Stop a deck file losing the lines that are not cards

[Aduneer]

Three ways a deck file could be damaged or a path mistyped, all found by pointing the app at files nobody had thought to point it at. Found while auditing before an announcement — the first one is the sort of thing that becomes the comment in the thread.

Lines that are not cards were silently deleted

Anything in a deck file that did not parse as a card was dropped on load, and the next save — after a single answered card — wrote the deck back without it. A heading, a note to yourself, a line with a typo in it: gone, with nothing on screen to say so.

$ cat spanish.txt              $ # answer one card, then:
# Chapter one                  hola,hello,,1,0,2,2026-08-23,2026-08-26,98911cdf12151615
hola,hello                     adios,goodbye,
TODO: add the rest
adios,goodbye

The worst version was a mistyped path. FlashTerm ~/notes.txt opened happily, reported Loaded 0 flashcards, and replaced the file's entire contents with the first card you added to it.

Non-card lines are now carried through load and save untouched, anchored to the card they sat above so headings stay above their section and notes stay at the bottom. Blank lines are carried the same way, so a deck with sections in it round trips byte for byte and the 0.3.1 no-op-save check still holds on it. Opening a deck that has any says so once, on the way in — which is also what tells you the file was never a deck:

$ FlashTerm notes.txt
Loaded 0 flashcards from notes.txt
2 lines are not flashcards. They are kept exactly as they are and written
back untouched — if you meant a different file, this is what that looks like.

Deliberately not a comment syntax: # would have been the obvious choice and it would have broken #include,preprocessor directive in exactly the kind of deck this project ships examples for. Preservation needs no format change, and covers lines nobody would have thought to mark.

A deck path that cannot hold a deck is refused up front

Naming a directory loaded an empty deck and opened as normal; so did naming a file under a directory that does not exist. Either way the problem only surfaced as a failed save, after a card had been typed in. Both are checked before the deck is opened now, for every mode, and exit 2 with the reason.

$ FlashTerm ~/decks
FlashTerm: /home/me/decks is a directory, not a deck file
$ FlashTerm missing/deck.txt
FlashTerm: cannot create missing/deck.txt: there is no directory missing

CRLF decks load clean

A deck written on Windows, or exported by a spreadsheet, kept the carriage return as part of each answer. Invisible on screen, but it went back to disk as a quoted "hello\r" and stayed there. Stripped where the line is parsed, so imports are covered as well as decks, and writing normalises to \n.

Tests

Five unit tests and three golden cases, all covering behaviour that had no coverage at all:

• test_foreign_lines_survive_saving — the round trip, including that an untouched file with comments in it is still not rewritten
• test_non_deck_file_is_not_emptied — the mistyped-path case
• test_foreign_lines_outlive_their_cards — anchors point at cards, so deleting every card must leave the lines somewhere rather than nowhere
• test_crlf_deck_loads_clean, test_deck_path_error
• golden deck-foreign-lines (two cards answered, all three foreign lines still in place in the transcript's file dump), deck-path-is-a-directory, deck-path-no-directory

1233 unit checks and 50 golden cases pass, in both the optimised and the -fsanitize=address,undefined build.