Add Scanner.scanStep(), the first increment of true chunked input streaming
(#4): Reader gains isFinal/append(_:)/markFinal(), and peek(_:) flags ranDry
instead of asserting end of input when the buffer runs out mid-fetch and more
chunks may still arrive. scanStep() snapshots the scanner before each
fetchNextToken() and restores it whenever a fetch runs dry, so a construct
spanning a chunk boundary is retried after more input lands rather than
misread as an early end of input. Every existing call site keeps the default
isFinal: true, so scanAll() and batch parsing are unchanged; ranDry can never
be set when isFinal is true, by construction. A token already queued is not
always safe to release, since libyaml's simple-key mechanism can still splice
a KEY token in front of it, so scanStep() ports libyaml's
yaml_parser_fetch_more_tokens release condition.
Proof: StreamingScannerDifferentialTests.swift drives scanStep() at several
chunk sizes over representative cases always, and over the entire
yaml-test-suite manifest (402 cases) behind PUREYAML_RUN_YAML_TEST_SUITE=1,
asserting the token sequence matches scanAll() exactly. That proof caught a
real bug: lastTokenForbidsInlineSequence() read indentEngine.tokens.last,
which goes empty once tokens are released incrementally, silently breaking
the key: - a rejection. Fixed by tracking IndentEngine.lastToken explicitly,
kept in sync with tokens.last so batch behavior (which never dequeues) is
unchanged.
Not yet wired into PureYAMLStreaming; scanStep() and its supporting types stay
internal until that integration is designed.
Also broaden the real-seed mutation property suite (#6) with the three
mutation classes it named but didn't yet cover: invalidControlCharacter,
mixedTabSpaceIndentation, and truncatedFlowCollection. The test now asserts
diagnostic quality, not just pass/fail: line must fall within the mutated
document, and column, when present, must be 1-based (some ParseError cases,
like tabIndentation, are line-only by design).
A tab is never indentation, so flow-collection content reached through a
tab below the continuation indent ('- [' / '\tfoo') is malformed even
though the tab inflates the column. A tab at or past the indent is
separation, and a tab-only blank line stays blank, so both remain valid.
Track a tab in a flow continuation's leading whitespace below the
parent + 1 indent and reject content (but not a closing bracket) reached
through it. Surpasses libyaml on Y79Y/003; whole-suite conformance
400/402, canonical events 308/308.
YAML requires multi-line flow content to be indented past the enclosing
block context (s-flow-line-prefix). libyaml does not enforce this, so
'flow: [a,' / 'b]' with 'b' in column 0, and the equivalent multi-line
flow mapping, wrongly parsed.
Track whether reaching a token crossed a line break, and in flow context
reject a token at or below the parent block indent. A closing ']' / '}'
or a leading ',' may sit at any indentation, so those are exempt (a flow
mapping's closing brace in column 0 stays valid).
Surpasses libyaml on 9C9N, VJP3/00; whole-suite conformance 394/402,
canonical events 308/308.
Per ns-plain-first a '-' starts a plain scalar in flow only when a
flow-safe character follows. A '-' before a flow indicator ('[-]',
'[-, -]') cannot start a node, but the port (like libyaml) read it as the
scalar '-'.
Reject that form in the indicator dispatch. Extract scan_to_next_token,
the comment validator, and blankLineAhead into LibYAML.Scanner.SkipToToken
to keep the fetch file within the length limit.
Surpasses libyaml on G5U8, YJV2; whole-suite conformance 390/402,
canonical events 308/308.
