feat(parsing): parse flow mappings with empty keys

A ':' value indicator with no preceding key is also valid inside a flow
mapping, denoting an entry whose key is empty (null). libyaml hands the
bare VALUE token to its node parser and fails with 'did not find expected
node content'. The flow-mapping-key parser now emits the empty scalar key
and lets the value state consume the indicator.

Fixes '{ : }' and '{ key: value, : empty key }', completing the
multi-document NKF9 case. Port score 282 -> 283.

CHANGELOG.md

@@ -31,6 +31,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   sequences (`- :`, `- ? : x`), and mixed empty-key/value entries. Surpasses
   libyaml on these seven test-suite cases (2JQS, 6M2F, M2N8/00, NHX8, S3PD,
   SM9W/01, UKK6/00).
+- Parse flow mappings with empty (null) keys. The same `:`-without-a-key entry
+  is valid inside `{ ... }`, where libyaml hands the bare VALUE token to its node
+  parser and fails with "did not find expected node content". The flow-mapping-key
+  parser now emits the empty scalar key for a leading `:`, so `{ : }` and
+  `{ key: value, : empty key }` parse, completing the multi-document NKF9 case.
 - Emit no document for a stream whose only content is comments or whitespace. A
   comment-only or empty stream has zero documents in YAML 1.2 (matching libyaml
   and the test suite), but the event stream was emitting a spurious empty null

Sources/Parsing/LibYAML.Parser.Flow.swift

@@ -96,6 +96,14 @@ extension PureYAML.Parsing.LibYAML.Parser {
             if isKey(token) {
                 return try flowMappingKeyEvent()
             }
+            // YAML 1.2 permits an empty (null) key in a flow mapping: a `:` with
+            // no preceding key denotes an entry whose key is empty. libyaml hands
+            // the VALUE token to `parse_node` and fails; instead emit the empty
+            // scalar key and let `parseFlowMappingValue` consume the indicator.
+            if isValue(token) {
+                state = .flowMappingValue
+                return processEmptyScalar(at: token.startMark)
+            }
             if !isFlowMappingEnd(token) {
                 states.append(.flowMappingEmptyValue)
                 return try parseNode(block: false, indentlessSequence: false)

Tests/CanonicalEventConformanceTests.swift

@@ -12,7 +12,7 @@ struct CanonicalEventConformanceTests {
     /// Valid suite cases whose event stream PureYAML does NOT yet emit exactly.
     /// This is the authoritative gap set: a fix removes an id (and the test fails
     /// until it is removed), a regression adds one (and the test fails). Of these,
-    /// 23 currently throw on valid input and 3 emit a wrong event stream.
+    /// 22 currently throw on valid input and 3 emit a wrong event stream.
     static let knownEventGaps: Set<String> = [
         // Gap set for the ported libyaml engine. Verified against the libyaml
         // reference dumper: on all but four of these the port matches libyaml
@@ -24,7 +24,7 @@ struct CanonicalEventConformanceTests {
         "4ABK", "58MP", "5T43", "6BCT", "6CA3",
         "96NN/00", "96NN/01", "A2M4", "CFD4", "DBG4", "DK3J", "DK95/00",
         "DK95/04", "FP8R", "FRK4", "HM87/00", "M7A3",
-        "NKF9", "Q5MG", "R4YG", "W4TN",
+        "Q5MG", "R4YG", "W4TN",
         "Y79Y/001", "Y79Y/010",
         // wrong event stream: the port matches libyaml but both deviate from the
         // canonical events (libyaml quirks to be surpassed).