## Introduction

At it’s most basic hashing is a technique to identify a particular value of a certain type among other values of the same type. To achieve this we typically make use of a hash function: a function that takes a value of the required type as input and generates a “key” that (hopefully) can uniquely identify this value. In this article we’ll look at how this function works, how it’s implemented in C++, and how to apply hashing to solve a Competitive Programming problem.

## Hashing in C++

C++ offers 2 containers in C++ that are often useful for hashing problems: map and unordered map. Both containers store elements in a “mapped fashion”: we associate a key value to a mapped value, typically (but not necessarily) of a different data type. Out of these two the unordered map is the one that is typically implemented using a traditional hash function whereas the map represents a balanced binary search tree.

### $std::map$

1. Implemented with red-black tree.
2. Elements are sorted.
3. Small memory usage.
4. O(log N) lookup time.


### $std::unordered$_$map$

1. Implemented with a hash function
2. Elements are not sorted.
3. Requires additional memory to keep the hash-table.
4. Lookup is usually O(1), dependent on hash function.


### Why use $std::map$ over $std::unordered$_$map$?

A quick look at the comparison between the two containers gives us the impression that we should use $std::unordered$_$map$ in most cases (except those where we require a sorted traversal through all keys), and we would be right to do so normally. After all, $O(1)$ lookup and insertion is a lot more appealing than the $O(log N)$ operations of $std::map$. However, there are a few things to keep in mind. Namely that the worst-case runtime of unordered_map is $O(N)$ not $O(1)$. This is because of how it is implemented. As we’ll see later, hash functions have a chance of “colliding”; that is, two distinct inputs to the hash function give the same output. This causes the unordered_map to store the associated values corresponding to these colliding keys in a list. In the worst case, when all keys collide, the lookup time for each key is $O(N)$.

For more:Reference 1 Reference 2

## A hash function

Let’s look at the case where we need to map a string to an integer: we need to take care of the distinct characters present in the string as well as their position in the string. A common strategy is to take the ASCII value of each character and multiply it by a prime p to the power of the index of the character. In order to ensure it fits in a C++ variable we typically take the hash mod a large prime M.

$$F(R) = hash(S[0,R]) = (\sum_{i=0}^{R}S_i * p^i)\% MOD$$

## Looking at a problem

Let’s try and apply the hashing technique to a competitive programming problem here. We are given a graph with a list of edges from $1 … M$. The queries are of the form $[L, R]$. For each query we look at the graph containing all the edges numbered from L to R. We need to determine whether all the connected components of such a graph have Euler cycles. The answer will then be “YES” if all vertex degrees are even, and “NO” otherwise. A naïve solution would be to store the parity of each vertex of the graph for all edges from $1 … X$, for all $X$ from $1$ to $M$. Using these prefix parity values we can say the answer for a range L to R is “YES” when $pref[R] = pref[L-1]$ and “NO” otherwise. Memory use here is $O(N^2)$ and query time is $O(N)$. This can be improved by storing each prefix parity value as a hash. In the above expression $S_i = 0$ if degree of $i^{th}$ vertex is even and 1 otherwise. This reduces memory use to $O(N)$ and query time to $O(1)$.