bitReverse

    pub fn bitReverse(r: *Mutable, a: Const, signedness: Signedness, bit_count: usize) void {
        if (bit_count == 0) {
            r.limbs[0] = 0;
            r.len = 1;
            r.positive = true;
            return;
        }

        r.copy(a);

        const limbs_required = calcTwosCompLimbCount(bit_count);

        if (!a.positive) {
            r.positive = true; // Negate.
            r.bitNotWrap(r.toConst(), .unsigned, bit_count); // Bitwise NOT.
            r.addScalar(r.toConst(), 1); // Add one.
        } else if (limbs_required > a.limbs.len) {
            // Zero-extend to our output length
            for (r.limbs[a.limbs.len..limbs_required]) |*limb| {
                limb.* = 0;
            }
            r.len = limbs_required;
        }

        // 0b0..01..1000 with @log2(@sizeOf(Limb)) consecutive ones
        const endian_mask: usize = (@sizeOf(Limb) - 1) << 3;

        const bytes = std.mem.sliceAsBytes(r.limbs);

        var k: usize = 0;
        while (k < ((bit_count + 1) / 2)) : (k += 1) {
            var i = k;
            var rev_i = bit_count - i - 1;

            // This "endian mask" remaps a low (LE) byte to the corresponding high
            // (BE) byte in the Limb, without changing which limbs we are indexing
            if (native_endian == .big) {
                i ^= endian_mask;
                rev_i ^= endian_mask;
            }

            const bit_i = std.mem.readPackedInt(u1, bytes, i, .little);
            const bit_rev_i = std.mem.readPackedInt(u1, bytes, rev_i, .little);
            std.mem.writePackedInt(u1, bytes, i, bit_rev_i, .little);
            std.mem.writePackedInt(u1, bytes, rev_i, bit_i, .little);
        }

        // Calculate signed-magnitude representation for output
        if (signedness == .signed) {
            const last_bit = switch (native_endian) {
                .little => std.mem.readPackedInt(u1, bytes, bit_count - 1, .little),
                .big => std.mem.readPackedInt(u1, bytes, (bit_count - 1) ^ endian_mask, .little),
            };
            if (last_bit == 1) {
                r.bitNotWrap(r.toConst(), .unsigned, bit_count); // Bitwise NOT.
                r.addScalar(r.toConst(), 1); // Add one.
                r.positive = false; // Negate.
            }
        }
        r.normalize(r.len);
    }