#+TITLE: Review History
#+DATE: [2020-08-01 Sat 10:47]
#+KEYWORDS: fc

The review history is stored in a tsv file, to avoid cluttering org
files. This makes it easy to calculate review statistics.

Columns:
1. Date in ISO8601 format, second precision
2. Filename
3. Card ID
4. Position
5. Ease (before review), used only by the SM2 algorithm
6. Box (before review), used only by the SM2 algorithm
7. Interval (before review), used only by the SM2 algorithm
8. Rating ("again", "hard", "good", "easy")
9. Seconds spent reviewing the card
10. [[file:repetition_spacing_algorithm.org][Repetition Spacing Algorithm]] used

More advanced review algorithms might need to use the review history
of a card. In this case, the card ID + position should be used to look
up the review history, as the filename can change when moving cards
from file to file.

Columns 5, 6 and 7 are considered legacy and should be left empty
by [[file:repetition_spacing_algorithm.org][Repetition Spacing Algorithms]] other than the initial SM2 variant
implemented by org-fc.

Columns 9 and 10 were added later, so it's possible users have review
files where these are missing. The review duration is unknown in this
case and we can assume the algorithm used was "sm2".

* Migrating Between Spacing Algorithms

The contents of the review history file should be sufficient
to re-computed the state of a card stored in the [[file:review_data.org][Review Data drawer]].

Depending on the spacing algorithm used, this history can also be used
to migrate the review data to the newer algorithm.