# Multi-threading

    library(a5R)

a5R can parallelise vectorised operations using multiple threads via rayon. By default a5R uses a single thread, so there is zero overhead. You opt in to parallelism when you need it.

## Setting the thread count

    # Check the current setting (default: 1)
    a5_get_threads()
    #> [1] 1

    # Use 4 threads
    a5_set_threads(4)
    a5_get_threads()
    #> [1] 4

You can also set threads at package load time via an R option or environment variable, useful for scripts and batch jobs:

    # In .Rprofile or at the top of a script
    options(a5R.threads = 4)

    # Or as an environment variable
    # Sys.setenv(A5R_NUM_THREADS = 4)

`a5_set_threads()` invisibly returns the previous value, making temporary changes easy:

    old <- a5_set_threads(4)
    # ... parallel work ...
    a5_set_threads(old)

## What gets parallelised

Threading applies to vectorised functions that process each element independently:

Function | Per-element cost | Benefit
--- | --- | ---
`a5_cell_to_boundary()` | Heavy (boundary + WKT/WKB) | High
`a5_polygon_to_cells()` | Heavy (per-part flood-fill, hole subtraction) | High
`a5_linestring_to_cells()` | Heavy (per-feature tracing) | High
`a5_lonlat_to_cell()` | Moderate (projection) | High
`a5_cell_distance()` | Moderate (2x projection + distance) | Medium
`a5_cell_to_lonlat()` | Moderate (reverse projection) | Medium
`a5_cell_to_parent()` | Light (bit ops + hex) | Low
`a5_get_resolution()` | Light (bit ops) | Low
`a5_is_valid()` | Light (hex parse) | Low

Scalar and bulk operations (`a5_cell_to_children()`, `a5_compact()`, `a5_cell_area()`, etc.) are unaffected: they are already fast or delegate to algorithms that don’t parallelise element-wise.

## When is it worthwhile?

Threading has a small fixed overhead (thread synchronisation, memory allocation for intermediate results). For small vectors this can outweigh the benefit. As a rule of thumb:

- < 1,000 elements: stick with 1 thread
- 1,000–10,000: 2-4 threads helps for heavy ops (boundary, indexing)
- > 10,000: use as many threads as you have cores

Here’s a quick comparison on 100k cells:

    cells <- a5_polygon_to_cells(wk::rct(-10, 50, 10, 60), resolution = 12)
    length(cells)
    #> [1] 704259

    a5_set_threads(1)
    system.time(a5_cell_to_boundary(cells, format = "wkt"))
    #>   user  system elapsed
    #>  3.124   0.000   3.122

    a5_set_threads(8)
    system.time(a5_cell_to_boundary(cells, format = "wkt"))
    #>   user  system elapsed
    #> 6.195   1.289   1.667

Note that `user` time increases (total CPU work across all threads) while `elapsed` (wall-clock) time decreases: that is the parallelism at work.

## Thread safety

a5R uses a dedicated rayon thread pool, separate from R’s own parallelism. It is safe to use alongside `future`, `mirai`, etc. but think carefully about this nested parallelism as it can, if overloaded, degrade performance.

The thread pool is rebuilt each time you call `a5_set_threads()`, so changing the count mid-session is fine (and cheap) but not free: ideally, do it once at the start of your workflow rather than toggling per-call.