The data model and Anki compatibility

The collection is one SQLite file, collection.sqlite, with Anki’s column names and units: a review Anki stored as “interval 14, ease 2500, due on day 812” is stored as those numbers. The schema is crates/loci-core/src/schema.sql.

Units

The collection’s creation day, crt, is the zero point for review due days. It lives in col_meta, together with the day rollover hour (4 by default), the schema version and a collection id.

The main tables

notes   (id ms, guid, ntid, mtime s, usn, tags, flds 0x1f-separated, sfld, csum, plain)
cards   (id ms, nid, did, ord, mtime, usn, type, queue, due, ivl, factor, reps, lapses,
         left, odue, odid, flags, data JSON)
revlog  (id ms, cid, usn, ease, ivl, last_ivl, factor, time ms, type)

• Notes hold the fields, joined by the unit separator U+001F as Anki does, and tags as a space-padded string (' tag1 tag2 '). sfld is the HTML-stripped sort field; csum is the first 8 hex digits of the SHA-1 of the stripped first field, used for duplicate checks; plain is all fields stripped of HTML, which feeds full-text search.
• Cards carry the scheduling columns. type is 0 new, 1 learning, 2 review, 3 relearning. queue adds the states a card can be parked in: −1 suspended, −2 buried as a sibling, −3 buried by hand, plus 3 for day-learning and 4 for preview. The low three bits of flags are the flag colour.
• Revlog has one row per answer. ease is the button (1 Again to 4 Easy; 0 for a manual reschedule), and type is 0 learn, 1 review, 2 relearn, 3 filtered/early, 4 manual, 5 rescheduled.

Note types, fields, templates, decks and deck presets are normalised tables (notetypes, fields, templates, decks, deck_configs) with typed JSON configuration. Media bodies are plain files in the media/ directory, indexed by a media(name, sha1, size, mtime) table.

Other tables: tags, saved_searches, undo_log (one row per undoable step, holding the inverse operation), optimise_log (every optimiser run, with parameters and metrics before and after), and graves (deleted ids, as Anki keeps them).

Search uses an FTS5 table, notes_fts, over notes.plain and the tags, with the trigram tokenizer. Trigrams give substring matching for Latin, Arabic and CJK text; queries shorter than three characters fall back to LIKE on notes.plain.

Sync. Sync stamps and deletions live in side tables (sync_rows and its neighbours); the Anki-compatible tables are untouched.

FSRS state lives where Anki puts it

FSRS memory state is stored in cards.data as JSON, with Anki’s keys: s (stability in days), d (difficulty, 1 to 10), dr (desired retention in effect), decay, and pos (the original new-card position). A card that goes Loci → Anki → Loci keeps both its FSRS state and its SM-2 columns.

Package layouts

Anki has three package layouts. A zip entry called meta (a small protobuf) says which one a file uses; when it is absent, the name of the collection file inside decides.

Loci reads all three, as .apkg (a deck) or .colpkg (a whole collection). zstd is decoded in pure Rust; protobuf fields are decoded by field number.

Loci writes one: LEGACY_2. An export is a schema-11 collection.anki21, meta = 08 02, a JSON media map with numbered media files, "schedVer": 2 in the collection config, and the small dummy collection.anki2 that every Anki package carries for very old clients. Anki 2.1 through 26.x and AnkiDroid import this layout. Loci never writes .anki21b.

What round-trips

Exporting from Loci and importing the result into a second Loci collection is lossless: notes, every scheduling column of every card, FSRS data, the review log and media bytes come back identical. The test suite checks this.

Out to Anki and back:

• The day numbering. If the collection began with an import from Anki, exports carry that Anki collection’s creation time (crt) and renumber due days to it, so a package returning to Anki has the day numbering it left with. Otherwise the package carries Loci’s own crt. Either way the day numbers and crt agree, so any importer lands each card on the same date.
• Deck presets are written as Anki deck options. A preset that came from Anki keeps its original Anki JSON, so options Loci does not model are not lost.

What does not round-trip:

• Filtered decks. Loci has no persistent filtered decks. Importing one returns its cards to their home decks. Use Custom session for study-ahead, forgotten cards, previews or an arbitrary search.
• The .anki21b layout. A collection imported from a LATEST package exports as LEGACY_2, with the same content.