Bitwise Fun with Elixir

For most web developers, bitwise operations are not very familiar, but in some situations they can be quite useful and they are by nature super performant. In this talk, I'll try to lead the audience to have some fun with bitwise operations in Elixir.
Presented at Elixir Australia December 2020 Meetup:
https://www.meetup.com/elixir-sydney/events/zrlnzrybcqbvb/

Recording video available at https://www.youtube.com/watch?v=NR_Jk3yUEqc

Transcript

1. Bitwise Fun with Elixir by Qing Wu
2. Why do I want to know these? 1. Useful • Bit fields • Communications Protocols • Embedded systems • Something fun 2. Performance
3. FlagShihTzu: Bit fields for ActiveRecord class Subscription < ActiveRecord::Base include FlagShihTzu ... has_flags 1 => :requires_shipping, 2 => :signup_only, 3 => :recurring, ... 20 => :za_3p, 21 => :combined, 22 => :tablet, ... end
4. Communications Protocols
5. Embedded systems
6. Retro Games
7. Basic bitwise operations • or • and • not • shift
8. Basic bitwise operations: or x y x or y 1 1 1 0 1 1 1 0 1 0 0 0
9. Basic bitwise operations: or property use Bitwise, only_operators: true # x is a bit x ||| 1 = 1 x ||| 0 = x
10. Basic bitwise operations: and x y x and y 1 1 1 1 0 0 0 1 0 0 0 0
11. Basic bitwise operations: and property # x is a bit x &&& 0 = 0 x &&& 1 = x
12. Basic bitwise operations: not ~~~0b1011_0100 # -> 0b0100_1011
13. Basic bitwise operations: shift # shift to the left by 1 0b0000_0001 <<< 1 # -> 0b0000_0010 # shift to the right by 4 0b0101_0000 >>> 4 # -> 0b0000_0101
14. Common Bitwise Operation Patterns • set • clear • read
15. Set fixed bit • Set second bit of 0b1011_0100
16. Set fixed bit x y x or y 1 0 1 0 0 0 1 0 1 1 0 1 0 0 0 1 0 1 0 1 1 0 0 0
17. Set fixed bit 0b1011_0100 ||| 0b0000_0010 # -> 0b1011_0110 # or 0b1011_0100 ||| (1 <<< 1) # -> 0b1011_0110
18. Set nth bit • Set nth bit of 0b1011_0100 0b1011_0100 ||| (1 <<< (n - 1))
19. Set nth to mth bits Set 2th to 4th bits of 0b1001_0110 0b1001_0110 ||| (0b111 <<< 1) # -> 0b1001_1110
20. Clear fixed bit Clear third bit of 0b1011_0100
21. Clear fixed bit x y x and y 1 1 1 0 1 0 1 1 1 1 1 1 0 1 0 1 0 0 0 1 0 0 1 0
22. Clear fixed bit 0b1011_0100 &&& 0b1111_1011 # -> 0b1011_0000 # or 0b1011_0100 &&& ~~~(1 <<< 2) # -> 0b1011_0000
23. Clear nth bit Clear nth bit of 0b1011_0100 0b1011_0100 &&& ~~~(1 <<< (n - 1))
24. Clear nth to mth bits Clear highest three bits of 0b1010_0101 0b1010_0101 &&& ~~~(0b111 <<< 5) # -> 0b0000_0101
25. Read fixed bit Read 4th bit from 0b1010_0101 0b1011_0100 &&& 0b0000_1000 > 0
26. Read nth bit Read nth bit from 0b1010_0101 0b1011_0100 &&& (1 <<< (n - 1)) > 0
27. Read a group of bits Read 5rd to 8th bits from 0b1010_0101 (0b1010_0101 &&& 0b1111_0000) >>> 4 # -> 0b1010
28. Exercism
29. Secret Handshake Determine the actions of a secret handshake based on the binary representation of the given code. If the following bits are set, include the corresponding action in your list of commands, in order from lowest to highest. 1 = wink 10 = double blink 100 = close your eyes 1000 = jump 10000 = Reverse the order of the operations in the secret handshake
30. Is the Bit Set? # code is the number we are given (code &&& 0b1) > 0 (code &&& 0b10) > 0 (code &&& 0b100) > 0 ...
31. Secret Handshake Solution defmodule SecretHandshake do use Bitwise, only_operators: true @actions %{ 0b1 => "wink", 0b10 => "double blink", 0b100 => "close your eyes", 0b1000 => "jump", 0b10000 => nil } defguard bit_set?(code, mask) when (code &&& mask) > 0 @spec commands(code :: integer) :: list(String.t()) def commands(code) do Enum.reduce(@actions, [], fn {0b10000, _}, acc when bit_set?(code, 0b10000) -> acc {0b10000, _}, acc -> Enum.reverse(acc) {mask, action}, acc when bit_set?(code, mask) -> [action | acc] _, acc -> acc end) end end
32. Summery • common use cases • basic bitwise operations • usage patterns • Exercism challenge
33. References Elixir Bitwise: https://hexdocs.pm/elixir/Bitwise.html Writing NES Games With Assembly: https://www.youtube.com/ watch?v=kXbMCKMJXXQ&feature=youtu.be&t=484 NES Emulator Part #1: Bitwise Basics & Overview: https:// www.youtube.com/watch?v=F8kx56OZQhg