2fe53fdee4
The ten single-byte ANSI pages join as a second enum alongside DosEncoding, mirroring its API. Errors now carry a public Encoding wrapper (Dos | Windows) so both families share the single-byte codec functions; Encoding::from_code_page resolves a raw code page number across families, DOS first. data/ is reorganized into pc/, windows/, and bestfit/, and the generated single-byte tables split into dos.rs and ansi.rs. Microsoft publishes an identical Thai table for both families, so DosEncoding:: Cp874 and WindowsEncoding::Cp874 agree byte-for-byte (pinned by test).
96 lines
3.9 KiB
Markdown
96 lines
3.9 KiB
Markdown
# msdos-encodings
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Bidirectional, emulator-grade conversion between Microsoft code pages and
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Unicode — without linking ICU. The crate owns the canonical Microsoft tables,
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so it stays small, self-contained, and cross-compiles cleanly.
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These are strictly **Microsoft's** mappings as published by the Unicode
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Consortium (`VENDORS/MICSFT/`) — not the IBM or Oracle variants, which
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differ in a handful of slots.
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```rust
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use msdos_encodings::{DosEncoding, Encoding, WindowsEncoding};
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let enc = DosEncoding::default(); // code page 437
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assert_eq!(enc.decode(b"Caf\x82").unwrap(), "Café");
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assert_eq!(enc.encode("Café").unwrap(), b"Caf\x82");
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// Pick by numeric code page (e.g. from CHCP or locale data):
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let enc = DosEncoding::from_code_page(866).unwrap();
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assert_eq!(enc.decode(&[0x80]).unwrap(), "А"); // DOS Cyrillic Russian
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// The Windows ANSI family is a separate enum:
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let enc = WindowsEncoding::Cp1252;
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assert_eq!(enc.decode(&[0x93, 0x80, 0x94]).unwrap(), "\u{201C}€\u{201D}");
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// Or resolve a raw code page number across both families:
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assert_eq!(Encoding::from_code_page(1251).unwrap().name(), "Windows Cyrillic");
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// Some pages leave bytes undefined; decode is fallible, or go lossy:
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assert!(DosEncoding::Cp874.decode(&[0xFF]).is_err());
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assert_eq!(DosEncoding::Cp874.decode_lossy(&[0xFF]), "\u{FFFD}");
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```
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## Status
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- **All sixteen single-byte DOS (OEM) pages** — implemented, decode + encode:
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437, 737, 775, 850, 852, 855, 857, 860, 861, 862, 863, 864, 865, 866, 869, 874.
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- **All four double-byte CJK pages** — implemented, decode + encode:
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932 (Shift-JIS), 936 (GBK), 949 (Unified Hangul), 950 (Big5). They sit
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behind the on-by-default `dbcs` feature; build with
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`default-features = false` for a lean single-byte-only crate.
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- **All ten single-byte Windows ANSI pages** — implemented, decode + encode:
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874 (Thai) and 1250–1258, as `WindowsEncoding`.
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## Fidelity notes
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- Every single-byte table is a verified bijection: anything a page decodes
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re-encodes to the identical bytes.
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- Many pages leave byte values undefined (DOS 857/864/869/874; every ANSI
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page except 1256 has holes in `0x80..=0x9F`), so `decode` returns a
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`Result`; `decode_lossy` substitutes U+FFFD.
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- Microsoft's CP864 (DOS Arabic) maps `0x25` to ARABIC PERCENT SIGN — the low
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half is *not* pure ASCII, and `%` itself is unmappable. This is faithful to
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the source table.
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- CP932 and CP950 each have a handful of byte pairs that decode to the same
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character (932's NEC/IBM extension overlap, 950's duplicated box-drawing
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rows). All of them decode; re-encoding picks the code Windows picks, as
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resolved from Microsoft's own `bestfit932/950.txt` WCTABLE (vendored in
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`data/bestfit/`, used *only* for duplicate resolution — no lossy best-fit
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mappings are imported).
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- Double-byte decode adds two failure modes: a lead byte at end of input
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(`TruncatedPair`) and an undefined lead/trail pair (`UndefinedPair`).
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`decode_lossy` emits one U+FFFD per failed pair.
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- The full decode direction of all four CJK pages agrees exactly with
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Python's `cp932`/`cp936`/`cp949`/`cp950` codecs (59k+ pairs, verified
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during development).
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- Microsoft publishes the identical Thai table for both families, so
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`DosEncoding::Cp874` and `WindowsEncoding::Cp874` agree byte-for-byte
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(pinned by a test).
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## Regenerating tables
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Everything in `src/tables/` except `mod.rs` is generated from the vendored
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mapping files in `data/` (`pc/` and `windows/` mapping tables, `bestfit/`
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for duplicate resolution; needs Python ≥ 3.10 and `rustfmt` on `PATH`):
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```sh
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uv run tools/gen_tables.py
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```
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## Scope
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Text-encoding conversion only. Filename conventions (8.3 space padding,
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case folding, the 0x05/0xE5 deleted-entry dance) are deliberately **out of
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scope** and left to consumers, as is any Vietnamese (CP1258) combining-mark
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normalization.
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## Consumers
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Designed to be shared by `fsinspect` and friends via path (and later git)
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dependency.
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## License
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MIT
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