# Encoding a 15bp DNA sequence into a shorter than 15 digits number

I have a series of 15bp DNA sequences from single-cell barcodes and I would like to encode them into a shorter than 15 digits number so that each unique 15bp DNA sequence will create a unique numeric value. What's the best way of achieving this?

What you are asking is called a "hash" (wiki).

In python, get your oligo as a string and use the standard library method hash(oligo), this is a number that gets used for a lot of operations. It is however always a 18+1 digit number in decimal, so a different solution is needed.

There are four nucleobases, so binary conversion from scratch is easy and best option and each solution will be unique and small. Each base takes up 2 bits and you concatenate these and convert them to hex or decimal —the length should be 10 decimal digits. One issue is when you have a oligo of a different length which start with adenosine or not.

One issue with this is the order. I am not sure where it comes from but the bases are nearly often expressed in the order ATGC. So...

mapping = {'A': '00', 'T': '01', 'G': '10', 'C': '11'}


It is satisfying how the first digit is the W/S, while the second is R/Y.

Now simply covert and add them compressed = functools.reduce(lambda acc,base : acc+mapping[base], oligo, '') and covert the string to decimal like so int(compressed, 2).

• The problem is hash collisions, there is no guarantee that the compression is unique – Chris_Rands Jun 19 '20 at 14:41
• Hash collision with the fake-hash would happen with two oligos that are the same but one has one or more extra adenosines. The oligo was said to be 15bp, so this does not apply. A workaround is to add a 1 at the start... – Matteo Ferla Jun 19 '20 at 15:00