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preallocate scanner tokens #2

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Merged
ncode merged 1 commit into from
May 10, 2026
Merged

preallocate scanner tokens #2

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19 changes: 19 additions & 0 deletions src/scanner/scanner.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,10 @@ const isMappingSeparatorAt = lex.isMappingSeparatorAt;
const isSeparatedIndicatorAt = lex.isSeparatedIndicatorAt;
const lineAt = lex.lineAt;

const min_preallocated_input_bytes: usize = 1024;
const token_capacity_input_divisor: usize = 4;
const max_initial_token_capacity: usize = 8 * 1024;

/// Tokenizes a YAML byte stream into lexical tokens.
///
/// UTF-8 input is borrowed for the lifetime of the returned `TokenStream`.
Expand All @@ -40,6 +44,10 @@ pub fn scan(allocator: std.mem.Allocator, input: []const u8) Error!TokenStream {
var tokens: std.ArrayList(Token) = .empty;
defer tokens.deinit(arena_allocator);

if (initialTokenCapacity(utf8_input.len)) |capacity| {
try tokens.ensureTotalCapacity(arena_allocator, capacity);
}

try tokens.append(arena_allocator, .stream_start);

var scanner_state: Scanner = .{
Expand Down Expand Up @@ -414,6 +422,11 @@ fn flowDepth(scanner: anytype) usize {
return scanner.square_depth + scanner.curly_depth;
}

fn initialTokenCapacity(input_len: usize) ?usize {
if (input_len < min_preallocated_input_bytes) return null;
return @min(max_initial_token_capacity, input_len / token_capacity_input_divisor);
}

fn colonStartsMappingValue(scanner: anytype) bool {
const in_flow = flowDepth(scanner) != 0;
if (lex.isMappingSeparatorAt(scanner.input, scanner.index, in_flow)) return true;
Expand Down Expand Up @@ -524,3 +537,9 @@ test "scanner indicator: compact sequence entry requires structural context afte
try std.testing.expectEqual(@as(usize, 2), fake.index);
try std.testing.expectEqual(@as(usize, 2), fake.tokens.items.len);
}

test "scanner token capacity estimate stays conservative" {
try std.testing.expectEqual(@as(?usize, null), initialTokenCapacity(97));
try std.testing.expectEqual(@as(?usize, 7509), initialTokenCapacity(30_036));
try std.testing.expectEqual(@as(?usize, 8192), initialTokenCapacity(1_000_000));
}
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