Watusi

A high-performance, native Elixir implementation for converting WebAssembly Text (WAT) to the WebAssembly Binary Format (WASM).

Watusi provides a pure-Elixir pipeline for transforming human-readable WebAssembly into the standard binary format, supporting modern extensions and ensuring full compatibility with official tooling.

Why Watusi?

• Native Elixir: No external dependencies or tools required - pure Elixir implementation.
• High Performance: Optimized lexer and encoder with compile-time code generation for maximum speed.
• Modern Standards: Core 1.0 plus Bulk Memory, Fixed-width SIMD, Threads/Atomics, Sign-extension, Non-trapping float-to-int, Exception Handling, Garbage Collection, Reference-Types, and Function-References proposals.
• Developer Friendly: Optional debug names and detailed identifier resolution.
• Spec Compliant: Tested against the official spec vectors (5,000+) with bit-for-bit parity to wasm-tools.

Performance

Watusi is optimized for speed with typical compilation times for real-world modules in the single-digit milliseconds range.

Specifications

Watusi adheres to the following standards:

• WebAssembly Core Specification 1.0 (Binary and Text formats).
• WebAssembly Bulk Memory Operations Extension.
• WebAssembly Fixed-width SIMD Extension.
• WebAssembly Threads/Atomics Extension.
• WebAssembly Sign-extension Operators.
• WebAssembly Nontrapping Float-to-int Conversions.
• WebAssembly Exception Handling Proposal.
• WebAssembly Garbage Collection Proposal: recursive/sub-typed type groups, struct/array types, and GC/ref instructions.
• WebAssembly Reference-Types and Function-References.
• IEEE 754-2019 for floating-point representation.

Installation

Add watusi to your list of dependencies in mix.exs:

def deps do
  [
    {:watusi, "~> 0.6.0"}
  ]
end

Usage

The primary entry point is Watusi.to_wasm/2. It accepts WAT source as a string or iodata and returns the compiled WASM binary.

wat = \"\"\"
(module
  (func (export \"add\") (param $a i32) (param $b i32) (result i32)
    local.get $a
    local.get $b
    i32.add)
)
\"\"\"

wasm = Watusi.to_wasm(wat)
# <<0, 97, 115, 109, 1, 0, 0, 0, ...>>

Binary Patching

For pre-compiled WASM templates, use Watusi.Patcher to replace data segments and global initializers:

# Compile template once
template = Watusi.to_wasm(interpreter_wat)

# Patch with runtime data
wasm = Watusi.Patcher.patch(template,
  data: [{0x00000, story_bytes}],
  globals: %{0 => version}
)

See doc/patcher.md for details.

Debug Names

You can include symbolic identifiers in the binary by passing debug_names: true. This adds a standard name custom section to the output.

wasm_with_names = Watusi.to_wasm(wat, debug_names: true)

Testing

Watusi is tested against the bytecodealliance/wasm-tools. The test suite compiles each .wat vector with wasm-tools parse (stripped with wasm-tools strip --all) and verifies bit-for-bit parity with Watusi's output, then validates generated binaries with wasm-tools validate --features all.

Over 5,000 official spec vectors are included, covering core instructions and advanced extensions, with the full suite at 5146/5146 passing against the pinned wasm-tools reference. See test/spec_vectors/README.md for details.