msdos-encodings/README.md

# msdos-encodings

Bidirectional, emulator-grade conversion between MS-DOS code pages and Unicode —
without linking ICU. The crate owns the canonical Microsoft tables, so it stays
small, self-contained, and cross-compiles cleanly.

These are strictly **Microsoft's** mappings as published by the Unicode
Consortium (`VENDORS/MICSFT/PC/`) — not the IBM or Oracle variants, which
differ in a handful of slots.

```rust
use msdos_encodings::DosEncoding;

let enc = DosEncoding::default(); // code page 437
assert_eq!(enc.decode(b"Caf\x82").unwrap(), "Café");
assert_eq!(enc.encode("Café").unwrap(), b"Caf\x82");

// Pick by numeric code page (e.g. from CHCP or locale data):
let enc = DosEncoding::from_code_page(866).unwrap();
assert_eq!(enc.decode(&[0x80]).unwrap(), "А"); // DOS Cyrillic Russian

// Four pages leave bytes undefined; decode is fallible, or go lossy:
assert!(DosEncoding::Cp874.decode(&[0xFF]).is_err());
assert_eq!(DosEncoding::Cp874.decode_lossy(&[0xFF]), "\u{FFFD}");
```

## Status

- **All sixteen single-byte DOS pages** — implemented, decode + encode:
  437, 737, 775, 850, 852, 855, 857, 860, 861, 862, 863, 864, 865, 866, 869, 874.
- **All four double-byte CJK pages** — implemented, decode + encode:
  932 (Shift-JIS), 936 (GBK), 949 (Unified Hangul), 950 (Big5). They sit
  behind the on-by-default `dbcs` feature; build with
  `default-features = false` for a lean single-byte-only crate.

## Fidelity notes

- Every single-byte table is a verified bijection: anything a page decodes
  re-encodes to the identical bytes.
- Code pages 857, 864, 869, and 874 leave some byte values undefined, so
  `decode` returns a `Result`; `decode_lossy` substitutes U+FFFD.
- Microsoft's CP864 (DOS Arabic) maps `0x25` to ARABIC PERCENT SIGN — the low
  half is *not* pure ASCII, and `%` itself is unmappable. This is faithful to
  the source table.
- CP932 and CP950 each have a handful of byte pairs that decode to the same
  character (932's NEC/IBM extension overlap, 950's duplicated box-drawing
  rows). All of them decode; re-encoding picks the code Windows picks, as
  resolved from Microsoft's own `bestfit932/950.txt` WCTABLE (vendored in
  `data/`, used *only* for duplicate resolution — no lossy best-fit mappings
  are imported).
- Double-byte decode adds two failure modes: a lead byte at end of input
  (`TruncatedPair`) and an undefined lead/trail pair (`UndefinedPair`).
  `decode_lossy` emits one U+FFFD per failed pair.
- The full decode direction of all four CJK pages agrees exactly with
  Python's `cp932`/`cp936`/`cp949`/`cp950` codecs (59k+ pairs, verified
  during development).

## Regenerating tables

Everything in `src/tables/` except `mod.rs` is generated from the vendored
mapping files in `data/`
(needs Python ≥ 3.10 and `rustfmt` on `PATH`):

```sh
uv run tools/gen_tables.py
```

## Scope

Text-encoding conversion only. Filename conventions (8.3 space padding,
case folding, the 0x05/0xE5 deleted-entry dance) are deliberately **out of
scope** and left to consumers.

## Consumers

Designed to be shared by `fsinspect` and friends via path (and later git)
dependency.

## License

MIT