How can I directly work with bits in Clojure?

Question

I began working through the first problem set over at https://cryptopals.com the other day. I'm trying to learn Clojure simultaneously, so I figured I'd implement all of the exercises in Clojure. These exercises are for learning purposes of course, but I'm going out of my way to not use any libraries besides clojure.core and the Java standard library. The first exercise asks you to write code that takes in a string encoded in hexadecimal and spit out a string encoded in base64. The algorithm for doing this is fairly straightforward:

1. Get the byte associated with each couplet of hex digits (for example, the hex 49 becomes 01001001 ).
2. Once all bytes for the hex string have been retrieved, turn the list of bytes into a sequence of individual bits.
3. For every 6 bits, return a base64 character (they're all represented as units of 6 bits).

I'm having trouble actually representing/working-with bits and bytes in Clojure (operating on raw bytes is one of the requirements of the exercise). I know I can do byte-array on the initial hex values and get back an array of bytes, but how do I access the raw bits so that I can translate from a series of bytes into a base64 encoded string? Any help or direction would be greatly appreciated.

Answer 1

Always keep a browser tab open to the Clojure CheatSheet .

For detailed bit work, you want functions like bit-and , bit-test , etc.

If you are just parsing a hex string, see java.lang.BigInteger withe the radix option: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/math/BigInteger.html#%3Cinit%3E(java.lang.String,int)

java.lang.Long/parse( string, radix ) is also useful.

For the base64 part, you may be interested in the tupelo.base64 functions . This library function is all you really need to convert a string of hex into a base-64 string, although it may not count for your homework!

Please note that Java includes base-64 functions:

https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/Base64.html

Remember, also, that you can get ideas by looking at the source code for both Clojure & the Tupelo lib.

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And also , keep in mind that one of Clojure's super-powers is the ability to write low-level or performance-critical code in native Java and then link all the *.clj and *.java files together into one program (you can use Leiningen to compile & link everything in one step).