Clean up Base* code

[Ericson2314]

Motivation

Make it separate from Hash, since other things can be base-encoded too.

This isn't really needed for Nix, but it makes the code easier to read e.g. for someone reimplementing this stuff in a different language. (Of course, Base16/Base64 should be gotten off-the-shelf, but now the hash code, which is more bespoke, is less cluttered with the parts that would be from some library.)

Many reimplementations of "Nix32" and our hash type already exist, so this cleanup is coming years too late, but I say better late than never / it is always good to nudge the code in the direction of being a "living spec".

Context

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[Mic92]

git-add?

       > In file included from ../ref-scan-bench.cc:3:
       > /nix/store/jbwx3hmahj4zihpdqxjmvdilsw12lv0b-nix-util-2.31.0pre-dev/include/nix/util/base-nix-32.hh:10:10: fatal error: 'nix/util/array-from-string-literal.hh' file not found
       >    10 | #include "nix/util/array-from-string-literal.hh"
       >       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       > 1 error generated.

[Copilot]

Pull Request Overview

This PR refactors base encoding functionality by extracting Base16/Base32/Base64 implementations from the hash module into dedicated base-n modules. The goal is to make the hash code less cluttered and create more reusable encoding utilities that can be used independently of hashing.

Key changes include:

• Extracted base64 encoding/decoding from util.cc into new base-n.cc module
• Moved base16 encoding logic from hash module to base-n.cc
• Enhanced base-nix-32 module with decode functionality
• Updated all call sites to use new namespaced functions (base64::encode, base16::encode, etc.)

Reviewed Changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/libutil/util.cc	Removed base64 encoding functions (moved to base-n.cc)
src/libutil/base-n.cc	New file containing base16 and base64 encoding/decoding implementations
src/libutil/include/nix/util/base-n.hh	New header defining base16 and base64 namespaces and function signatures
src/libutil/include/nix/util/array-from-string-literal.hh	New utility for converting string literals to arrays without null terminator
src/libutil/base-nix-32.cc	Added decode function and updated to use std::byte
src/libutil/include/nix/util/base-nix-32.hh	Updated signatures and added decode function declaration
src/libutil/hash.cc	Refactored to use new base-n functions and simplified hash parsing logic
src/libutil/include/nix/util/hash.hh	Removed base encoding length calculation methods
Multiple files	Updated includes and function calls to use new base-n namespaces

[xokdvium]

Can you also add a test where there are extra trailing padding = characters at the end? Both snix/lix ran into the fact that parser isn't entirely correct in that regard and nixpkgs relies on some of the broken behaviour.

[xokdvium]

Here's what I'm talking about: https://git.snix.dev/snix/snix/src/commit/2a29b90c7f3f3c52b5bdae50260fb0bd903c6b38/snix/nix-compat/src/nixhash/mod.rs#L431

[Ericson2314]

OK I think this is ready.

[Ericson2314]

OK, think this is ready now?

[xokdvium]

Reviewed with --color-moved. Most of the diff is just moving code around.