The algorithm (FSRS) supports reviewing in advance or delay. It's free for users to decide the time of review. And it will adapt to the user's memory.
Meanwhile, spaced repetition is one essential technology to achieve free learning.
FSRS runs entirely locally and has no risk under others' control.
FSRS is based on the DSR model proposed by Piotr Wozniak, the author of SuperMemo. FSRS is improved with the DHP model introduced in the paper: A Stochastic Shortest Path Algorithm for Optimizing Spaced Repetition Scheduling.
The model considers three variables that affect memory: difficulty, stability, and retrievability.
Stability refers to the storage strength of memory; the higher it is, the slower it is forgotten. Retrievability refers to memory's retrieval strength; the lower it is, the higher the probability that the memory will be forgotten.
In the present model, the following memory laws are considered:
The formula of memory on FSRS: The Algorithm
Scheduler:
Optimizer: