16-bytes ArrayList

mg979 · December 25, 2025, 12:16am

I made an ArrayList variant that uses internally a many-items pointer, and whose len and capacity are u32 instead of usize, so that the total size is 16 bytes where the normal ArrayList would be 24. Not sure if it’s a good idea/useful (I was planning to use it actually), but anyway.

This is the unmanaged ArrayList (from 0.15.2 std) for comparison. I removed unmanaged version (and relative tests) to make comparison easier and diff minimal between the two:

gist.github.com

https://gist.github.com/mg979/8d7a9e0e0a17ac7803db6e19e398e8ea

array_list.zig

const std = @import("std");
const builtin = @import("builtin");

const debug = std.debug;
const assert = debug.assert;
const testing = std.testing;
const mem = std.mem;
const math = std.math;
const Allocator = mem.Allocator;

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This is the 16-bytes version is here:

gist.github.com

https://gist.github.com/mg979/632800bd05941f879d92206418ac2c73

array_list16.zig

const std = @import("std");
const debug = std.debug;
const assert = debug.assert;
const testing = std.testing;
const mem = std.mem;
const math = std.math;
const Allocator = mem.Allocator;

// Note: here the main concern is that, being the underlying array
// a many-pointer array, it's not automatically bound-checked when indexed, and

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I called it ArrayList16, but probably it’s badly named because it will be smaller on 16 bit systems. I didn’t change documentation from the original (it still mostly applies).

I left some FIXME comments for stuff I should fix/work out better, but tests are passing.
I’d be grateful for opinions. The most problematic function seems print() because I’m using the deprecated writer() function.

About AI: I asked initial refactor, but scrapped entirely, only kept some helper functions, refactor is manual, function per function. I later used AI again to check errors/bound checks/assertions but that was it. Well the current print() function was suggested by AI.