Advent of Code http://adventofcode.com/
Glyphs and Warnings: This is how I write code I will throw away, this isn't how I write code I want to share with other people or to maintain myself.
The AoC challenge gives you a single fixed input so you don't have to write code that can parse a class of inputs that sorta look like the thing, just the one thing they give you (and just once correctly). Sameishly you don't have to allow a general set of limits, just the upper/lower bound of the one thing they give you. All that allows/demands a fast and loose style. If you want something more (but not a lot more) rigorous then check out Project Euler (https://projecteuler.net/). It has the same kinds of challenges but there are hundreds and they are repetitive - so writing more robust tests and utility functions will pay off in the long run.