🧠 FSRS Scheduler (Version 6) – Kotlin Implementation

This repository contains a Kotlin implementation of the Free Spaced Repetition Scheduler (FSRS), version 6. FSRS is a modern algorithm for spaced repetition, optimized using machine learning to improve long-term retention and scheduling efficiency.

🔬 FSRS v6 is the latest official version of the algorithm, offering better accuracy and flexibility compared to older algorithms like SM-2.

✅ Features

📌 Based on the official FSRS v6 specification
🔄 Supports all review responses: Again, Hard, Good, Easy
🧮 Returns updated stability, difficulty, and interval
🧱 Written in idiomatic Kotlin, easy to integrate into Android or JVM projects
🕒 Uses LocalDateTime for precise due date handling
🧪 Suitable for experimentation or direct production use

📦 Use Cases

This FSRS implementation can be used in:

Flashcard apps (Anki-like)
Educational software for medical, language, and test prep
Any Kotlin-based spaced repetition project

🚀 Example Usage

//create instance, use deck default retention(0.9) and default params
//FSRS-6 sepecific params = [0.212, 1.2931, 2.3065, 8.2956, 6.4133, 0.8334, 3.0194, 0.001, 1.8722, 0.1666, 0.796, 1.4835, 0.0614, 0.2629, 1.6483, 0.6014, 1.8729, 0.5425, 0.0912, 0.0658, 0.1542]
val fsrs = FSRS(deck.retention, deck.params)

//calculate a list contains stability, difficulty, interval, durationMillis and text to be shown for each button
//see models.kt file
val gradeList = fsrs.calculate(flashCard)

//to update card, just add interval to current time
fun addMillisToNow(millis: Long): LocalDateTime {
        val nowInstant = Instant.now()
        val newInstant = nowInstant.plusMillis(millis)
        return LocalDateTime.ofInstant(newInstant, ZoneId.systemDefault())
    }

📚 References

🛠 License

This implementation is provided under the MIT License. Attribution to the original FSRS author is appreciated.

🙌 Contributing

Feel free to fork the repo and submit pull requests if you’d like to add enhancements or bug fixes. Issues and suggestions are always welcome.