---
title: "Backend Selection"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Backend Selection}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r, include = FALSE}
knitr::opts_chunk$set(collapse = TRUE, comment = "#>")
library(Rsassy)
```

Rsassy installs a C shim plus Rust backend libraries. One backend is selected
per R process.

## Auto selection

`backend = "auto"` uses the first installed and supported backend in this order:

1. `wasm_simd128`
2. `avx512`
3. `avx2`
4. `neon`
5. `scalar`

```{r}
sassy_features()
```

## Explicit selection

Call `sassy_set_backend()` before any other Rsassy native call. Backend loading
is one-shot, so test another backend in a new R process.

```{r}
script <- tempfile(fileext = ".R")
writeLines(c(
  "library(Rsassy)",
  "sassy_set_backend('scalar')",
  "print(sassy_features())"
), script)
cat(system2(file.path(R.home("bin"), "Rscript"), c("--vanilla", script), stdout = TRUE), sep = "\n")
unlink(script)
```

## Why separate backend libraries?

The `sassy` crate uses build-time SIMD configuration and SIMD-dependent types.
The R package therefore builds separate Rust libraries for the supported backend
families instead of switching individual functions at runtime.