WebAssembly Wizardry

Tiny programming challenges designed for learning about WebAssembly by writing it by hand.

Each challenge has a .wat file where you'll need to implement a WebAssembly module to pass the tests.

Run node tests.mjs to run the tests and to see the recommended order for solving the challenges.

The challenges have descriptions within their .wat files, but it some cases it will also be useful to look inside tests.mjs to better understand the different test cases.

Remove the ;; SOLVE comment when you're ready to test your solution.

If you get stuck or want to compare notes check out my solutions branch.

These challenges assume you have a programming background and are comfortable with some low-level concepts such as binary representations, bitwise operations, memory and pointers.

Dependencies

To run the tests and compile .wat to .wasm, you'll need the following dependencies.

• node
• wabt

Resources

The challenges aren't designed to teach you WAT syntax or WebAssembly instructions directly, so you'll probably need to do some reading alongside.

• Understanding WebAssembly text format
• WebAssembly instruction reference
• WebAssembly specification

Whilst not technically a reference, I found the WebAssembly core test suite incredibly helpful for some exhaustive learning about the various instructions. The files are .wast and they include some testing directives, but otherwise they are all valid .wat.

Debugging

Every challenge can import functions for logging debug values at runtime.

(module
  ;; Add any of the following imports to your .wat module
  (import "debug" "i32" (func $debug_i32 (param i32)))
  (import "debug" "i64" (func $debug_i64 (param i64)))
  (import "debug" "f32" (func $debug_f32 (param f32)))
  (import "debug" "f64" (func $debug_f64 (param f64)))
  (import "debug" "bool" (func $debug_bool (param i32)))
  (import "debug" "char" (func $debug_char (param i32)))
  (import "debug" "byte" (func $debug_byte (param i32)))
  (import "debug" "bits" (func $debug_bits (param i32)))

  ;; ...

  ;; Then call it with the appropriate value type in your program.
  (call $debug_f64 (f64.const 123.456))
)

Poker Notes

The poker exercises require you to read values from five cards, which are stored in memory as pairs of u8 values. The first is the rank value and the second is the suit value.

Decimal Value	Rank Value
1	A
2	2
3	3
...	...
10	10
11	J
12	Q
13	K

Decimal Value	Suit Value
0	♠ Spades
1	♣ Clubs
2	♥ Hearts
3	♦ Diamonds

So the hand A♠ 2♣️ 3♥ 4♦ 5♠️ would be stored in memory as:

A	♠	2	♣️	3	♥	4	♦	5	♠ ️
0x1	0x0	0x2	0x1	0x3	0x2	0x4	0x3	0x5	0x0

String Notes

The string challenges all work with null-terminated ASCII strings for simplicity, unless stated explicitly otherwise.

Compared to UTF-8 and other multi-byte encodings, ASCII is generally easier to work with, because every single character can be represented with a single byte in memory.

For example, the string "Hello, world" would be stored in memory as:

H	e	l	l	o	,		w	o	r	l	d	END
0x48	0x65	0x6C	0x6C	0x6F	0x2C	0x20	0x77	0x6F	0x72	0x6C	0x64	0x00

The strings are null-terminated, which means they always end with a NUL byte (0x00).