Deterministic Shuffling Using Hashes

Whenever I create my yearly reading list, I need a way to order the books. Sure, I could just shuffle them normally, but that leads me to the temptation of cheating and re-shuffling them so that the books I want to read most are first. What I really need is a shuffle that will shuffle the same way every time.

Enter: hashsort

The basic idea is to take a list of items, hash each of them, and sort them based on the hash values.

import fileinput

inputs = set()

for line in fileinput.input():
    line = line.strip()
    if line:
        inputs.add(line)

hashed = {(hash(line), line) for line in inputs}

for _, line in sorted(hashed):
    print(line)

Unfortunately, the built in hash function isn’t deterministic (I think it has something to do with memory addresses?):

$ python3 -c 'print(hash("test"))'

7048182159058774479

$ python3 -c 'print(hash("test"))'

7534055093572699423

So we need a custom hash function. While we’re at it, let’s deal with multiple possibly input types:

import hashlib

def hash(line):
    if isinstance(line, bytes):
        pass
    elif isinstance(line, str):
        line = line.encode()
    else:
        line = repr(line).encode()

    hasher = hashlib.md5()
    hasher.update(line)
    return hasher.hexdigest()

That always uses hashlib.md5 to sort the lists. The md5 hash has some issues from a security perspective, but for our purposes here it works just fine. Let’s try it out:

$ cat fruit

Apple
Banana
Cherry
Dragonfrut
Elderberry
Fig
Grape
Huckleberry
Kiwi

$ hashsort fruit

Grape
Elderberry
Fig
Huckleberry
Dragonfrut
Cherry
Apple
Banana
Kiwi

And no matter how many times you run it with the same input, you’ll get the same values. Even better, if you add more values, they won’t change the existing order, they’ll just be inserted somewhere in the list:

$ echo 'Lime' >> fruit

$ echo 'Mango' >> fruit

$ hashsort fruit

Grape
Elderberry
Mango
Fig
Huckleberry
Dragonfrut
Cherry
Lime
Apple
Banana
Kiwi

That’s pretty cool.

But what if I still want to cheat and have some little control over the ‘random’ ordering? Well, hashlib has a small pile of hash functions. Let’s add the ability to list what’s available and choose one:

import argparse
import hashlib
import sys

algorithms = {algorithm.lower() for algorithm in hashlib.algorithms_available}

parser = argparse.ArgumentParser()
parser.add_argument('--hash', default = 'sha256', choices = algorithms, help = 'Hash to set, any hash in hashlib can be used')
parser.add_argument('--list', action = 'store_true', help = 'Display available hashes and exit')
parser.add_argument('files', metavar='FILE', nargs='*', help='Files to read, if empty use stdin')
args = parser.parse_args()

if args.list:
    for algorithm in sorted(algorithms):
        print(algorithm)
    sys.exit(0)

argparse is pretty wonderful when it comes to creating simple UNIX style applications. In this case, we’ll generate the list of algorithms using hashlib.algorithms_available and both give them as an argparse choices parameter (which will warn the user if they specify one not on the list) and also as a separate --list parameter, which lets us do this (I use the Fish shell):

$ for hash in (hashsort --list)
      echo === $hash ===
      hashsort --hash $hash fruit | head -n 2
      echo
  end

=== blake2b ===
Mango
Kiwi

=== blake2s ===
Fig
Elderberry
...

Pretty cool. 😄

If you’d like to see the full source, it’s part of my dotfiles on GitHub: hashsort

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