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120 lines
5.2 KiB
Markdown
120 lines
5.2 KiB
Markdown
# Known divergences
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Places where the Rust and the Python deliberately do not agree, each with the
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reason it is acceptable. A divergence that is not written down here is a bug,
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and loosening a test instead of adding an entry here is how a port rots.
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Format:
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```
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## <function> — <one-line summary>
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**Python:** what it does.
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**Rust:** what it does instead.
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**Why this is acceptable:** the argument, including what input class is
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affected and how often it occurs in practice.
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**Measured:** how many records out of how many differ.
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```
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---
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## cuenorm — a cue line with trailing settings still reads
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**Original:** the timestamp line is split on the arrow and each half is read
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whole, so `00:00:01,000 --> 00:00:03,000 X1:040 X2:560` gives an end time
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that is not a number and the cue is dropped entirely.
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**Rust:** each half is reduced to the one token that could be a timestamp, so
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the trailing text is ignored and the cue reads.
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**Why this is acceptable:** it is what makes the same reader take WebVTT,
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where positioning settings after the end time are not a malformation but the
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format — `00:00:01.000 --> 00:00:03.000 line:0 position:20%` is an ordinary
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cue. Without it the tool would need a second parser that differed in one
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line. The divergence can only ever recover a cue that the original discarded;
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there is no input where it changes a cue the original read.
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**Measured:** `parity/cases/cuenorm/coordinates.srt` — one cue of two
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recovered. Across the other eight cases, including deliberately malformed
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timestamps, the two agree cue for cue.
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## mdscrub — un-paginated text is not given a page 1 marker
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**Python:** `_normalize_marker_pages` ends by wrapping any text it found no
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pagination in as `<!-- Page 1 -->\n\n…`, so that downstream can assume a
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marker exists.
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**Rust:** text with no page separators and no form feeds is returned
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unchanged.
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**Why this is acceptable:** a cleanup filter has to be safe to run twice, and
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the Python's version is not — a second pass wraps the first pass's marker in
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another one. The assumption it exists to serve belongs to the converter that
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knows it just read a PDF, not to a filter that is handed markdown of unknown
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provenance. The chunker already handles a document with no page markers by
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emitting chunks with no page number, which is the honest answer.
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**Measured:** not exercised by any parity case, since the Python function is
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only reachable from the PDF conversion path. Covered by
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`mdscrub::pages::tests::text_with_no_page_boundaries_is_left_exactly_as_it_was`.
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## Every ported pattern — the two engines disagree about `\d` and `(?i)`
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**Python:** `re` compiles against the Unicode version of the interpreter;
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`(?i)` folds `ı` and `İ` onto `i`.
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**Rust:** `regex` compiles against the Unicode version it was built with, and
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`(?i)` uses simple case folding, which leaves the dotless and dotted `i`
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alone.
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**Why this is acceptable:** `\s`, `\w` and `\b` were the differences that
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mattered and they are closed — every ported pattern now spells Python's
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classes explicitly, from `textkit::pattern`. What is left is a `\d` that
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disagrees only on decimal digits added between the two Unicode versions
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(Garay, added in Unicode 16, is the current example), and a case fold that
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disagrees only for Turkish dotless `i`. A chapter heading reading
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`Bıblıography` is classified as body text here and as a bibliography there;
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a page marker written in Garay digits reads here and does not there.
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Both will drift again at every dependency bump, which is the real reason this
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entry exists: the golden files pin one pair of Unicode versions, and a change
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to either shows up as a parity failure rather than as a silent difference.
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**Measured:** zero differences over the parity corpus. Found by brute-forcing
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every code point below `U+11000` through both engines: `\d` differs on ten,
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`(?i)` on three.
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## chunkmd — a page number too large for the field saturates
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**Python:** integers are unbounded, so `<!-- Page 99999999999 -->` records
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page 99,999,999,999.
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**Rust:** the page field is a 32-bit number and saturates at 4,294,967,295.
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**Why this is acceptable:** the number is a page number. A document with four
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billion pages does not exist, and the alternative — a 64-bit field — would
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make every chunk record wider to carry a value nothing can produce. The
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saturation is deliberate rather than a wrap, so the wrong answer is at least
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obviously wrong.
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**Measured:** no document in any corpus this was run against has a page
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marker above five digits.
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## chunkmd — a hard maximum of zero yields nothing rather than raising
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**Python:** `_enforce_hard_max` with `hard_max=0` raises `ValueError:
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range() arg 3 must not be zero`.
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**Rust:** `chunkmd::chunk` returns no chunks, since no piece could satisfy
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the limit.
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**Why this is acceptable:** the command-line tool rejects it before reading
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any input, so this is reachable only by a library caller, and a filter that
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returns nothing for a budget that permits nothing is the more useful answer
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of the two. Every other degenerate budget — overlap at or above target, a
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hard maximum below the target, a minimum of zero — agrees with the Python
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exactly.
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**Measured:** checked against the oracle for all of those combinations; only
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the zero hard maximum differs.
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