12735888711

Committed 12 Jan 2025 06:06PM UTC coverage: 92.973% (-6.9%) from 99.866%

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Merge 2bba403ef into f4478d7eb

Pull Request Pull Request #319: Update to rust 1.84

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98.0

/src/scheduler.rs

//! Defines and implements the data structures used to schedule batches of exercises to show to the
//! user. This module is the core mechanism of how Trane guides students to mastery of the material.
//!
//! The scheduler has a few core goals:
//! 1. Schedule exercises the user has practiced before in order to improve them and keep them up to
//!    date if they have been mastered already.
//! 2. Once the current material has been sufficiently mastered, schedule exercises that list the
//!    current material as a dependency.
//! 3. Optimize the difficulty of the schedule exercises so that the user is neither frustrated
//!    because many of the exercises are too difficult or bored because they have become too easy.
//!    The optimal are lies slightly outside their current comfort zone.
//! 4. Record the scores self-reported by the user to use them to drive the decisions done in
//!    service of all the other goals.
//!
//! In more formal terms, the scheduler's job is to plan the most optimal traversal of the graph of
//! skills as the student's performance blocks or unblocks certain paths. The current implementation
//! uses depth-first search to traverse the graph and collect a large pool of exercises, a multiple
//! of the actual exercises included in the final batch. From this large pool, the candidates are
//! split in groups of exercises which each match a disjoint range of scores to be randomly selected
//! into a list of fixed size. The result is combined, shuffled, and becomes the final batch
//! presented to the student.

mod cache;
pub mod data;
mod filter;

use anyhow::Result;
use chrono::Utc;
use rand::{seq::SliceRandom, thread_rng};
use ustr::{Ustr, UstrMap, UstrSet};

use crate::{
    data::{
        filter::{ExerciseFilter, KeyValueFilter, UnitFilter},
        ExerciseManifest, MasteryScore, SchedulerOptions, UnitType,
    },
    error::ExerciseSchedulerError,
    scheduler::{cache::ScoreCache, data::SchedulerData, filter::CandidateFilter},
};

/// The scheduler returns early if the search reaches a dead end and the number of candidates is
/// bigger than the multiple of the final batch size and this value. This is to avoid the need to
/// search the entire graph if the search already found a decently sized pool of candidates.
const MAX_CANDIDATE_FACTOR: usize = 10;

/// The trait that defines the interface for the scheduler. Contains functions to request a new
/// batch of exercises and to provide Trane the self-reported scores for said exercises.
pub trait ExerciseScheduler {
    /// Gets a new batch of exercises scheduled for a new trial. Contains an optimal filter to
    /// restrict the units visited during the search with the purpose of allowing students to choose
    /// which material to practice. If the filter is not provided, the scheduler will search the
    /// entire graph.
    fn get_exercise_batch(
        &self,
        filter: Option<ExerciseFilter>,
    ) -> Result<Vec<ExerciseManifest>, ExerciseSchedulerError>;

    /// Records the score of the given exercise's trial. The scores are used by the scheduler to
    /// decide when to stop traversing a path and how to sort and filter all the found candidates
    /// into a final batch.
    fn score_exercise(
        &self,
        exercise_id: Ustr,
        score: MasteryScore,
        timestamp: i64,
    ) -> Result<(), ExerciseSchedulerError>;

    /// Removes any cached scores for the given unit. The score will be recomputed the next time the
    /// score is needed.
    ///
    /// The scores for lessons and exercises are cached to save a large amount of unnecessary
    /// computation. Without the caller manually invalidating the cache using this call, it is not
    /// possible to know when the cached value becomes outdated with the current interface. The
    /// reason is that the calls to modify the blacklist are not known by the scheduler.
    ///
    /// However, the final users of Trane do not need to call this function because the `Trane`
    /// object in lib.rs takes care of clearing the cache when exposing the interface that modifies
    /// the blacklist.
    fn invalidate_cached_score(&self, unit_id: Ustr);

    /// Removes any cached scores from units with the given prefix. The same considerations as
    /// `invalidate_cached_score` apply.
    fn invalidate_cached_scores_with_prefix(&self, prefix: &str);

    /// Returns the options used to control the behavior of the scheduler.
    fn get_scheduler_options(&self) -> SchedulerOptions;

    /// Sets the options used to control the behavior of the scheduler.
    fn set_scheduler_options(&mut self, options: SchedulerOptions);

    /// Resets the options used to control the behavior of the scheduler to their default values.
    fn reset_scheduler_options(&mut self);
}

/// An item in the stack of units that are scheduled for traversal during the process of scheduling
/// the next batch of exercises.
struct StackItem {
    /// The ID of the unit contained in the item.
    unit_id: Ustr,

    /// The depth of this unit from the starting unit. That is, the number of hops the graph search
    /// needed to reach this exercise.
    depth: usize,
}

/// An exercise selected during the initial phase of the search and which will be grouped with all
/// the other candidates which fall in the same mastery window and filtered and randomly selected
/// to form the final batch.
#[derive(Clone, Debug)]
struct Candidate {
    /// The ID of the exercise.
    exercise_id: Ustr,

    // The ID of the exercise's lesson.
    lesson_id: Ustr,

    /// The depth of this unit from the starting unit. That is, the number of hops the graph search
    /// needed to reach this exercise.
    depth: f32,

    /// The score assigned to the exercise represented as a float number between 0.0 and 5.0
    /// inclusive. This score will be computed from the previous trials of this exercise.
    score: f32,

    /// The number of previous trials that have been recorded for this exercise.
    num_trials: usize,

    /// The number of times this exercise has been scheduled during the run of this scheduler. This
    /// value will be used to assign more weight to exercises that have been scheduled less often.
    frequency: usize,
}

/// An implementation of [`ExerciseScheduler`] based on depth-first search.
pub struct DepthFirstScheduler {
    /// The external data used by the scheduler. Contains pointers to the graph, blacklist, and
    /// course library and provides convenient functions.
    data: SchedulerData,

    /// A cache of unit scores. Scores are cached to avoid unnecessary computation, an issue that
    /// was found during profiling of Trane's performance. The memory footprint of Trane is low, so
    /// the trade-off is worth it.
    score_cache: ScoreCache,

    /// The filter used to build the final batch of exercises among the candidates found during the
    /// graph search.
    filter: CandidateFilter,
}

impl DepthFirstScheduler {
    /// Creates a new scheduler.
    #[must_use]
    pub fn new(data: SchedulerData) -> Self {
        Self {
            data: data.clone(),
            score_cache: ScoreCache::new(data.clone(), data.options.clone()),
            filter: CandidateFilter::new(data),
        }
    }

    /// Shuffles the units and pushes them to the given stack. Used with the goal of ensuring that
    /// the units are traversed in a different order each time a new batch is requested.
    fn shuffle_to_stack(curr_unit: &StackItem, mut units: Vec<Ustr>, stack: &mut Vec<StackItem>) {
        units.shuffle(&mut thread_rng());
        stack.extend(units.iter().map(|id| StackItem {
            unit_id: *id,
            depth: curr_unit.depth + 1,
        }));
    }

    /// Returns all the courses and lessons without dependencies which are used to initialize a
    /// search of the entire graph.
    fn get_all_starting_units(&self) -> UstrSet {
        // Replace any missing units with their dependents and repeat this process until there are
        // no missing courses.
        let mut starting_courses = self.data.unit_graph.read().get_dependency_sinks();
        loop {
            let mut new_starting_courses = UstrSet::default();
            for course_id in &starting_courses {
                if self.data.unit_exists(*course_id).unwrap_or(false) {
                    new_starting_courses.insert(*course_id);
                } else {
                    new_starting_courses.extend(self.data.get_all_dependents(*course_id).iter());
                }
            }
            if new_starting_courses.len() == starting_courses.len() {
                break;
            }
            starting_courses = new_starting_courses;
        }

        // Some courses added to the original list in the previous steps might have other
        // dependencies, some of which exist in the course library. This means they cannot be
        // considered a starting course, so remove them from the final output.
        starting_courses
            .into_iter()
            .filter(|course_id| {
                self.data
                    .unit_graph
                    .read()
                    .get_dependencies(*course_id)
                    .unwrap_or_default()
                    .iter()
                    .all(|id| !self.data.unit_exists(*id).unwrap())
            })
            .collect()
    }

    /// Returns the lessons in the course that have no dependencies with other lessons in the course
    /// and whose dependencies are satisfied.
    pub fn get_course_valid_starting_lessons(
        &self,
        course_id: Ustr,
        depth: usize,
        metadata_filter: Option<&KeyValueFilter>,
    ) -> Result<Vec<Ustr>> {
        Ok(self
            .data
            .unit_graph
            .read()
            .get_starting_lessons(course_id)
            .unwrap_or_default()
            .into_iter()
            .filter(|id| {
                // Filter out lessons whose dependencies are not satisfied. Otherwise, those lessons
                // would be traversed prematurely.
                self.all_satisfied_dependencies(*id, depth, metadata_filter)
            })
            .collect())
    }

    //@<lp-example-1
    /// Returns an initial stack with all the starting units in the graph that are used to search
    /// the entire graph.
    fn get_initial_stack(&self, metadata_filter: Option<&KeyValueFilter>) -> Vec<StackItem> {
        // First get all the starting units and then all of their starting lessons.
        let starting_units = self.get_all_starting_units();
        let mut initial_stack: Vec<StackItem> = vec![];
        for course_id in starting_units {
            // Set the depth to zero since all the starting units are at the same depth.
            let lesson_ids = self
                .get_course_valid_starting_lessons(course_id, 0, metadata_filter)
                .unwrap_or_default();

            if lesson_ids.is_empty() {
                // For units with no lessons, insert the unit itself as a starting unit so that its
                // dependents are traversed.
                initial_stack.push(StackItem {
                    unit_id: course_id,
                    depth: 0,
                });
            } else {
                // Insert all the starting lessons in the stack.
                initial_stack.extend(
                    lesson_ids
                        .into_iter()
                        .map(|unit_id| StackItem { unit_id, depth: 0 }),
                );
            }
        }

        // Shuffle the lessons to follow a different ordering each time a new batch is requested.
        initial_stack.shuffle(&mut thread_rng());
        initial_stack
    }
    //>@lp-example-1

    /// Returns the list of candidates selected from the given lesson along with the average score.
    /// The average score is used to help decide whether to continue searching a path in the graph.
    fn get_candidates_from_lesson_helper(&self, item: &StackItem) -> Result<(Vec<Candidate>, f32)> {
        // Retrieve the lesson's exercises.
        let exercises = self.data.all_valid_exercises_in_lesson(item.unit_id);
        if exercises.is_empty() {
            // Return early to avoid division by zero later on.
            return Ok((vec![], 0.0));
        }

        // Generate a list of candidates from the lesson's exercises.
        let candidates = exercises
            .into_iter()
            .map(|exercise_id| {
                Ok(Candidate {
                    exercise_id,
                    lesson_id: item.unit_id, // It's assumed that the item is a lesson.
                    depth: (item.depth + 1) as f32,
                    score: self
                        .score_cache
                        .get_unit_score(exercise_id)?
                        .unwrap_or_default(),
                    num_trials: self
                        .score_cache
                        .get_num_trials(exercise_id)?
                        .unwrap_or_default(),
                    frequency: self.data.get_exercise_frequency(exercise_id),
                })
            })
            .collect::<Result<Vec<Candidate>>>()?;

        // Calculate the average score of the candidates.
        let avg_score = candidates.iter().map(|c| c.score).sum::<f32>() / (candidates.len() as f32);
        Ok((candidates, avg_score))
    }

    /// Returns whether the given dependency can be considered as satisfied. If all the dependencies
    /// of a unit are met, the search can continue with the unit.
    fn satisfied_dependency(
        &self,
        dependency_id: Ustr,
        depth: usize,
        metadata_filter: Option<&KeyValueFilter>,
    ) -> bool {
        // Dependencies which do not pass the metadata filter are considered as satisfied, so the
        // search can continue past them.
        let passes_filter = self
            .data
            .unit_passes_filter(dependency_id, metadata_filter)
            .unwrap_or(false);
        if !passes_filter {
            return true;
        }

        // Dependencies in the blacklist are considered as satisfied, so the search can continue
        // past them.
        let blacklisted = self.data.blacklisted(dependency_id);
        if blacklisted.unwrap_or(false) {
            return true;
        }

        // Dependencies which are a lesson belonging to a blacklisted course are considered as
        // satisfied, so the search can continue past them.
        let course_id = self
            .data
            .get_lesson_course(dependency_id)
            .unwrap_or_default();
        if self.data.blacklisted(course_id).unwrap_or(false) {
            return true;
        }

        // The dependency is considered as satisfied if it's been superseded by another unit.
        let superseding = self.score_cache.get_superseding_recursive(dependency_id);
        if let Some(superseding) = superseding {
            if self.score_cache.is_superseded(dependency_id, &superseding) {
                return true;
            }
        }

        // Finally, dependencies with a score equal or greater than the passing score are considered
        // as satisfied.
        let score = self.score_cache.get_unit_score(dependency_id);
        if let Ok(Some(score)) = score {
            score >= self.data.options.passing_score.compute_score(depth)
        } else {
            true
        }
    }

    /// Returns whether all the dependencies of the given unit are satisfied.
    fn all_satisfied_dependencies(
        &self,
        unit_id: Ustr,
        depth: usize,
        metadata_filter: Option<&KeyValueFilter>,
    ) -> bool {
        self.data
            .unit_graph
            .read()
            .get_dependencies(unit_id)
            .unwrap_or_default()
            .into_iter()
            .all(|dependency_id| self.satisfied_dependency(dependency_id, depth, metadata_filter))
    }

    /// Returns the valid dependents which can be visited after the given unit. A valid dependent is
    /// a unit whose full dependencies are met.
    fn get_valid_dependents(
        &self,
        unit_id: Ustr,
        depth: usize,
        metadata_filter: Option<&KeyValueFilter>,
    ) -> Vec<Ustr> {
        self.data
            .get_all_dependents(unit_id)
            .into_iter()
            .filter(|unit_id| self.all_satisfied_dependencies(*unit_id, depth, metadata_filter))
            .collect()
    }

    // Returns whether the given course should be skipped during the search. If so, the valid
    /// dependents of the course should be added to the stack.
    fn skip_course(
        &self,
        course_id: Ustr,
        metadata_filter: Option<&KeyValueFilter>,
        pending_course_lessons: &mut UstrMap<usize>,
    ) -> bool {
        // Check if the course is blacklisted.
        let blacklisted = self.data.blacklisted(course_id).unwrap_or(false);

        // Check if the course passes the metadata filter.
        let passes_filter = self
            .data
            .unit_passes_filter(course_id, metadata_filter)
            .unwrap_or(true);

        // Check the number of pending lessons in the course.
        let pending_lessons = pending_course_lessons
            .entry(course_id)
            .or_insert_with(|| self.data.get_num_lessons_in_course(course_id));

        // Check if the course has been superseded by another unit.
        let superseding_units = self
            .score_cache
            .get_superseding_recursive(course_id)
            .unwrap_or_default();
        let is_superseded = self
            .score_cache
            .is_superseded(course_id, &superseding_units);

        // The course should be skipped if the course is blacklisted, does not pass the filter, has
        // no pending lessons, or if it' been superseded.
        blacklisted || !passes_filter || *pending_lessons == 0 || is_superseded
    }

    /// Returns whether the given lesson should be skipped during the search. If so, the valid
    /// dependents of the lesson should be added to the stack.
    fn skip_lesson(&self, lesson_id: Ustr, metadata_filter: Option<&KeyValueFilter>) -> bool {
        // Check if the lesson is blacklisted.
        let blacklisted = self.data.blacklisted(lesson_id).unwrap_or(false);

        // Check if the lesson passes the metadata filter.
        let passes_filter = self
            .data
            .unit_passes_filter(lesson_id, metadata_filter)
            .unwrap_or(true);

        // Check if the lesson has been superseded by another unit.
        let superseding_units = self
            .score_cache
            .get_superseding_recursive(lesson_id)
            .unwrap_or_default();
        let is_lesson_superseded = self
            .score_cache
            .is_superseded(lesson_id, &superseding_units);

        // Check if the lesson's course has been superseded by another unit.
        let course_id = self.data.get_lesson_course(lesson_id).unwrap_or_default();
        let superseding_units = self
            .score_cache
            .get_superseding_recursive(course_id)
            .unwrap_or_default();
        let is_course_superseded = self
            .score_cache
            .is_superseded(course_id, &superseding_units);

        // The lesson should be skipped if it is blacklisted, does not pass the filter or if it or
        // its course have been superseded.
        blacklisted || !passes_filter || is_lesson_superseded || is_course_superseded
    }

    /// Searches for candidates across the entire graph. An optional metadata filter can be used to
    /// only select exercises from the courses and lessons that match the filter and ignore the rest
    /// of the graph while still respecting the dependency relationships.
    fn get_candidates_from_graph(
        &self,
        initial_stack: Vec<StackItem>,
        metadata_filter: Option<&KeyValueFilter>,
    ) -> Result<Vec<Candidate>> {
        // Initialize the stack with every starting lesson, which are those units with no
        // dependencies that are needed to reach all the units in the graph.
        let mut stack: Vec<StackItem> = Vec::new();
        stack.extend(initial_stack);

        // Initialize the list of candidates and the set of visited units.
        let max_candidates = self.data.options.batch_size * MAX_CANDIDATE_FACTOR;
        let mut all_candidates: Vec<Candidate> = Vec::new();
        let mut visited = UstrSet::default();

        // The dependency relationships between a course and its lessons are not explicitly encoded
        // in the graph. While this would simplify this section of the search logic, it would
        // require that courses are represented by two nodes. The first incoming node would connect
        // the course dependencies to the first lessons in the course. The second outgoing node
        // would connect the last lessons in the course to the course dependents.
        //
        // To get past this limitation, the search will only add the course dependents until all of
        // its lessons have been visited and mastered. This value is tracked by the
        // `pending_course_lessons` map.
        let mut pending_course_lessons: UstrMap<usize> = UstrMap::default();

        // Perform a depth-first search of the graph.
        while let Some(curr_unit) = stack.pop() {
            // Immediately skip the item if it has been visited.
            if visited.contains(&curr_unit.unit_id) {
                continue;
            }

            // The logic past this point depends on the type of the unit.
            let unit_type = self.data.get_unit_type(curr_unit.unit_id);
            if unit_type.is_none() {
                // The type of the unit is unknown. This can happen when a unit depends on some
                // unknown unit.
                continue;
            }
            let unit_type = unit_type.unwrap();

            // Handle exercises. All of them should be skipped as the search only considers lessons
            // and courses.
            if unit_type == UnitType::Exercise {
                continue;
            }

            // Handle courses.
            if unit_type == UnitType::Course {
                // Retrieve the starting lessons in the course and add them to the stack.
                let starting_lessons: Vec<Ustr> = self
                    .get_course_valid_starting_lessons(
                        curr_unit.unit_id,
                        curr_unit.depth,
                        metadata_filter,
                    )
                    .unwrap_or_default();
                Self::shuffle_to_stack(&curr_unit, starting_lessons, &mut stack);

                // The course can be skipped. Add it to the visited set, push its valid dependents
                // onto the stack, and continue.
                if self.skip_course(
                    curr_unit.unit_id,
                    metadata_filter,
                    &mut pending_course_lessons,
                ) {
                    visited.insert(curr_unit.unit_id);
                    let valid_deps = self.get_valid_dependents(
                        curr_unit.unit_id,
                        curr_unit.depth,
                        metadata_filter,
                    );
                    Self::shuffle_to_stack(&curr_unit, valid_deps, &mut stack);
                    continue;
                }

                // The course has pending lessons, so it cannot be marked as visited yet. Simply
                // continue with the search.
                continue;
            }

            // If the searched reached this point, the unit must be a lesson.
            visited.insert(curr_unit.unit_id);

            // Update the number of lessons pending to be processed.
            let course_id = self.data.get_course_id(curr_unit.unit_id)?;
            let pending_lessons = pending_course_lessons
                .entry(course_id)
                .or_insert_with(|| self.data.get_num_lessons_in_course(course_id));
            *pending_lessons -= 1;

            // Check whether there are pending lessons.
            if *pending_lessons == 0 {
                // Once all the lessons in the course have been visited, re-add the course to the
                // stack, so the search can continue exploring its dependents.
                stack.push(StackItem {
                    unit_id: course_id,
                    depth: curr_unit.depth + 1,
                });
            }

            // Retrieve the valid dependents of the lesson, and directly skip the lesson if needed.
            let valid_deps =
                self.get_valid_dependents(curr_unit.unit_id, curr_unit.depth, metadata_filter);
            if self.skip_lesson(curr_unit.unit_id, metadata_filter) {
                Self::shuffle_to_stack(&curr_unit, valid_deps, &mut stack);
                continue;
            }

            // Retrieve the candidates from the lesson and add them to the list of candidates.
            let (candidates, avg_score) = self.get_candidates_from_lesson_helper(&curr_unit)?;
            let num_candidates = candidates.len();
            all_candidates.extend(candidates);

            // The average score is considered valid only if at least one candidate was retrieved.
            // Compare it against the passing score to decide whether the search should continue
            // past this lesson.
            if num_candidates > 0
                && avg_score
                    < self
                        .data
                        .options
                        .passing_score
                        .compute_score(curr_unit.depth)
            {
                // If the search reaches a dead-end and there are already enough candidates,
                // terminate the search. Otherwise, continue with the search.
                if all_candidates.len() >= max_candidates {
                    break;
                }
                continue;
            }

            // The search should continue past this lesson. Add its valid dependents to the stack.
            Self::shuffle_to_stack(&curr_unit, valid_deps, &mut stack);
        }

        Ok(all_candidates)
    }

    /// Searches for candidates from the given course.
    fn get_candidates_from_course(&self, course_ids: &[Ustr]) -> Result<Vec<Candidate>> {
        // Initialize the set of visited units and the stack with the starting lessons from the
        // courses. Add all starting lessons, even if their dependencies are not satisfied, because
        // the user specifically asked for questions from these courses.
        let mut stack: Vec<StackItem> = Vec::new();
        let mut visited = UstrSet::default();
        for course_id in course_ids {
            let starting_lessons = self
                .data
                .unit_graph
                .read()
                .get_starting_lessons(*course_id)
                .unwrap_or_default()
                .into_iter()
                .map(|id| StackItem {
                    unit_id: id,
                    depth: 0,
                });
            stack.extend(starting_lessons);
            visited.insert(*course_id);
        }

        // Initialize the list of candidates.
        let max_candidates = self.data.options.batch_size * MAX_CANDIDATE_FACTOR;
        let mut all_candidates: Vec<Candidate> = Vec::new();

        // Perform a depth-first search to find the candidates.
        while let Some(curr_unit) = stack.pop() {
            // Continue if the unit has been visited and update the list of visited units.
            if visited.contains(&curr_unit.unit_id) {
                continue;
            }
            visited.insert(curr_unit.unit_id);

            // The logic past this point depends on the type of the unit.
            let unit_type = self.data.get_unit_type(curr_unit.unit_id);
            if unit_type.is_none() {
                // The type of the unit is unknown. This can happen when a unit depends on some
                // unknown unit.
                continue;
            }
            let unit_type = unit_type.unwrap();

            // Handle courses. They should be skipped, as all the courses that should be considered
            // were already handled when their starting lessons were added to the stack.
            if unit_type == UnitType::Course {
                continue;
            }

            // Handle exercises. They should be skipped as well, as only lessons should be
            // traversed.
            if unit_type == UnitType::Exercise {
                continue;
            }

            // If the searched reached this point, the unit must be a lesson. Ignore lessons from
            // other courses that might have been added to the stack.
            let lesson_course_id = self
                .data
                .get_lesson_course(curr_unit.unit_id)
                .unwrap_or_default();
            if !course_ids.contains(&lesson_course_id) {
                continue;
            }

            // Retrieve the valid dependents of the lesson, and directly skip the lesson if needed.
            let valid_deps = self.get_valid_dependents(curr_unit.unit_id, curr_unit.depth, None);
            if self.skip_lesson(curr_unit.unit_id, None) {
                Self::shuffle_to_stack(&curr_unit, valid_deps, &mut stack);
                continue;
            }

            // Retrieve the candidates from the lesson and add them to the list of candidates.
            let (candidates, avg_score) = self.get_candidates_from_lesson_helper(&curr_unit)?;
            let num_candidates = candidates.len();
            all_candidates.extend(candidates);

            // The average score is considered valid only if at least one candidate was retrieved.
            // Compare it against the passing score to decide whether the search should continue
            // past this lesson.
            if num_candidates > 0
                && avg_score
                    < self
                        .data
                        .options
                        .passing_score
                        .compute_score(curr_unit.depth)
            {
                // If the search reaches a dead-end and there are already enough candidates,
                // terminate the search. Continue otherwise.
                if all_candidates.len() >= max_candidates {
                    break;
                }
                continue;
            }

            // Add the lesson's valid dependents to the stack.
            Self::shuffle_to_stack(&curr_unit, valid_deps, &mut stack);
        }

        Ok(all_candidates)
    }

    /// Searches for candidates from the given lesson.
    fn get_candidates_from_lesson(&self, lesson_id: Ustr) -> Result<Vec<Candidate>> {
        let (candidates, _) = self.get_candidates_from_lesson_helper(&StackItem {
            unit_id: lesson_id,
            depth: 0,
        })?;
        Ok(candidates)
    }

    /// Searches from candidates from the units in the review list. This mode allows the student to
    /// exclusively practice the courses, lessons, and exercises they have marked for review.
    fn get_candidates_from_review_list(&self) -> Result<Vec<Candidate>> {
        // Retrieve candidates from each entry in the review list.
        let mut candidates = vec![];
        let review_list = self.data.review_list.read().get_review_list_entries()?;
        for unit_id in review_list {
            match self.data.get_unit_type_strict(unit_id)? {
                UnitType::Course => {
                    // If the unit is a course, use the course scheduler to retrieve candidates.
                    let course_ids = vec![unit_id];
                    candidates.extend(self.get_candidates_from_course(&course_ids)?);
                }
                UnitType::Lesson => {
                    // If the unit is a lesson, use the lesson scheduler to retrieve candidates.
                    candidates.extend(self.get_candidates_from_lesson(unit_id)?);
                }
                UnitType::Exercise => {
                    // Retrieve the exercise's lesson ID.
                    let lesson_id = self
                        .data
                        .unit_graph
                        .read()
                        .get_exercise_lesson(unit_id)
                        .unwrap_or_default();

                    // If the unit is an exercise, directly add it to the list of candidates.
                    candidates.push(Candidate {
                        exercise_id: unit_id,
                        lesson_id,
                        depth: 0.0,
                        score: self
                            .score_cache
                            .get_unit_score(unit_id)?
                            .unwrap_or_default(),
                        num_trials: self
                            .score_cache
                            .get_num_trials(unit_id)?
                            .unwrap_or_default(),
                        frequency: *self.data.frequency_map.read().get(&unit_id).unwrap_or(&0),
                    });
                }
            }
        }

        Ok(candidates)
    }

    /// Retrieves an initial batch of candidates based on the given filter.
    fn get_initial_candidates(&self, filter: Option<ExerciseFilter>) -> Result<Vec<Candidate>> {
        // Retrieve an initial list of candidates based on the type of the filter.
        let candidates = match filter {
            None => {
                // If the filter is empty, retrieve candidates from the entire graph. This mode is
                // Trane's default.
                let initial_stack = self.get_initial_stack(None);
                self.get_candidates_from_graph(initial_stack, None)?
            }
            Some(filter) => match filter {
                // Otherwise, use the given filter to select how candidates are retrieved.
                ExerciseFilter::UnitFilter(filter) => match filter {
                    UnitFilter::CourseFilter { course_ids } => {
                        self.get_candidates_from_course(&course_ids[..])?
                    }
                    UnitFilter::LessonFilter { lesson_ids } => {
                        let mut candidates = Vec::new();
                        for lesson_id in lesson_ids {
                            candidates
                                .extend(self.get_candidates_from_lesson(lesson_id)?.into_iter());
                        }
                        candidates
                    }
                    UnitFilter::MetadataFilter { filter } => {
                        let initial_stack = self.get_initial_stack(Some(&filter));
                        self.get_candidates_from_graph(initial_stack, Some(&filter))?
                    }
                    UnitFilter::ReviewListFilter => self.get_candidates_from_review_list()?,
                    UnitFilter::Dependents { unit_ids } => {
                        let initial_stack = unit_ids
                            .iter()
                            .map(|unit_id| StackItem {
                                unit_id: *unit_id,
                                depth: 0,
                            })
                            .collect();
                        self.get_candidates_from_graph(initial_stack, None)?
                    }
                    UnitFilter::Dependencies { unit_ids, depth } => {
                        let dependencies: Vec<Ustr> = unit_ids
                            .iter()
                            .flat_map(|unit_id| {
                                self.data.get_dependencies_at_depth(*unit_id, depth)
                            })
                            .collect();
                        let initial_stack = dependencies
                            .iter()
                            .map(|unit_id| StackItem {
                                unit_id: *unit_id,
                                depth: 0,
                            })
                            .collect();
                        self.get_candidates_from_graph(initial_stack, None)?
                    }
                },
                ExerciseFilter::StudySession(session_data) => {
                    let unit_filter = self
                        .data
                        .get_session_filter(&session_data, Utc::now())?
                        .map(ExerciseFilter::UnitFilter);
                    self.get_initial_candidates(unit_filter)?
                }
            },
        };

        Ok(candidates)
    }
}

impl ExerciseScheduler for DepthFirstScheduler {
    fn get_exercise_batch(
        &self,
        filter: Option<ExerciseFilter>,
    ) -> Result<Vec<ExerciseManifest>, ExerciseSchedulerError> {
        // Retrieve an initial batch of candidates based on the type of the filter.
        let initial_candidates = self
            .get_initial_candidates(filter)
            .map_err(ExerciseSchedulerError::GetExerciseBatch)?;

        // Sort the candidates into buckets, select the right number from each, and convert them
        // into a final batch of exercises.
        let final_candidates = self
            .filter
            .filter_candidates(&initial_candidates)
            .map_err(ExerciseSchedulerError::GetExerciseBatch)?;

        // Increment the frequency of the exercises in the batch. These exercises will have a lower
        // chance of being selected in the future,so that exercises that have not been selected as
        // often have a higher chance of being selected.
        for exercise_manifest in &final_candidates {
            self.data.increment_exercise_frequency(exercise_manifest.id);
        }

        Ok(final_candidates)
    }

    fn score_exercise(
        &self,
        exercise_id: Ustr,
        score: MasteryScore,
        timestamp: i64,
    ) -> Result<(), ExerciseSchedulerError> {
        // Write the score to the practice stats database.
        self.data
            .practice_stats
            .write()
            .record_exercise_score(exercise_id, score, timestamp)
            .map_err(|e| ExerciseSchedulerError::ScoreExercise(e.into()))?;

        // Any cached score for this exercise and its parent lesson is now invalid. Remove it from
        // the exercise and lesson caches.
        self.score_cache.invalidate_cached_score(exercise_id);
        Ok(())
    }

    fn invalidate_cached_score(&self, unit_id: Ustr) {
        self.score_cache.invalidate_cached_score(unit_id);
    }

    fn invalidate_cached_scores_with_prefix(&self, prefix: &str) {
        self.score_cache
            .invalidate_cached_scores_with_prefix(prefix);
    }

    fn get_scheduler_options(&self) -> SchedulerOptions {
        self.data.options.clone()
    }

    fn set_scheduler_options(&mut self, options: SchedulerOptions) {
        self.data.options = options;
    }

    fn reset_scheduler_options(&mut self) {
        self.data.options = SchedulerOptions::default();
    }
}

trane-project / trane / 12735888711

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