Feat/support load balance and easy days in rescheduling (#3815)

* Feat/support load balance and easy days in rescheduling

* ./ninja fix:minilints

* apply clippy

* reuse calculate_easy_days_modifiers()

* consider LoadBalancerEnabled

* move calculate_easy_days_modifiers out of struct

* improve naming & add comments

* apply clippy

* reschedule if easy days settings are changed

* Minor simplification

* refactor to share code between load balancer and rescheduler

* intervals_and_params -> intervals_and_weights

* find_best_interval -> select_weighted_interval

* cargo clippy

* add warning about easyDaysChanged

* compare arrays directly

* Don't show warning if fsrs+rescehdule is already enabled

---------

Co-authored-by: Damien Elmes <gpg@ankiweb.net>
Co-authored-by: Jake Probst <jake.probst@gmail.com>

ftl/core/deck-config.ftl

@@ -293,6 +293,7 @@ deck-config-easy-days-normal = Normal
 deck-config-easy-days-reduced = Reduced
 deck-config-easy-days-minimum = Minimum
 deck-config-easy-days-no-normal-days = At least one day should be set to '{ deck-config-easy-days-normal }'.
+deck-config-easy-days-change = Existing reviews will not be rescheduled unless '{ deck-config-reschedule-cards-on-change }' is enabled in the FSRS options.
 
 ## Adding/renaming
 

rslib/src/deckconfig/update.rs

@@ -215,6 +215,7 @@ impl Collection {
                     .unwrap_or_default();
                 let previous_params = previous_config.map(|c| c.fsrs_params());
                 let previous_retention = previous_config.map(|c| c.inner.desired_retention);
+                let previous_easy_days = previous_config.map(|c| &c.inner.easy_days_percentages);
 
                 // if a selected (sub)deck, or its old config was removed, update deck to point
                 // to new config
@@ -242,9 +243,11 @@ impl Collection {
                 // if params differ, memory state needs to be recomputed
                 let current_params = current_config.map(|c| c.fsrs_params());
                 let current_retention = current_config.map(|c| c.inner.desired_retention);
+                let current_easy_days = current_config.map(|c| &c.inner.easy_days_percentages);
                 if fsrs_toggled
                     || previous_params != current_params
                     || previous_retention != current_retention
+                    || (req.fsrs_reschedule && previous_easy_days != current_easy_days)
                 {
                     decks_needing_memory_recompute
                         .entry(current_config_id)

rslib/src/scheduler/answering/mod.rs

@@ -608,7 +608,7 @@ impl Card {
 /// Return a consistent seed for a given card at a given number of reps.
 /// If for_reschedule is true, we use card.reps - 1 to match the previous
 /// review.
-fn get_fuzz_seed(card: &Card, for_reschedule: bool) -> Option<u64> {
+pub(crate) fn get_fuzz_seed(card: &Card, for_reschedule: bool) -> Option<u64> {
     let reps = if for_reschedule {
         card.reps.saturating_sub(1)
     } else {

rslib/src/scheduler/fsrs/memory_state.rs

@@ -10,10 +10,12 @@ use fsrs::FSRS;
 use itertools::Itertools;
 
 use super::params::ignore_revlogs_before_ms_from_config;
+use super::rescheduler::Rescheduler;
 use crate::card::CardType;
 use crate::prelude::*;
 use crate::revlog::RevlogEntry;
 use crate::revlog::RevlogReviewKind;
+use crate::scheduler::answering::get_fuzz_seed;
 use crate::scheduler::fsrs::params::reviews_for_fsrs;
 use crate::scheduler::fsrs::params::Params;
 use crate::scheduler::states::fuzz::with_review_fuzz;
@@ -69,6 +71,10 @@ impl Collection {
             } else {
                 None
             };
+            let mut rescheduler = self
+                .get_config_bool(BoolKey::LoadBalancerEnabled)
+                .then(|| Rescheduler::new(self))
+                .transpose()?;
             let fsrs = FSRS::new(req.as_ref().map(|w| &w.params[..]).or(Some([].as_slice())))?;
             let historical_retention = req.as_ref().map(|w| w.historical_retention);
             let items = fsrs_items_for_memory_states(
@@ -99,6 +105,10 @@ impl Collection {
                                 if let Some(state) = &card.memory_state {
                                     // or in (re)learning
                                     if card.ctype == CardType::Review {
+                                        let deck = self
+                                            .get_deck(card.original_or_current_deck_id())?
+                                            .or_not_found(card.original_or_current_deck_id())?;
+                                        let deckconfig_id = deck.config_id().unwrap();
                                         // reschedule it
                                         let original_interval = card.interval;
                                         let interval = fsrs.next_interval(
@@ -106,19 +116,41 @@ impl Collection {
                                             card.desired_retention.unwrap(),
                                             0,
                                         );
-                                        card.interval = with_review_fuzz(
+                                        card.interval = rescheduler
-                                            card.get_fuzz_factor(true),
+                                            .as_mut()
-                                            interval,
+                                            .and_then(|r| {
-                                            1,
+                                                r.find_interval(
-                                            req.max_interval,
+                                                    interval,
-                                        );
+                                                    1,
+                                                    req.max_interval,
+                                                    days_elapsed as u32,
+                                                    deckconfig_id,
+                                                    get_fuzz_seed(&card, true),
+                                                )
+                                            })
+                                            .unwrap_or_else(|| {
+                                                with_review_fuzz(
+                                                    card.get_fuzz_factor(true),
+                                                    interval,
+                                                    1,
+                                                    req.max_interval,
+                                                )
+                                            });
                                         let due = if card.original_due != 0 {
                                             &mut card.original_due
                                         } else {
                                             &mut card.due
                                         };
-                                        *due = (timing.days_elapsed as i32) - days_elapsed
+                                        let new_due = (timing.days_elapsed as i32) - days_elapsed
                                             + card.interval as i32;
+                                        if let Some(rescheduler) = &mut rescheduler {
+                                            rescheduler.update_due_cnt_per_day(
+                                                *due,
+                                                new_due,
+                                                deckconfig_id,
+                                            );
+                                        }
+                                        *due = new_due;
                                         // Add a rescheduled revlog entry if the last entry wasn't
                                         // rescheduled
                                         if !last_info.last_revlog_is_rescheduled {

rslib/src/scheduler/fsrs/mod.rs

@@ -4,6 +4,7 @@
 mod error;
 pub mod memory_state;
 pub mod params;
+pub mod rescheduler;
 pub mod retention;
 pub mod simulator;
 pub mod try_collect;

rslib/src/scheduler/fsrs/rescheduler.rs

@@ -0,0 +1,182 @@
+// Copyright: Ankitects Pty Ltd and contributors
+// License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
+use std::collections::HashMap;
+
+use chrono::Datelike;
+
+use crate::prelude::*;
+use crate::scheduler::states::fuzz::constrained_fuzz_bounds;
+use crate::scheduler::states::load_balancer::build_easy_days_percentages;
+use crate::scheduler::states::load_balancer::calculate_easy_days_modifiers;
+use crate::scheduler::states::load_balancer::select_weighted_interval;
+use crate::scheduler::states::load_balancer::EasyDay;
+use crate::scheduler::states::load_balancer::LoadBalancerInterval;
+
+pub struct Rescheduler {
+    today: i32,
+    next_day_at: TimestampSecs,
+    due_cnt_per_day_by_preset: HashMap<DeckConfigId, HashMap<i32, usize>>,
+    due_today_by_preset: HashMap<DeckConfigId, usize>,
+    reviewed_today_by_preset: HashMap<DeckConfigId, usize>,
+    easy_days_percentages_by_preset: HashMap<DeckConfigId, [EasyDay; 7]>,
+}
+
+impl Rescheduler {
+    pub fn new(col: &mut Collection) -> Result<Self> {
+        let timing = col.timing_today()?;
+        let deck_stats = col.storage.get_deck_due_counts()?;
+        let deck_map = col.storage.get_decks_map()?;
+        let did_to_dcid = deck_map
+            .values()
+            .filter_map(|deck| Some((deck.id, deck.config_id()?)))
+            .collect::<HashMap<_, _>>();
+
+        let mut due_cnt_per_day_by_preset: HashMap<DeckConfigId, HashMap<i32, usize>> =
+            HashMap::new();
+        for (did, due_date, count) in deck_stats {
+            let deck_config_id = did_to_dcid[&did];
+            due_cnt_per_day_by_preset
+                .entry(deck_config_id)
+                .or_default()
+                .entry(due_date)
+                .and_modify(|e| *e += count)
+                .or_insert(count);
+        }
+
+        let today = timing.days_elapsed as i32;
+        let due_today_by_preset = due_cnt_per_day_by_preset
+            .iter()
+            .map(|(deck_config_id, config_dues)| {
+                let due_today = config_dues
+                    .iter()
+                    .filter(|(&due, _)| due <= today)
+                    .map(|(_, &count)| count)
+                    .sum();
+                (*deck_config_id, due_today)
+            })
+            .collect();
+
+        let next_day_at = timing.next_day_at;
+        let reviewed_stats = col.storage.studied_today_by_deck(timing.next_day_at)?;
+        let mut reviewed_today_by_preset: HashMap<DeckConfigId, usize> = HashMap::new();
+        for (did, count) in reviewed_stats {
+            if let Some(&deck_config_id) = &did_to_dcid.get(&did) {
+                *reviewed_today_by_preset.entry(deck_config_id).or_default() += count;
+            }
+        }
+
+        let easy_days_percentages_by_preset =
+            build_easy_days_percentages(col.storage.get_deck_config_map()?)?;
+
+        Ok(Self {
+            today,
+            next_day_at,
+            due_cnt_per_day_by_preset,
+            due_today_by_preset,
+            reviewed_today_by_preset,
+            easy_days_percentages_by_preset,
+        })
+    }
+
+    pub fn update_due_cnt_per_day(
+        &mut self,
+        due_before: i32,
+        due_after: i32,
+        deck_config_id: DeckConfigId,
+    ) {
+        if let Some(counts) = self.due_cnt_per_day_by_preset.get_mut(&deck_config_id) {
+            if let Some(count) = counts.get_mut(&due_before) {
+                *count -= 1;
+            }
+            *counts.entry(due_after).or_default() += 1;
+        }
+
+        if due_before <= self.today && due_after > self.today {
+            if let Some(count) = self.due_today_by_preset.get_mut(&deck_config_id) {
+                *count -= 1;
+            }
+        }
+        if due_before > self.today && due_after <= self.today {
+            *self.due_today_by_preset.entry(deck_config_id).or_default() += 1;
+        }
+    }
+
+    fn due_today(&self, deck_config_id: DeckConfigId) -> usize {
+        *self.due_today_by_preset.get(&deck_config_id).unwrap_or(&0)
+    }
+
+    fn reviewed_today(&self, deck_config_id: DeckConfigId) -> usize {
+        *self
+            .reviewed_today_by_preset
+            .get(&deck_config_id)
+            .unwrap_or(&0)
+    }
+
+    pub fn find_interval(
+        &self,
+        interval: f32,
+        minimum: u32,
+        maximum: u32,
+        days_elapsed: u32,
+        deckconfig_id: DeckConfigId,
+        fuzz_seed: Option<u64>,
+    ) -> Option<u32> {
+        let (before_days, after_days) = constrained_fuzz_bounds(interval, minimum, maximum);
+
+        // Don't reschedule the card when it's overdue
+        if after_days < days_elapsed {
+            return None;
+        }
+        // Don't reschedule the card to the past
+        let before_days = before_days.max(days_elapsed);
+
+        // Generate possible intervals and their review counts
+        let possible_intervals: Vec<u32> = (before_days..=after_days).collect();
+        let review_counts: Vec<usize> = possible_intervals
+            .iter()
+            .map(|&ivl| {
+                if ivl > days_elapsed {
+                    let check_due = self.today + ivl as i32 - days_elapsed as i32;
+                    *self
+                        .due_cnt_per_day_by_preset
+                        .get(&deckconfig_id)
+                        .and_then(|counts| counts.get(&check_due))
+                        .unwrap_or(&0)
+                } else {
+                    // today's workload is the sum of backlogs, cards due today and cards reviewed
+                    // today
+                    self.due_today(deckconfig_id) + self.reviewed_today(deckconfig_id)
+                }
+            })
+            .collect();
+        let weekdays: Vec<usize> = possible_intervals
+            .iter()
+            .map(|&ivl| {
+                self.next_day_at
+                    .adding_secs(days_elapsed as i64 * -86400)
+                    .adding_secs((ivl - 1) as i64 * 86400)
+                    .local_datetime()
+                    .unwrap()
+                    .weekday()
+                    .num_days_from_monday() as usize
+            })
+            .collect();
+
+        let easy_days_load = self.easy_days_percentages_by_preset.get(&deckconfig_id)?;
+        let easy_days_modifier =
+            calculate_easy_days_modifiers(easy_days_load, &weekdays, &review_counts);
+
+        let intervals =
+            possible_intervals
+                .iter()
+                .enumerate()
+                .map(|(interval_index, &target_interval)| LoadBalancerInterval {
+                    target_interval,
+                    review_count: review_counts[interval_index],
+                    sibling_modifier: 1.0,
+                    easy_days_modifier: easy_days_modifier[interval_index],
+                });
+
+        select_weighted_interval(intervals, fuzz_seed)
+    }
+}

rslib/src/scheduler/states/load_balancer.rs

@@ -44,7 +44,7 @@ impl From<f32> for EasyDay {
 }
 
 impl EasyDay {
-    fn load_modifier(&self) -> f32 {
+    pub(crate) fn load_modifier(&self) -> f32 {
         match self {
             // this is a non-zero value so if all days are minimum, the load balancer will
             // proceed as normal
@@ -161,27 +161,7 @@ impl LoadBalancer {
                 },
             );
         let configs = storage.get_deck_config_map()?;
-
+        let easy_days_percentages_by_preset = build_easy_days_percentages(configs)?;
-        let easy_days_percentages_by_preset = configs
-            .into_iter()
-            .map(|(dcid, conf)| {
-                let easy_days_percentages: [EasyDay; 7] =
-                    if conf.inner.easy_days_percentages.is_empty() {
-                        [EasyDay::Normal; 7]
-                    } else {
-                        TryInto::<[_; 7]>::try_into(conf.inner.easy_days_percentages)
-                            .map_err(|_| {
-                                AnkiError::from(InvalidInputError {
-                                    message: "expected 7 days".into(),
-                                    source: None,
-                                    backtrace: None,
-                                })
-                            })?
-                            .map(EasyDay::from)
-                    };
-                Ok((dcid, easy_days_percentages))
-            })
-            .collect::<Result<HashMap<_, [EasyDay; 7]>, AnkiError>>()?;
 
         Ok(LoadBalancer {
             days_by_preset,
@@ -254,87 +234,30 @@ impl LoadBalancer {
             })
             .unzip();
 
-        // Determine which days to schedule to with respect to Easy Day settings
-        // If a day is Normal, it will always be an option to schedule to
-        // If a day is Minimum, it will almost never be an option to schedule to
-        // If a day is Reduced, it will look at the amount of cards due in the fuzz
-        //    range to determine if scheduling a card on that day would put it
-        //    above the reduced threshold or not.
-        // the resulting easy_days_modifier will be a vec of 0.0s and 1.0s, to be
-        //    used when calculating the day's weight. This turns the day on or off.
-        //    Note that it does not actually set it to 0.0, but a small
-        //    0.0-ish number (see EASY_DAYS_MINIMUM_LOAD) to remove the need to
-        //    handle a handful of zero-related corner cases.
         let easy_days_load = self.easy_days_percentages_by_preset.get(&deckconfig_id)?;
-        let total_review_count: usize = review_counts.iter().sum();
+        let easy_days_modifier =
-        let total_percents: f32 = weekdays
+            calculate_easy_days_modifiers(easy_days_load, &weekdays, &review_counts);
-            .iter()
-            .map(|&weekday| easy_days_load[weekday].load_modifier())
-            .sum();
-        let easy_days_modifier = weekdays
-            .iter()
-            .zip(review_counts.iter())
-            .map(|(&weekday, &review_count)| {
-                let day = match easy_days_load[weekday] {
-                    EasyDay::Reduced => {
-                        const HALF: f32 = 0.5;
-                        let other_days_review_total = (total_review_count - review_count) as f32;
-                        let other_days_percent_total = total_percents - HALF;
-                        let normalized_count = review_count as f32 / HALF;
-                        let reduced_day_threshold =
-                            other_days_review_total / other_days_percent_total;
-                        if normalized_count > reduced_day_threshold {
-                            EasyDay::Minimum
-                        } else {
-                            EasyDay::Normal
-                        }
-                    }
-                    other => other,
-                };
-                day.load_modifier()
-            })
-            .collect::<Vec<_>>();
 
-        // calculate params for each day
+        let intervals = interval_days
-        let intervals_and_params = interval_days
             .iter()
             .enumerate()
             .map(|(interval_index, interval_day)| {
-                let target_interval = interval_index as u32 + before_days;
+                LoadBalancerInterval {
+                    target_interval: interval_index as u32 + before_days,
+                    review_count: review_counts[interval_index],
+                    // if there is a sibling on this day, give it a very low weight
+                    sibling_modifier: note_id
+                        .and_then(|note_id| {
+                            interval_day
+                                .has_sibling(&note_id)
+                                .then_some(SIBLING_PENALTY)
+                        })
+                        .unwrap_or(1.0),
+                    easy_days_modifier: easy_days_modifier[interval_index],
+                }
+            });
 
-                // if there is a sibling on this day, give it a very low weight
+        select_weighted_interval(intervals, fuzz_seed)
-                let sibling_multiplier = note_id
-                    .and_then(|note_id| {
-                        interval_day
-                            .has_sibling(&note_id)
-                            .then_some(SIBLING_PENALTY)
-                    })
-                    .unwrap_or(1.0);
-
-                let weight = match review_counts[interval_index] {
-                    0 => 1.0, // if theres no cards due on this day, give it the full 1.0 weight
-                    card_count => {
-                        let card_count_weight = (1.0 / card_count as f32).powi(2);
-                        let card_interval_weight = 1.0 / target_interval as f32;
-
-                        card_count_weight
-                            * card_interval_weight
-                            * sibling_multiplier
-                            * easy_days_modifier[interval_index]
-                    }
-                };
-
-                (target_interval, weight)
-            })
-            .collect::<Vec<_>>();
-
-        let mut rng = StdRng::seed_from_u64(fuzz_seed?);
-
-        let weighted_intervals =
-            WeightedIndex::new(intervals_and_params.iter().map(|k| k.1)).ok()?;
-
-        let selected_interval_index = weighted_intervals.sample(&mut rng);
-        Some(intervals_and_params[selected_interval_index].0)
     }
 
     pub fn add_card(&mut self, cid: CardId, nid: NoteId, dcid: DeckConfigId, interval: u32) {
@@ -354,6 +277,118 @@ impl LoadBalancer {
     }
 }
 
+/// Build a mapping of deck config IDs to their easy days settings.
+/// For each deck config, maintains an array of 7 EasyDay values representing
+/// the load modifier for each day of the week.
+pub(crate) fn build_easy_days_percentages(
+    configs: HashMap<DeckConfigId, DeckConfig>,
+) -> Result<HashMap<DeckConfigId, [EasyDay; 7]>> {
+    configs
+        .into_iter()
+        .map(|(dcid, conf)| {
+            let easy_days_percentages: [EasyDay; 7] = if conf.inner.easy_days_percentages.is_empty()
+            {
+                [EasyDay::Normal; 7]
+            } else {
+                TryInto::<[_; 7]>::try_into(conf.inner.easy_days_percentages)
+                    .map_err(|_| {
+                        AnkiError::from(InvalidInputError {
+                            message: "expected 7 days".into(),
+                            source: None,
+                            backtrace: None,
+                        })
+                    })?
+                    .map(EasyDay::from)
+            };
+            Ok((dcid, easy_days_percentages))
+        })
+        .collect()
+}
+
+// Determine which days to schedule to with respect to Easy Day settings
+// If a day is Normal, it will always be an option to schedule to
+// If a day is Minimum, it will almost never be an option to schedule to
+// If a day is Reduced, it will look at the amount of cards due in the fuzz
+//    range to determine if scheduling a card on that day would put it
+//    above the reduced threshold or not.
+// the resulting easy_days_modifier will be a vec of 0.0s and 1.0s, to be
+//    used when calculating the day's weight. This turns the day on or off.
+//    Note that it does not actually set it to 0.0, but a small
+//    0.0-ish number (see EASY_DAYS_MINIMUM_LOAD) to remove the need to
+//    handle a handful of zero-related corner cases.
+pub(crate) fn calculate_easy_days_modifiers(
+    easy_days_load: &[EasyDay; 7],
+    weekdays: &[usize],
+    review_counts: &[usize],
+) -> Vec<f32> {
+    let total_review_count: usize = review_counts.iter().sum();
+    let total_percents: f32 = weekdays
+        .iter()
+        .map(|&weekday| easy_days_load[weekday].load_modifier())
+        .sum();
+
+    weekdays
+        .iter()
+        .zip(review_counts.iter())
+        .map(|(&weekday, &review_count)| {
+            let day = match easy_days_load[weekday] {
+                EasyDay::Reduced => {
+                    const HALF: f32 = 0.5;
+                    let other_days_review_total = (total_review_count - review_count) as f32;
+                    let other_days_percent_total = total_percents - HALF;
+                    let normalized_count = review_count as f32 / HALF;
+                    let reduced_day_threshold = other_days_review_total / other_days_percent_total;
+                    if normalized_count > reduced_day_threshold {
+                        EasyDay::Minimum
+                    } else {
+                        EasyDay::Normal
+                    }
+                }
+                other => other,
+            };
+            day.load_modifier()
+        })
+        .collect()
+}
+
+pub struct LoadBalancerInterval {
+    pub target_interval: u32,
+    pub review_count: usize,
+    pub sibling_modifier: f32,
+    pub easy_days_modifier: f32,
+}
+
+pub fn select_weighted_interval(
+    intervals: impl Iterator<Item = LoadBalancerInterval>,
+    fuzz_seed: Option<u64>,
+) -> Option<u32> {
+    let intervals_and_weights = intervals
+        .map(|interval| {
+            let weight = match interval.review_count {
+                0 => 1.0, // if theres no cards due on this day, give it the full 1.0 weight
+                card_count => {
+                    let card_count_weight = (1.0 / card_count as f32).powi(2);
+                    let card_interval_weight = 1.0 / interval.target_interval as f32;
+
+                    card_count_weight
+                        * card_interval_weight
+                        * interval.sibling_modifier
+                        * interval.easy_days_modifier
+                }
+            };
+
+            (interval.target_interval, weight)
+        })
+        .collect::<Vec<_>>();
+
+    let mut rng = StdRng::seed_from_u64(fuzz_seed?);
+
+    let weighted_intervals = WeightedIndex::new(intervals_and_weights.iter().map(|k| k.1)).ok()?;
+
+    let selected_interval_index = weighted_intervals.sample(&mut rng);
+    Some(intervals_and_weights[selected_interval_index].0)
+}
+
 fn interval_to_weekday(interval: u32, next_day_at: TimestampSecs) -> usize {
     let target_datetime = next_day_at
         .adding_secs((interval - 1) as i64 * 86400)

rslib/src/storage/card/deck_due_counts.sql

@@ -0,0 +1,14 @@
+SELECT CASE
+    WHEN odid == 0 THEN did
+    ELSE odid
+  END AS original_did,
+  CASE
+    WHEN odid == 0 THEN due
+    ELSE odue
+  END AS true_due,
+  COUNT() AS COUNT
+FROM cards
+WHERE type = 2
+  AND queue != -1
+GROUP BY original_did,
+  true_due

rslib/src/storage/card/mod.rs

@@ -624,6 +624,15 @@ impl super::SqliteStorage {
             ))
     }
 
+    pub(crate) fn get_deck_due_counts(&self) -> Result<Vec<(DeckId, i32, usize)>> {
+        self.db
+            .prepare(include_str!("deck_due_counts.sql"))?
+            .query_and_then([], |row| -> Result<_> {
+                Ok((DeckId(row.get(0)?), row.get(1)?, row.get(2)?))
+            })?
+            .collect()
+    }
+
     pub(crate) fn congrats_info(&self, current: &Deck, today: u32) -> Result<CongratsInfo> {
         // NOTE: this line is obsolete in v3 as it's run on queue build, but kept to
         // prevent errors for v1/v2 users before they upgrade

rslib/src/storage/revlog/mod.rs

@@ -201,6 +201,18 @@ impl SqliteStorage {
             .map_err(Into::into)
     }
 
+    pub(crate) fn studied_today_by_deck(
+        &self,
+        day_cutoff: TimestampSecs,
+    ) -> Result<Vec<(DeckId, usize)>> {
+        let start = day_cutoff.adding_secs(-86_400).as_millis();
+        self.db
+            .prepare_cached(include_str!("studied_today_by_deck.sql"))?
+            .query_and_then([start.0], |row| -> Result<_> {
+                Ok((DeckId(row.get(0)?), row.get(1)?))
+            })?
+            .collect()
+    }
     pub(crate) fn upgrade_revlog_to_v2(&self) -> Result<()> {
         self.db
             .execute_batch(include_str!("v2_upgrade.sql"))

rslib/src/storage/revlog/studied_today_by_deck.sql

@@ -0,0 +1,14 @@
+SELECT CASE
+    WHEN c.odid == 0 THEN c.did
+    ELSE c.odid
+  END AS original_did,
+  COUNT(DISTINCT r.cid) AS cnt
+FROM revlog AS r
+  JOIN cards AS c ON r.cid = c.id
+WHERE r.id > ?
+  AND r.ease > 0
+  AND (
+    r.type < 3
+    OR r.factor != 0
+  )
+GROUP BY original_did

ts/routes/deck-options/EasyDays.svelte

@@ -13,17 +13,29 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
     export let state: DeckOptionsState;
     export let api: Record<string, never>;
 
+    const fsrsEnabled = state.fsrs;
+    const reschedule = state.fsrsReschedule;
     const config = state.currentConfig;
     const defaults = state.defaults;
+    const prevEasyDaysPercentages = $config.easyDaysPercentages.slice();
 
     $: if ($config.easyDaysPercentages.length !== 7) {
         $config.easyDaysPercentages = defaults.easyDaysPercentages.slice();
     }
 
+    $: easyDaysChanged = $config.easyDaysPercentages.some(
+        (value, index) => value !== prevEasyDaysPercentages[index],
+    );
+
     $: noNormalDay = $config.easyDaysPercentages.some((p) => p === 1.0)
         ? ""
         : tr.deckConfigEasyDaysNoNormalDays();
 
+    $: rescheduleWarning =
+        easyDaysChanged && !($fsrsEnabled && $reschedule)
+            ? tr.deckConfigEasyDaysChange()
+            : "";
+
     const easyDays = [
         tr.deckConfigEasyDaysMonday(),
         tr.deckConfigEasyDaysTuesday(),
@@ -81,6 +93,9 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
         <Item>
             <Warning warning={noNormalDay} />
         </Item>
+        <Item>
+            <Warning warning={rescheduleWarning} />
+        </Item>
     </DynamicallySlottable>
 </TitledContainer>