Stage 6a: a tool that times the age writer

tool/age_writer_bench.dart times the header of each age file of a
capsule, the file of an author key and a large PAYLOAD_AGE in streaming,
with the CSPRNG of the platform. On the VM, 64 MiB take 1.38 s, 46
MiB/s; compiled to JavaScript, 16 MiB take 0.33 s, 49 MiB/s. The header
of PAYLOAD_AGE costs 4 ms, INNER_ACCESS_AGE with 16 stanzas about 45 ms
on the VM and 35 ms in Node.js, OUTER_TIME_AGE 40 ms and 0.55 s, and an
author key file with scrypt of logN 16, 0.6 s and 0.85 s.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tool/age_writer_bench.dart

@@ -0,0 +1,89 @@
+// Times the age writer of datekeys-dart, stage 6a, on the VM and compiled
+// to JavaScript: the throughput of a large PAYLOAD_AGE encrypted in
+// streaming, in pieces of 1 MiB, 64 MiB on the VM and 16 MiB in Node.js; and
+// the header of each file of a capsule: PAYLOAD_AGE for one X25519
+// recipient, INNER_ACCESS_AGE for 16 and a control of 103 bytes,
+// OUTER_TIME_AGE for the tlock recipient, and the file of an author key, a
+// scrypt recipient with logN 16. Each is the median of a few runs. The
+// random values come from the CSPRNG of the platform, as in production.
+//
+//   dart run tool/age_writer_bench.dart
+//
+//   dart compile js -O2 -o age_writer_bench.js tool/age_writer_bench.dart
+//   node -e "globalThis.self = globalThis; require('./age_writer_bench.js')"
+//
+// ignore_for_file: avoid_print
+import 'dart:typed_data';
+
+import 'package:datekeys/datekeys.dart' show quicknet;
+import 'package:datekeys/src/age_writer.dart';
+import 'package:datekeys/src/random.dart';
+import 'package:datekeys/src/recipient.dart';
+
+const isWeb = identical(0, 0.0);
+
+double median(List<double> xs) => (xs..sort())[xs.length ~/ 2];
+
+/// The median time of [f], in milliseconds, over [runs] runs.
+double time(int runs, void Function() f) {
+  final times = <double>[];
+  for (var i = 0; i < runs; i++) {
+    final sw = Stopwatch()..start();
+    f();
+    times.add(sw.elapsedMicroseconds / 1000);
+  }
+  return median(times);
+}
+
+void main() {
+  final id = generateX25519Identity();
+  final payload = X25519Recipient.of(id);
+  final slots = [
+    for (var i = 0; i < 16; i++) X25519Recipient.of(generateX25519Identity()),
+  ];
+  final tlock = TimeRecipient(quicknet(), 1000);
+  final scrypt = ScryptRecipient('a passphrase', workFactor: 16);
+  final control = Uint8List(103);
+
+  print('age writer, ${isWeb ? 'Node.js' : 'VM'}');
+  print(
+    'PAYLOAD_AGE header, one X25519 stanza: '
+    '${time(15, () => AgeEncryptor([payload]).close()).toStringAsFixed(2)} ms',
+  );
+  print(
+    'INNER_ACCESS_AGE, 16 X25519 stanzas, 103 bytes: '
+    '${time(7, () => ageEncrypt(control, slots)).toStringAsFixed(1)} ms',
+  );
+  print(
+    'OUTER_TIME_AGE, the tlock stanza of round 1000, 2128 bytes: '
+    '${time(5, () => ageEncrypt(Uint8List(1773), [tlock])).toStringAsFixed(0)} ms',
+  );
+  print(
+    'an author key file, scrypt with logN 16: '
+    '${time(3, () => ageEncrypt(Uint8List(120), [scrypt])).toStringAsFixed(0)} ms',
+  );
+
+  // The throughput in streaming, in pieces of 1 MiB.
+  final mib = isWeb ? 16 : 64;
+  final piece = Uint8List(1 << 20);
+  for (var i = 0; i < piece.length; i++) {
+    piece[i] = (31 * i + 7) & 0xff;
+  }
+  final ms = time(3, () {
+    final e = AgeEncryptor([payload], random: secureRandom);
+    var n = e.header.length + e.nonce.length;
+    for (var i = 0; i < mib; i++) {
+      for (final c in e.add(piece)) {
+        n += c.length;
+      }
+    }
+    n += e.close().length;
+    if (n != ageFileLength([x25519StanzaLength], mib << 20)) {
+      throw StateError('$n bytes');
+    }
+  });
+  print(
+    '$mib MiB in streaming: ${(ms / 1000).toStringAsFixed(2)} s, '
+    '${(mib / ms * 1000).toStringAsFixed(1)} MiB/s',
+  );
+}