alloc

    fn alloc(gpa: Allocator, new_bit_index: u8) Allocator.Error!*IndexHeader {
        const len = @as(usize, 1) << @as(math.Log2Int(usize), @intCast(new_bit_index));
        const index_size = hash_map.capacityIndexSize(new_bit_index);
        const nbytes = @sizeOf(IndexHeader) + index_size * len;
        const bytes = try gpa.alignedAlloc(u8, .of(IndexHeader), nbytes);
        @memset(bytes[@sizeOf(IndexHeader)..], 0xff);
        const result: *IndexHeader = @ptrCast(@alignCast(bytes.ptr));
        result.* = .{
            .bit_index = new_bit_index,
        };
        return result;
    }