Improve randomisation of answers

DESIGN.md

@@ -480,9 +480,10 @@ amateurfunk-anki [--data DIR] [--out DIR] [--seed STR] [--epoch INT]
 - `--seed STR` — deterministic seed for answer shuffling. Omit it
   for the normal study-deck behavior: a fresh seed is generated for
   each build, so answers are reshuffled every time decks are rebuilt.
-- `--epoch INT` — override the package timestamp epoch. By default we
-  derive it from the manifest's `fetched_at`; this flag is mainly for
-  tests and explicit rebuilds.
+- `--epoch INT` — override the package and note-modification timestamp
+  epoch. By default we use the current rebuild time so Anki recognizes
+  reshuffled notes as newer on re-import; this flag is mainly for tests
+  and explicit reproducible builds.
 
 Exit codes: `0` success, `1` configuration / catalog / build error.
 There is no Stage-2 equivalent of the fetcher's `EXIT_BAD_STATE` —
@@ -602,13 +603,14 @@ same `.apkg` bytes out. Determinism rests on three things:
 2. **Stable shuffle.** `randomized_answers()` builds a per-question
    `random.Random` seeded from SHA-256 of
    `f"{cli_seed}:{question_number}"`.
-3. **Stable timestamps.** Every `now` value in the collection (the
-   `mod` columns, the JSON config blob timestamps) is fixed to
-   `build_epoch` — derived from the manifest's `fetched_at`, or
-   overridden via `--epoch`. ZIP member timestamps are also fixed
-   via `ZipInfo(name, zip_datetime(build_epoch))`. Without this
-   last step, the inner SQLite would be identical but the
-   archive's per-entry mtimes would still vary between runs.
+3. **Controlled timestamps.** Every `now` value in the collection (the
+   `mod` columns and JSON config blob timestamps) uses `build_epoch`.
+   A normal rebuild sets it to the current time because the fresh
+   shuffle changes note HTML and Anki only updates an existing note
+   when the imported copy is newer. `--epoch` fixes it explicitly for
+   reproducible builds. ZIP member timestamps use the same value via
+   `ZipInfo(name, zip_datetime(build_epoch))`; otherwise the inner
+   SQLite could be identical while the archive bytes still differed.
 
 The combined effect: two runs with the same `data/`, `--seed`, and
 timestamp inputs produce byte-identical sha256 on each `.apkg`.

amateurfunk_anki.py

@@ -37,6 +37,7 @@ import secrets
 import shutil
 import sqlite3
 import sys
+import time
 import unicodedata
 import zipfile
 from pathlib import Path
@@ -1790,6 +1791,19 @@ def resolve_shuffle_seed(seed):
     return secrets.token_hex(16)
 
 
+def build_epoch_for_rebuild(override_epoch=None):
+    """Return the modification epoch for a multiple-choice deck build.
+
+    A normal rebuild reshuffles the answer HTML, so its notes must carry
+    the current time for Anki to recognize the imported copies as newer.
+    `--epoch` remains the explicit escape hatch for byte-reproducible
+    builds, where the caller intentionally fixes every timestamp.
+    """
+    if override_epoch is not None:
+        return int(override_epoch)
+    return int(time.time())
+
+
 def build_all(data_dir, out_dir, seed, override_epoch=None, explanations_path=None):
     """Build every category's `.apkg` and return their result dicts.
 
@@ -1798,8 +1812,8 @@ def build_all(data_dir, out_dir, seed, override_epoch=None, explanations_path=No
     `.apkg` each. Raises `AnkiBuildError` on configuration / catalog
     / explanation-schema problems.
     """
-    edition_dir, manifest, catalog = load_latest_catalog(data_dir)
-    build_epoch = build_epoch_from_manifest(manifest, override_epoch)
+    edition_dir, _manifest, catalog = load_latest_catalog(data_dir)
+    build_epoch = build_epoch_for_rebuild(override_epoch)
     explanations = load_explanations(
         explanations_path if explanations_path is not None
         else DEFAULT_EXPLANATIONS_PATH
@@ -1866,8 +1880,8 @@ def _parse_args(argv):
         type=int,
         default=None,
         help=(
-            "override the package timestamp epoch; by default this is "
-            "derived from manifest.json's fetched_at"
+            "override the package and note modification timestamp epoch; "
+            "by default the current rebuild time is used"
         ),
     )
     parser.add_argument(

test_amateurfunk_anki.py

@@ -127,6 +127,19 @@ class TestResolveShuffleSeed(unittest.TestCase):
         self.assertNotEqual(first, second)
 
 
+class TestBuildEpochForRebuild(unittest.TestCase):
+    """Multiple-choice rebuilds must look newer to Anki by default."""
+
+    def test_default_uses_current_time(self):
+        with patch("amateurfunk_anki.time.time", return_value=1777777777.9):
+            self.assertEqual(aa.build_epoch_for_rebuild(), 1777777777)
+
+    def test_explicit_epoch_is_preserved_for_reproducible_builds(self):
+        with patch("amateurfunk_anki.time.time") as current_time:
+            self.assertEqual(aa.build_epoch_for_rebuild(1234567890), 1234567890)
+        current_time.assert_not_called()
+
+
 class TestAnkiBuild(unittest.TestCase):
     def setUp(self):
         self.tmp = tempfile.TemporaryDirectory()
@@ -662,6 +675,42 @@ class TestAnkiBuild(unittest.TestCase):
         ]
         self.assertFalse(all(byte_equal))
 
+    def test_cli_default_rebuild_stamps_notes_as_newer(self):
+        out_a = self.root / "anki-current-a"
+        out_b = self.root / "anki-current-b"
+        common = [
+            "--data", str(self.data_dir),
+            "--explanations", str(self.explanations_path),
+        ]
+
+        with (
+            patch(
+                "amateurfunk_anki.resolve_shuffle_seed",
+                side_effect=["fresh-a", "fresh-b"],
+            ),
+            patch(
+                "amateurfunk_anki.time.time",
+                side_effect=[1777777777.9, 1777777788.1],
+            ),
+        ):
+            aa.main([*common, "--out", str(out_a)])
+            aa.main([*common, "--out", str(out_b)])
+
+        def note_mods(out_dir):
+            apkg = out_dir / "amateurfunk-technische-kenntnisse-n.apkg"
+            with tempfile.TemporaryDirectory() as tmp:
+                db_path, _media, _names = extract_collection(apkg, Path(tmp))
+                conn = sqlite3.connect(db_path)
+                try:
+                    return {
+                        row[0] for row in conn.execute("select mod from notes")
+                    }
+                finally:
+                    conn.close()
+
+        self.assertEqual(note_mods(out_a), {1777777777})
+        self.assertEqual(note_mods(out_b), {1777777788})
+
 
 class TestIntrinsicSvgSize(unittest.TestCase):
     """Parsing of native figure dimensions — SVG only."""