Improve the hash func in C++ hash table

Author: rp-code9

Operating-System-Concepts/hash-table/hash-table-using-stl/hashmap.h

@@ -10,20 +10,20 @@
 #include <iostream>
 #include <cstddef>
 
-template<typename K, std::size_t size>
+template<typename K>
 struct KeyHash
 {
-    unsigned long operator() (const K& key)
+    unsigned long operator() (const K& key, size_t tableSize)
     {
-        return reinterpret_cast<unsigned long>(key) % size;
+        return reinterpret_cast<unsigned long>(key) % tableSize;
     }
 };
 
-template<typename K, typename V, std::size_t size = 1024, typename F = KeyHash<K, size>>
+template<typename K, typename V, std::size_t tableSize = 1024, typename F = KeyHash<K>>
 class HashMap
 {
 private:
-    std::array<std::vector<HashNode<K, V>>, size> table { };
+    std::array<std::vector<HashNode<K, V>>, tableSize> table { };
     F hashFunc;
 
 public:
@@ -32,8 +32,9 @@ class HashMap
 
     void put(const K& key, const V& value)
     {
-        unsigned long hash = hashFunc(key);
+        unsigned long hash = hashFunc(key, tableSize);
 
+        std::cout << key << " , " << value << " , " << hash << "\n";
         for (auto node : table[hash]) {
             if (node.getKey() == key) {
                 // key already present in map. Simply overwrite value
@@ -49,7 +50,7 @@ class HashMap
 
     void remove(const K& key)
     {
-        unsigned long hash = hashFunc(key);
+        unsigned long hash = hashFunc(key, tableSize);
 
         if (table[hash].size() == 1) {
             if (table[hash][0].getKey() == key) {
@@ -67,7 +68,7 @@ class HashMap
 
     bool get(const K& key, V& value)
     {
-        unsigned long hash = hashFunc(key);
+        unsigned long hash = hashFunc(key, tableSize);
 
         for (auto node : table[hash]) {
             if (node.getKey() == key) {

Operating-System-Concepts/hash-table/hash-table-using-stl/test.cpp

@@ -13,24 +13,30 @@
 #include <string>
 #include <cstddef>
 
-struct MyKeyHash {
-    unsigned long operator()(const int& k) const
+struct StringKeyHash {
+    unsigned long operator()(const std::string& key, size_t tableSize) const
     {
-        return k % 10;
+        unsigned long hash = 5381;
+
+        for (auto ch : key) {
+            hash = ((hash << 5) + hash) + static_cast<int>(ch);
+        }
+
+        return hash % tableSize;
     }
 };
 
 int main()
 {
-    HashMap<int, std::string, 10, MyKeyHash> map;
-    map.put(1, "alice");
-    map.put(2, "bob");
-    map.put(3, "charlie");
-    map.put(4, "eve");
+    HashMap<std::string, std::string, 1024, StringKeyHash> map;
+    map.put("abc", "alice");
+    map.put("xyz", "bob");
+    map.put("mno", "charlie");
+    map.put("pqr", "eve");
 
-    map.remove(4);
+    map.remove("pqr");
     std::string  value;
-    map.get(3, value);
+    map.get("abc", value);
     std::cout << value << "\n";
 
     return 0;