reduce flow parser event allocations

src/parser/flow.zig

@@ -189,26 +189,28 @@ fn appendSequenceEntryEvents(
         return true;
     }
 
-    var key_events: EventBuilder = .{};
-    defer key_events.deinit(allocator);
-    try key_events.ensureTotalCapacity(allocator, @min(end - index.*, 4));
-
+    const key_checkpoint = events.checkpoint();
+    var key_committed = false;
+    errdefer if (!key_committed) events.rollback(key_checkpoint);
     const key_start = index.*;
-    if (!try appendNodeEventsWithOptions(allocator, tokens, index, end, &key_events, depth, directives, .{
+    if (!try appendNodeEventsWithOptions(allocator, tokens, index, end, events, depth, directives, .{
         .allow_plain_indented_continuations = true,
-    })) return false;
+    })) {
+        events.rollback(key_checkpoint);
+        return false;
+    }
 
     token_cursor.skipComments(tokens, index, end);
     if (token_cursor.flowMappingValueFollowsLineBreak(tokens, index.*, end)) return ParseError.InvalidSyntax;
     if (index.* >= end or tokens[index.*] != .flow_mapping_value) {
-        try events.appendSlice(allocator, key_events.slice());
+        key_committed = true;
         return true;
     }
 
     try implicit_key.validateImplicitTokenKeyLength(tokens, key_start, index.*);
     index.* += 1;
-    try events.append(allocator, .{ .mapping_start = .{ .style = .flow } });
-    try events.appendSlice(allocator, key_events.slice());
+    try events.insert(allocator, key_checkpoint.len, .{ .mapping_start = .{ .style = .flow } });
+    key_committed = true;
 
     token_cursor.skipFlowInsignificant(tokens, index, end);
     if (token_cursor.isFlowSequenceEntryBoundary(tokens, index.*, end)) {

src/parser/internal.zig

@@ -54,11 +54,26 @@ pub const EventBuilder = struct {
     list: std.ArrayList(Event) = .empty,
     stats: EventStats = .{},
 
+    pub const Checkpoint = struct {
+        len: usize,
+        stats: EventStats,
+    };
+
     pub fn deinit(self: *EventBuilder, allocator: std.mem.Allocator) void {
         self.list.deinit(allocator);
         self.* = .{};
     }
 
+    pub fn checkpoint(self: *const EventBuilder) Checkpoint {
+        return .{ .len = self.list.items.len, .stats = self.stats };
+    }
+
+    pub fn rollback(self: *EventBuilder, saved: Checkpoint) void {
+        std.debug.assert(saved.len <= self.list.items.len);
+        self.list.items.len = saved.len;
+        self.stats = saved.stats;
+    }
+
     pub fn ensureTotalCapacity(self: *EventBuilder, allocator: std.mem.Allocator, capacity: usize) std.mem.Allocator.Error!void {
         try self.list.ensureTotalCapacity(allocator, capacity);
     }
@@ -73,13 +88,23 @@ pub const EventBuilder = struct {
         self.stats.observeSlice(events);
     }
 
+    pub fn insert(self: *EventBuilder, allocator: std.mem.Allocator, index: usize, event: Event) std.mem.Allocator.Error!void {
+        try self.list.insert(allocator, index, event);
+        self.recomputeStats();
+    }
+
     pub fn toOwnedSlice(self: *EventBuilder, allocator: std.mem.Allocator) std.mem.Allocator.Error![]const Event {
         return self.list.toOwnedSlice(allocator);
     }
 
     pub fn slice(self: *const EventBuilder) []const Event {
         return self.list.items;
     }
+
+    fn recomputeStats(self: *EventBuilder) void {
+        self.stats = .{};
+        self.stats.observeSlice(self.list.items);
+    }
 };
 
 test "event builder tracks event stats while appending" {
@@ -101,6 +126,26 @@ test "event builder tracks event stats while appending" {
     try std.testing.expect(!builder.stats.malformed_nesting);
 }
 
+test "event builder rollback restores events and stats" {
+    var builder: EventBuilder = .{};
+    defer builder.deinit(std.testing.allocator);
+
+    try builder.append(std.testing.allocator, .stream_start);
+    try builder.append(std.testing.allocator, .{ .sequence_start = .{ .style = .flow } });
+    try builder.append(std.testing.allocator, .{ .scalar = .{ .value = "kept" } });
+    const checkpoint = builder.checkpoint();
+
+    try builder.append(std.testing.allocator, .{ .mapping_start = .{ .style = .flow } });
+    try builder.append(std.testing.allocator, .{ .scalar = .{ .value = "discarded" } });
+    try builder.append(std.testing.allocator, .mapping_end);
+    builder.rollback(checkpoint);
+
+    try std.testing.expectEqual(@as(usize, 3), builder.slice().len);
+    try std.testing.expectEqual(@as(usize, 3), builder.stats.event_count);
+    try std.testing.expectEqual(@as(usize, 4), builder.stats.max_scalar_bytes);
+    try std.testing.expectEqual(@as(usize, 1), builder.stats.current_nesting_depth);
+}
+
 pub const Line = struct {
     start: usize,
     end: usize,

src/parser/parser.zig

@@ -494,3 +494,17 @@ test "parseTokensWithStats reports private event stats" {
     try std.testing.expectEqual(@as(usize, 0), parsed.stats.current_nesting_depth);
     try std.testing.expect(!parsed.stats.malformed_nesting);
 }
+
+test "parseTokensWithStats reports flow sequence implicit mapping stats" {
+    var token_stream = try scanner.scan(std.testing.allocator, "[plain, key: [nested], tail]\n");
+    defer token_stream.deinit();
+
+    var parsed = try parseTokensWithStats(std.testing.allocator, token_stream.tokens);
+    defer parsed.stream.deinit();
+
+    try std.testing.expectEqual(parsed.stream.events.len, parsed.stats.event_count);
+    try std.testing.expectEqual(@as(usize, 6), parsed.stats.max_scalar_bytes);
+    try std.testing.expectEqual(@as(usize, 3), parsed.stats.max_nesting_depth);
+    try std.testing.expectEqual(@as(usize, 0), parsed.stats.current_nesting_depth);
+    try std.testing.expect(!parsed.stats.malformed_nesting);
+}

tests/allocation/failure_injection_test.zig

@@ -74,6 +74,9 @@ fn checkParseEventsAllocationFailure(failing_allocator: std.mem.Allocator) !void
         \\  ? !e!key "quoted\nkey"
         \\  : [*root, {plain: value}]
         \\
+        ,
+        \\&root [plain, key: &node !<tag:example.com,2000:seq> [*node, {inner: value}], ? explicit : , : empty]
+        \\
     };
 
     for (inputs) |input| {

tests/unit/api/parse_test.zig

@@ -30,6 +30,22 @@ test "parseEvents preserves escaped whitespace before folded double quoted line
     try std.testing.expectEqualStrings("kept \xc2\xa0 next", stream.events[2].scalar.value);
 }
 
+test "parseEvents owns flow sequence speculative event strings" {
+    const input = try std.testing.allocator.dupe(u8, "&root [plain, key: &node !<tag:example.com,2000:seq> [*node]]\n");
+    defer std.testing.allocator.free(input);
+
+    var stream = try parseEvents(std.testing.allocator, input);
+    defer stream.deinit();
+    @memset(input, 'x');
+
+    try std.testing.expectEqualStrings("root", stream.events[2].sequence_start.anchor.?);
+    try std.testing.expectEqualStrings("plain", stream.events[3].scalar.value);
+    try std.testing.expectEqualStrings("key", stream.events[5].scalar.value);
+    try std.testing.expectEqualStrings("node", stream.events[6].sequence_start.anchor.?);
+    try std.testing.expectEqualStrings("tag:example.com,2000:seq", stream.events[6].sequence_start.tag.?);
+    try std.testing.expectEqualStrings("node", stream.events[7].alias);
+}
+
 test "Parser.init reports token and event count limit diagnostics" {
     const input =
         \\- one
@@ -80,6 +96,30 @@ test "Parser.init reports input scalar and nesting limit diagnostics" {
     try std.testing.expectEqual(@as(usize, 10), diagnostic.offset);
 }
 
+test "Parser.init preserves flow sequence implicit mapping limit diagnostics" {
+    const input = "[plain, key: [nested], tail]\n";
+    var diagnostic: Diagnostic = .{};
+    try std.testing.expectError(ParseError.Unsupported, yaml.Parser.init(std.testing.allocator, input, .{
+        .max_event_count = 13,
+        .diagnostic = &diagnostic,
+    }));
+    try std.testing.expectEqualStrings("event count exceeds configured limit", diagnostic.message);
+
+    diagnostic = .{};
+    try std.testing.expectError(ParseError.Unsupported, yaml.Parser.init(std.testing.allocator, input, .{
+        .max_scalar_bytes = 5,
+        .diagnostic = &diagnostic,
+    }));
+    try std.testing.expectEqualStrings("scalar exceeds configured size limit", diagnostic.message);
+
+    diagnostic = .{};
+    try std.testing.expectError(ParseError.Unsupported, yaml.Parser.init(std.testing.allocator, input, .{
+        .max_nesting_depth = 2,
+        .diagnostic = &diagnostic,
+    }));
+    try std.testing.expectEqualStrings("nesting depth exceeds configured limit", diagnostic.message);
+}
+
 test "load keeps hash characters inside double quoted block mapping scalars" {
     var document = try load(std.testing.allocator,
         \\key: "value # not a comment"

tests/unit/parser/flow_test.zig

@@ -1098,6 +1098,43 @@ test "parseTokens parses a trailing empty flow mapping value" {
     try std.testing.expect(event_stream.events[7] == .stream_end);
 }
 
+test "parseTokens preserves mixed flow sequence event equivalence" {
+    var token_stream = try scanner.scan(std.testing.allocator, "&root [plain, key: &node !<tag:example.com,2000:seq> [*node, {inner: value}], ? explicit : , : empty]\n");
+    defer token_stream.deinit();
+
+    var event_stream = try parseTokens(std.testing.allocator, token_stream.tokens);
+    defer event_stream.deinit();
+
+    const expected = [_]types.Event{
+        .stream_start,
+        .{ .document_start = .{} },
+        .{ .sequence_start = .{ .style = .flow, .anchor = "root" } },
+        .{ .scalar = .{ .value = "plain" } },
+        .{ .mapping_start = .{ .style = .flow } },
+        .{ .scalar = .{ .value = "key" } },
+        .{ .sequence_start = .{ .style = .flow, .anchor = "node", .tag = "tag:example.com,2000:seq" } },
+        .{ .alias = "node" },
+        .{ .mapping_start = .{ .style = .flow } },
+        .{ .scalar = .{ .value = "inner" } },
+        .{ .scalar = .{ .value = "value" } },
+        .mapping_end,
+        .sequence_end,
+        .mapping_end,
+        .{ .mapping_start = .{ .style = .flow } },
+        .{ .scalar = .{ .value = "explicit" } },
+        .{ .scalar = .{ .value = "" } },
+        .mapping_end,
+        .{ .mapping_start = .{ .style = .flow } },
+        .{ .scalar = .{ .value = "" } },
+        .{ .scalar = .{ .value = "empty" } },
+        .mapping_end,
+        .sequence_end,
+        .{ .document_end = .{} },
+        .stream_end,
+    };
+    try std.testing.expectEqualDeep(&expected, event_stream.events);
+}
+
 test "parseTokens does not overallocate temporary flow sequence entry events" {
     const item_count = 96;
 
@@ -1118,7 +1155,7 @@ test "parseTokens does not overallocate temporary flow sequence entry events" {
     defer event_stream.deinit();
 
     try std.testing.expectEqual(@as(usize, item_count + 6), event_stream.events.len);
-    try std.testing.expect(counted.allocated_bytes <= input.items.len * 256);
+    try std.testing.expect(counted.allocated_bytes <= input.items.len * 72);
 }
 
 const CountingAllocator = struct {