# How a5R stores cell IDs without strings

## The problem

An A5 cell ID is a 64-bit unsigned integer (`u64`). R has no native `u64` type: its integers are 32-bit signed (`-2^31` to `2^31 - 1`), and its doubles are 64-bit floating point. A `double` can only represent integers exactly up to 2^53, while a `u64` can go up to 2^64 - 1.

The obvious workaround is to store cell IDs as hex strings (`"0800000000000006"`). This works, but every trip across the R / Rust boundary requires hex parsing and formatting: O(n) string allocation that dominates the cost of lightweight operations like `a5_get_resolution()` or `a5_cell_to_parent()`.

## The solution: eight raw-byte fields

A `u64` is exactly 8 bytes. We store each byte of the little-endian representation as a separate `raw` vector field in a vctrs record type:

    cell_id (u64):  0x0800000000000006

    little-endian bytes:
      b1 = 0x06, b2 = 0x00, b3 = 0x00, b4 = 0x00,
      b5 = 0x00, b6 = 0x00, b7 = 0x00, b8 = 0x08

This is lossless: the eight bytes are the exact same bits as the original `u64`, just stored across eight contiguous `raw` vectors. No precision loss, no special-case handling. On the Rust side, reconstructing the `u64` from the eight byte slices is a single `u64::from_le_bytes()` call. This also avoids pointers, so there is no need to think about serialization when saving an `a5_cell` object to disk.

## R-side: a vctrs record type

On the R side, `a5_cell` is a vctrs record (`vctrs::new_rcrd()`) with eight fields (`b1` through `b8`):

    library(a5R)
    cell <- a5_lonlat_to_cell(-3.19, 55.95, resolution = 10)
    vctrs::field(cell, "b1")
    #> [1] 00
    vctrs::field(cell, "b8")
    #> [1] 63

Each field is a plain `raw` vector: a contiguous block of memory with no per-element overhead. Subsetting, combining, and NA propagation are all handled automatically by vctrs.

Hex strings are only produced on demand:

    # Display calls format(), which converts to hex for readability
    cell
    #> <a5_cell[1]>
    #> [1] 6344be8000000000

    # Explicit conversion
    a5_u64_to_hex(cell)
    #> [1] "6344be8000000000"

    # Round-trip from hex
    a5_cell("0800000000000006")
    #> <a5_cell[1]>
    #> [1] 0800000000000006

## Why this matters

Compare memory for one million cells:

    set.seed(42)
    cells <- a5_lonlat_to_cell(
      runif(1e6, -180, 180),
      runif(1e6, -80, 80),
      resolution = 10
    )

    # rcrd: eight contiguous raw vectors (8 × 1 byte × 1M ≈ 7.6 MB)
    format(object.size(cells), units = "MB")
    #> [1] "7.6 Mb"

    # equivalent hex strings would be ~81 MB
    # (16 chars + 56-byte SEXP header per string)
    hex <- a5_u64_to_hex(cells)
    format(object.size(hex), units = "MB")
    #> [1] "81 Mb"

## NA handling

A5 cell IDs use 60 “quintants” (values 0–59) in their top 6 bits. Quintant 63 (binary `111111`) is invalid in the A5 system, so we use `0xFC00000000000000` as a sentinel value for `NA`. In little-endian, the last byte (`b8`) is `0xFC`, making NA detection a fast single-byte check.

On the Rust side, the sentinel is detected and mapped to `None`. Standard R idioms work as expected:

    cells_with_na <- a5_cell(c("0800000000000006", NA))
    is.na(cells_with_na)
    #> [1] FALSE  TRUE

## Summary

Aspect | Hex strings | Raw bytes
--- | --- | ---
R type | `character` vector | `vctrs_rcrd` (eight `raw` fields)
Memory (1M cells) | ~81 MB | ~7.6 MB
R-Rust crossing | O(n) hex parse/format | Zero-copy byte access
Human-readable | Always | On `format()` / `print()`
Lossless | Yes | Yes (exact byte representation)