While doing Advent of Code, when I tried to use `StaticBitSet`

, I was surprised it was missing a function to determine if one `StaticBitSet`

is the subset of another.

My solution required to check if a pair of sets were either a subset of one or the other. I.E. given sets `a`

, `b`

: `a ⊂ b or b ⊂ a`

.

Using existing `StaticBitSet`

functionality, I implemented this as:

```
const a: std.StaticBitSet...
const b: std.StaticBitSet...
var ab_intersection = a;
ab_intersection.setIntersection(b);
const either_subset_of = ab_intersection.mask == a.mask or
ab_intersection.mask == b.mask;
```

The next day I set out to see how difficult it would be to make a contribution to `std.StaticBitSet`

to add `subsetOf`

. It turns out the process is pretty simple and the actual experience was amazing.

The `std`

is well organized and `zig test lib/std/bit_set.zig`

just works.

I was able hammer out a complete pull request with full documentation and tests.

I decided to add a series of pure functions:

```
fn eql(self: Self, other: Self) bool
fn subsetOf(self: Self, other: Self) bool
fn supersetOf(self: Self, other: Self) bool
fn complement(self: Self) Self
fn unionWith(self: Self, other: Self) Self
fn intersectWith(self: Self, other: Self) Self
fn xorWith(self: Self, other: Self) Self
fn differenceWith(self: Self, other: Self) Self
```

Aside: I couldn't use the fn name

`union`

as that is a keyword. For consistency I decided to append`With`

for all the set operators.

Since `std.StaticBitSet`

is `comptime`

, there are far fewer edge cases to handle. In other languages to implement `eql`

one would need to ensure the `bit_length`

of the two sets are the same, but since `bit_length`

is `comptime`

if one tried to write `std.StaticBitSet(1).initFull().eql(std.StaticBitSet(2).initFull())`

, one would get a compiler error instead of a run-time error!

With these new functions, my original subset problem is simplified down to:

```
const a: std.StaticBitSet...
const b: std.StaticBitSet...
const either_subset_of = a.subsetOf(b) or b.subsetOf(a);
```

## Discussion (2)

Nice post, and glad to know that contributing to std is such a smooth experience.

Just a small nitpick: do you really need to compute the set intersection twice to perform the required test? It seems to me that computing it once and than comparing with both a and b could be enough.

Good point. I blindly inlined the implementation of

`subsetOf`

, but you are right. I'll fix it.