initial hashmap code

notes/hash-table.md

@@ -0,0 +1,19 @@
+# Hash Table
+Well, the moment has come. I never made a Hash Map algorithm, but in this scenario I have to save both fd and sockaddr (I could make a simply linked list, but it's a way to learn new things).
+
+Let's start with a little bit of theory.
+
+A *Hash Table* is a data structure also called **dictionary** or **map**. It maps *keys* to *values* thanks to a **hash function** that computes and *index* (**hash code**) into an array of **buckets**.
+
+One problem could be the *hash collision* where the hash function computes the same index for different values. A fix could be the **chaining** method, where for the same hash code you can make a linked list and append the index. Then the lookup function will go through the list and find the key.
+
+In a Hash Map you can insert, delete and lookup (simply search).
+
+### Hash function
+The easiest way... don't judge me.
+
+- Integer keys:
+$$ hash(\text{key}) = \text{key} \mod \text{table\_dim} $$
+
+- String keys:
+$$ hash(key) = \sum_{i=0}^{len(key) - 1} ascii\_value(key[i]) * prime\_number$$