Added Horspool algorithm (#1071)

* feat: add Horspool algorithm

* Fixed conversion from unsigned long to int with static casting

* Removed changes from CMakeLists.txt file

* Update strings/horspool.cpp
Co-authored-by: NDavid Leal <halfpacho@gmail.com>

* Update strings/horspool.cpp
Co-authored-by: NDavid Leal <halfpacho@gmail.com>

* Documentation, test function and namespaces added

* Documentation, test function and namespaces added

* Update strings/horspool.cpp
Co-authored-by: NDavid Leal <halfpacho@gmail.com>

* Update strings/horspool.cpp
Co-authored-by: NDavid Leal <halfpacho@gmail.com>

* Update strings/horspool.cpp
Co-authored-by: NDavid Leal <halfpacho@gmail.com>

* Update strings/horspool.cpp
Co-authored-by: NDavid Leal <halfpacho@gmail.com>

* Update strings/horspool.cpp
Co-authored-by: NDavid Leal <halfpacho@gmail.com>

* Update strings/horspool.cpp
Co-authored-by: NDavid Leal <halfpacho@gmail.com>

* Removed blank rows

* Commit to see if the problem with automated tests is solved
Co-authored-by: NDavid Leal <halfpacho@gmail.com>

strings/horspool.cpp

+/**
+ * @file
+ * @brief Horspool's algorithm that finds if a string contains a substring (https://en.wikipedia.org/wiki/Boyer%E2%80%93Moore%E2%80%93Horspool_algorithm)
+ * @author [Harry Kontakis](https://github.com/ckontakis)
+ */
+
+#include <iostream>
+#include <unordered_map>
+#include <cassert>
+
+/**
+ * @namespace strings
+ * @brief Algorithms with strings
+ */
+namespace strings {
+/**
+ * @namespace horspool
+ * @brief Functions for [Horspool's](https://en.wikipedia.org/wiki/Boyer%E2%80%93Moore%E2%80%93Horspool_algorithm) algorithm
+ */
+namespace horspool {
+/**
+ * A function that finds the shift table of the given prototype string that we need in Horpool's algorithm.
+ * @param prototype is the substring that we use to find shift table
+ * @return Shift Table of Horspool's algorithm
+ */
+std::unordered_map<char, int> findShiftTable(const std::string &prototype) {
+    std::unordered_map<char, int>
+        shiftTable;  // A HashMap for shift table that has characters for keys and integers for values
+
+    for (int i = 0; i < prototype.size();
+         i++) {  // Checking all characters of prototype string
+        if (shiftTable.find(prototype[i]) ==
+            shiftTable.end()) {  // If character does not exist in HashMap
+            if (i != prototype.size() - 1) {
+                shiftTable.insert(std::make_pair(
+                    prototype[i], prototype.size() - i -
+                                      1));  // Insert the character as key and the size of prototype string - index of character - 1 as value
+            } else {
+                shiftTable.insert(std::make_pair(
+                    prototype[i],
+                    prototype.size()));  // Insert the character as key and the size of prototype string as value
+            }
+        } else {
+            if (i != prototype.size() - 1) {
+                shiftTable[prototype[i]] = prototype.size() - i - 1;
+            }
+        }
+    }
+    return shiftTable;
+}
+
+/**
+ * A function that implements Horspool's algorithm.
+ * @param text is the string that we are searching if there is a substring
+ * @param prototype is the substring that we are searching in text
+ * @returns true if text string contains prototype string
+ * @returns false if text string does not contain prototype string
+ */
+bool horspool(const std::string &text, const std::string &prototype) {
+    std::unordered_map<char, int> shiftTable = findShiftTable(
+        prototype);  // Initialise shift table calling findShiftTable function
+
+    int i = static_cast<int>(
+        prototype.size() -
+        1);  // Index that we shift in text to find the substring
+    while (i < text.size()) {
+        int j = i, k = 0;
+        bool flag = true;
+
+        for (int z = static_cast<int>(prototype.size() - 1); z >= 0 && flag;
+             z--) {  // Checking if all characters of substring are equal with all characters of string
+            if (text[j] == prototype[z]) {
+                k++;
+                j--;
+            } else {
+                flag = false;  // If two characters are not equal set flag to false and break from loop
+            }
+        }
+
+        if (k ==
+            prototype.size()) {  // If all characters match then return true
+            return true;
+        } else {
+            if (shiftTable.find(text[i]) != shiftTable.end()) {
+                i += shiftTable[text[i]];  // If shift table contains the character then shift index as many steps as value
+            } else {
+                i += prototype.size();  // If character does not exist in shift table then shift index as many steps as size of prototype string
+            }
+        }
+    }
+    return false;
+}
+} // namespace horspool
+} // namespace strings
+
+/**
+ * @brief Function with test cases for Horspool's algorithm
+ * @returns void
+ */
+static void test(){
+    assert(strings::horspool::horspool("Hello World","World") == true);
+    assert(strings::horspool::horspool("Hello World"," World") == true);
+    assert(strings::horspool::horspool("Hello World","ello") == true);
+    assert(strings::horspool::horspool("Hello World","rld") == true);
+    assert(strings::horspool::horspool("Hello","Helo") == false);
+    assert(strings::horspool::horspool("c++_algorithms","c++_algorithms") == true);
+    assert(strings::horspool::horspool("c++_algorithms","c++_") == true);
+    assert(strings::horspool::horspool("Hello","Hello World") == false);
+    assert(strings::horspool::horspool("c++_algorithms","") == false);
+    assert(strings::horspool::horspool("c++","c") == true);
+    assert(strings::horspool::horspool("3458934793","4793") == true);
+    assert(strings::horspool::horspool("3458934793","123") == false);
+}
+
+/**
+ * @brief Main Function that calls test function
+ * @returns 0 on exit
+ */
+int main(){
+    test();
+    return 0;
+}