# 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.
- **Double-byte CJK pages** (932 Shift-JIS, 936 GBK, 949 Korean, 950 Big5) —
  planned, to be codegen'd the same way and feature-gated.

## Fidelity notes

- Every 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.

## Regenerating tables

`src/tables.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