Mihaela Mihaljevic
c3033c356d
PureYAML's scanner is a hand-rolled approximation of libyaml's algorithm; its indentation and simple-key bugs are the gap between the two. Rather than keep patching the approximation, re-derive the scanner as a faithful, function-by-function Swift port of libyaml's scanner.c (parser.c to follow), so correctness is by construction. The no-C-runtime constraint is preserved: this translates the algorithm into pure Swift, it does not bind to libyaml. First piece: the indentation engine under PureYAML.Parsing.LibYAML, a line-for-line port of yaml_parser_roll_indent and yaml_parser_unroll_indent. A block collection opens only when content is strictly more indented than the current level, which fixes the '- a:' / same-column 'b:' mis-nesting by construction. Unit-tested against that exact case (sibling vs nested vs flow-inert). Lands as a parallel implementation verified against libyaml built from the CYaml source; swapped in only once it matches the reference with no regression. docs/LIBYAML_PORT.md records the plan. No public behavior changed yet.
50 lines
2.7 KiB
Markdown
50 lines
2.7 KiB
Markdown
# Faithful libyaml port
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## Why
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PureYAML's original scanner is a hand-rolled approximation of libyaml's
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algorithm. Its bugs (block-mapping indentation, simple keys, empty values) are
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the gap between that approximation and the proven reference. Rather than keep
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patching the approximation, the scanner is being re-derived as a faithful,
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function-by-function Swift port of libyaml's `scanner.c`, with the parser to
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follow from `parser.c`. Correctness is then by construction: each Swift function
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mirrors its C counterpart and is verified against libyaml directly.
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The constraint that PureYAML ships no C or generated runtime is preserved: this
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is a translation of the algorithm into pure Swift, not a binding to libyaml.
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## Reference and verification
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- Source of truth: `PureYAMLResearch/Sources/CYaml/src/scanner.c` and `parser.c`
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(libyaml 0.2.5).
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- Oracle: the libyaml differential harness (`scratchpad/run_events.c` compiled
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against the same `CYaml` source emits the canonical yeast event stream). Each
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ported stage is checked token-for-token / event-for-event against it, in
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addition to the vendored `test.event` suite ground truth.
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- The port lands as a parallel implementation under `PureYAML.Parsing.LibYAML`
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and is only swapped in for the current scanner once it matches the reference on
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the full suite and the real-world corpus, so the working parser is never
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regressed mid-port.
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## Order (each step verified before the next)
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1. **Indent engine** (DONE): `yaml_parser_roll_indent` / `yaml_parser_unroll_indent`
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and the `indents` stack. A block collection opens only when content is strictly
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more indented than the current level, which fixes the `- a:` / same-column `b:`
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bug by construction. Implemented in `Sources/Parsing/LibYAML.IndentEngine.swift`,
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unit-tested in `Tests/LibYAMLIndentEngineTests.swift`.
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2. **Simple-key mechanism**: `save_simple_key`, `remove_simple_key`,
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`stale_simple_keys`, the `simple_keys` stack and `simple_key_allowed` flag.
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3. **Token-fetch core**: `fetch_more_tokens`, `fetch_next_token`,
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`scan_to_next_token`, and the `fetch_*` dispatch (`fetch_key`, `fetch_value`
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with its empty-scalar and roll-indent logic, `fetch_block_entry`,
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`fetch_flow_*`, document indicators, stream start/end).
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4. **Scalar scanners**: `scan_plain_scalar`, `scan_flow_scalar`,
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`scan_block_scalar` (+ breaks), `scan_anchor`, `scan_tag`, `scan_directive`.
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5. **Reader**: confirm the existing `Reader` covers `CACHE`/`SKIP`/`READ` and
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UTF-8 marks, or port the needed pieces.
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6. **Parser**: port `parser.c` to compose events from the token stream.
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7. **Integration**: route `parseEvents` / `parseStream` through the port; delete
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the approximation once the canonical event gate and libyaml differential are
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green with zero regressions.
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