Add comments to HashTable hash function.

Author: trekhleb

src/data-structures/hash-table/HashTable.js

@@ -25,6 +25,16 @@ export default class HashTable {
    * @return {number}
    */
   hash(key) {
+    // For simplicity reasons we will just use character codes sum of all characters of the key
+    // to calculate the hash.
+    //
+    // But you may also use more sophisticated approaches like polynomial string hash to reduce the
+    // number of collisions:
+    //
+    // hash = charCodeAt(0) * PRIME^(n-1) + charCodeAt(1) * PRIME^(n-2) + ... + charCodeAt(n-1)
+    //
+    // where charCodeAt(i) is the i-th character code of the key, n is the length of the key and
+    // PRIME is just any prime number like 31.
     const hash = Array.from(key).reduce(
       (hashAccumulator, keySymbol) => (hashAccumulator + keySymbol.charCodeAt(0)),
       0,