734)
+ */
+
+#include <iostream>
+#ifdef _MSC_VER
+#include <string>  // use this for MS Visual C++
+#else
+#include <cstring>
+#endif
+#include <vector>
+# include <cassert>
+
+/**
+ * Generate the Z-function for the inputted string.
+ * \param[in] pattern text on which to apply the Z-function
+ * \returns the Z-function output as a vector array
+ */
+std::vector<int> Z_function(const std::string &pattern) {
+    int pattern_length = pattern.size();
+    std::vector<int> z(pattern_length,0);
+
+    for (int i = 1,l = 0,r = 0; i < pattern_length; i++) {
+        if(i<=r)z[i]=std::min(r-i+1,z[i-l]);
+        while(i+z[i]<pattern_length && pattern[z[i]]==pattern[i+z[i]]) z[i]++;
+        if(i+z[i]-1>r)r=i+z[i]-1;
+    }
+    return z;
+}
+
+/**
+ * Using Z_function to find a pattern in a text
+ * \param[in] pattern string pattern to search
+ * \param[in] text text in which to search
+ * \returns a vector of starting indexes where pattern is found in the text 
+ */
+std::vector<int> find_pat_in_text(const std::string &pattern, const std::string &text) {
+    int text_length = text.size(), pattern_length = pattern.size();
+    std::vector<int> z = Z_function(pattern+'#'+text);
+    std::vector<int> matching_indexes;
+
+    for (int i = 0; i < text_length; i++) {
+        if(z[i+pattern_length+1]==pattern_length) matching_indexes.push_back(i);   
+    }
+    return matching_indexes;
+}  
+
+void testing(){
+
+    // usual case
+    std::string text1 = "alskfjaldsabc1abc1abcbksbcdnsdabcabc";
+    std::string pattern1 = "abc";
+
+    std::vector<int> matching_indexes1=find_pat_in_text(pattern1,text1);
+    assert((matching_indexes1 == std::vector<int>{10,14,18,30,33}));
+
+    // corner case
+    std::string text2 = "greengrass";
+    std::string pattern2 = "abc";
+
+    std::vector<int> matching_indexes2=find_pat_in_text(pattern2,text2);
+    assert((matching_indexes2 == std::vector<int>{}));
+}
+
+/** Main function */
+int main() {
+
+    testing();  
+    return 0;
+}