Implements an Anki-style spaced repetition scheduler for active recall prompts.

In a learning system that uses active recall, learners are presented with a prompt and rate their ability to recall the corresponding information. SpacedRepetitionScheduler recommends a time interval to wait before showing a learner the same prompt again, given his/her history of recalling the information associated with this prompt and how well he/she did recalling the associated information this time. The recommended time intervals for a given prompt will increase the more frequently the learner recalls the information associated with a prompt.

SpacedRepetitionScheduler considers a prompt to be in one of two modes when making time interval recommendations: learning or review.

• In the learning mode, the learner must successfully recall the corresponding information a specific number of times with specific time intervals between recall attempts. After successfully recalling the associated information the specified number of times, the prompt graduates to the review state.
• In the review mode, the amount of time between successive reviews of a prompt increases by a geometric progression with each successful recall.

• PromptSchedulingMetadata -- Information needed to determine the recommended time to review a prompt again.
• PromptSchedulingMode -- Whether the prompt is in learning or review mode.

• RecallEase -- Rating for how easy it is to recall the information associated with a prompt.
• SchedulingParameters -- Holds parameters used to determine the recommended time to schedule the next review of a prompt.