Merge pull request #263 from nirmalkatiyar/master

Added Z function

Author: master-fury

Pattern_Matching/src/Z_Function_Efficient_Approach.cpp

@@ -0,0 +1,56 @@
+/*For the sake of brevity, let's call segment matches those substrings that coincide with a prefix of s. For example, the value of the desired Z-function z[i] is the length of the segment match starting at position i (and that ends at position i+z[i]−1).
+To do this, we will keep the [l,r] indices of the rightmost segment match. That is, among all detected segments we will keep the one that ends rightmost. In a way, the index r can be seen as the "boundary" to which our string s has been scanned by the algorithm; everything beyond that point is not yet known.
+Then, if the current index (for which we have to compute the next value of the Z-function) is i, we have one of two options:
+i>r -- the current position is outside of what we have already processed.
+We will then compute z[i] with the trivial algorithm (that is, just comparing values one by one). Note that in the end, if z[i]>0, we'll have to update the indices of the rightmost segment, because it's guaranteed that the new r=i+z[i]−1 is better than the previous r.
+i≤r -- the current position is inside the current segment match [l,r].
+Then we can use the already calculated Z-values to "initialize" the value of z[i] to something (it sure is better than "starting from zero"), maybe even some big number.
+For this, we observe that the substrings s[l…r] and s[0…r−l] match. This means that as an initial approximation for z[i] we can take the value already computed for the corresponding segment s[0…r−l], and that is z[i−l].
+However, the value z[i−l] could be too large: when applied to position i it could exceed the index r. This is not allowed because we know nothing about the characters to the right of r: they may differ from those required.
+*/
+
+
+#include <bits/stdc++.h>
+using namespace std;
+//fuction to calculate z array/vector.
+//O(N+m) N,m are length of text and patterns respectively.
+vector<int> Z_array(string s)
+{
+
+    int n = s.size();
+    vector<int> z_array(n);
+    int ss = 0, es = 0;
+    // ss== start segment es== end segment 
+    //(ss and es is range of current segment which is prevoiusly occured/found.)
+    for (int i = 1; i < n; ++i)
+    {
+        if (i <= es)
+            z_array[i] = min(es - i + 1, z_array[i - ss]);
+        while (i + z_array[i] < n && s[z_array[i]] == s[i + z_array[i]])
+        //for maching characters we have to increase our range of segment and update the z_array
+            z_array[i]++;
+        if (i + z_array[i] - 1 > es)   
+            //if we have already processed then we have to use prevoius value
+            //for maintaining the condition l<=i<=r<=n ;
+            ss = i, es = i + z_array[i] - 1;
+    }
+    return z_array;
+}
+
+int main()
+{
+    string text, pat;
+    cin >> text >> pat;
+    string total = pat + "@" + text;
+    vector<int> output = Z_array(total); 
+    for (int i = 0; i < output.size(); i++)
+    {
+        if (output[i] == pat.size())
+        //To obtain the actual position of pattern in a given string.
+        //we have to subtract the pattern length +1(1 for special character)
+            cout << (i - pat.length() - 1) << " "; 
+            
+    }
+
+    return 0;
+}

Pattern_Matching/src/Z_function_Naive_Approach.cpp

@@ -0,0 +1,39 @@
+#include<bits/stdc++.h>
+using namespace std;
+//naive approach for finding Z  function array(i.e. z[i] is the length of the longest string that is, at the same time, a prefix of s and a prefix of the suffix of s starting at i.)
+// i-th element is equal to the greatest number of characters starting from the position i that coincide with the first characters of s.
+//O(n^2)
+vector<int>Z_function(string s){
+    int n = s.size();
+    //vector z store the  z  values 
+    //i.e.(i-th element is equal to the greatest number of characters starting from the position i that coincide with the first characters of s)
+    vector<int> z(n,0); 
+    //Traversing the whole string 
+    for (int i = 1; i < n; ++i)
+    //comparing the characters and incresing the range i.e. length of substring which previously exists 
+        while (i + z[i] < n && s[z[i]] == s[i + z[i]])
+            ++z[i];
+    return z;
+}
+
+int main(){
+    string text;
+    string pat;
+    cin>>text>>pat;
+    //making a temporary string total
+    string total=pat+"$"+text;
+    vector<int>OutputIndices=Z_function(total);
+    //Z_function will return the array/vector of indices ie
+    //i-th element is equal to the greatest number of characters starting from the position i that coincide with the first characters of temporary string total.
+    //if size of pattern matches with any value of vector returned by z_function it means our pattern found in temporary string 
+    // but to  get real index of pattern in given text we have to substract  pattern length and 1 (for special character) from the index of vector returned by Z_function.
+    for(auto i=0;i<OutputIndices.size();i++){
+        if(OutputIndices[i]==pat.size()){
+            
+            cout<<(i-pat.size()-1)<<" ";
+        }
+    }
+    
+    
+    return 0;
+}