From 69c4efea8afd30b7060020f9b8cb56dd9723e422 Mon Sep 17 00:00:00 2001
From: Damien Elmes <gpg@ankiweb.net>
Date: Sun, 24 Sep 2023 13:07:11 +1000
Subject: [PATCH] Use memory state to calculate relative overdueness

---
 rslib/src/storage/card/mod.rs | 13 ++++++++-
 rslib/src/storage/sqlite.rs   | 51 +++++++++++++++++++++++++++++++++++
 2 files changed, 63 insertions(+), 1 deletion(-)

diff --git a/rslib/src/storage/card/mod.rs b/rslib/src/storage/card/mod.rs
index c17479128..3313ca27b 100644
--- a/rslib/src/storage/card/mod.rs
+++ b/rslib/src/storage/card/mod.rs
@@ -707,6 +707,9 @@ enum ReviewOrderSubclause {
     RelativeOverdueness {
         today: u32,
     },
+    RelativeOverduenessFsrs {
+        today: u32,
+    },
 }
 
 impl fmt::Display for ReviewOrderSubclause {
@@ -726,6 +729,10 @@ impl fmt::Display for ReviewOrderSubclause {
                 temp_string = format!("ivl / cast({today}-due+0.001 as real)", today = today);
                 &temp_string
             }
+            ReviewOrderSubclause::RelativeOverduenessFsrs { today } => {
+                temp_string = format!("extract_fsrs_relative_overdueness(data, due, {today}) desc");
+                &temp_string
+            }
         };
         write!(f, "{}", clause)
     }
@@ -751,7 +758,11 @@ fn review_order_sql(order: ReviewCardOrder, today: u32, fsrs: bool) -> String {
             ReviewOrderSubclause::EaseDescending
         }],
         ReviewCardOrder::RelativeOverdueness => {
-            vec![ReviewOrderSubclause::RelativeOverdueness { today }]
+            vec![if fsrs {
+                ReviewOrderSubclause::RelativeOverduenessFsrs { today }
+            } else {
+                ReviewOrderSubclause::RelativeOverdueness { today }
+            }]
         }
         ReviewCardOrder::Random => vec![],
     };
diff --git a/rslib/src/storage/sqlite.rs b/rslib/src/storage/sqlite.rs
index ddf81be68..30390a954 100644
--- a/rslib/src/storage/sqlite.rs
+++ b/rslib/src/storage/sqlite.rs
@@ -73,6 +73,7 @@ fn open_or_create_collection_db(path: &Path) -> Result<Connection> {
     add_extract_custom_data_function(&db)?;
     add_extract_fsrs_variable(&db)?;
     add_extract_fsrs_retrievability(&db)?;
+    add_extract_fsrs_relative_overdueness(&db)?;
 
     db.create_collation("unicase", unicase_compare)?;
 
@@ -306,6 +307,56 @@ fn add_extract_fsrs_retrievability(db: &Connection) -> rusqlite::Result<()> {
     )
 }
 
+/// eg. extract_fsrs_retrievability(card.data, card.due, timing.days_elapsed) ->
+/// float | null. The higher the number, the more overdue.
+fn add_extract_fsrs_relative_overdueness(db: &Connection) -> rusqlite::Result<()> {
+    db.create_scalar_function(
+        "extract_fsrs_relative_overdueness",
+        3,
+        FunctionFlags::SQLITE_DETERMINISTIC,
+        move |ctx| {
+            assert_eq!(ctx.len(), 3, "called with unexpected number of arguments");
+
+            let Ok(card_data) = ctx.get_raw(0).as_str() else {
+                return Ok(None);
+            };
+            if card_data.is_empty() {
+                return Ok(None);
+            }
+            let card_data = &CardData::from_str(card_data);
+            let Ok(due) = ctx.get_raw(1).as_i64() else {
+                return Ok(None);
+            };
+            if due > 365_000 {
+                // learning card
+                return Ok(None);
+            }
+            let Ok(days_elapsed) = ctx.get_raw(2).as_i64() else {
+                return Ok(None);
+            };
+            let Some(state) = card_data.memory_state() else {
+                return Ok(None);
+            };
+            let Some(mut desired_retrievability) = card_data.fsrs_desired_retention else {
+                return Ok(None);
+            };
+            // avoid div by zero
+            desired_retrievability = desired_retrievability.max(0.0001);
+
+            let review_day = due.saturating_sub(state.stability as i64);
+            let days_elapsed = days_elapsed.saturating_sub(review_day) as u32;
+            let current_retrievability = FSRS::new(None)
+                .unwrap()
+                .current_retrievability(state.into(), days_elapsed)
+                .max(0.0001);
+
+            Ok(Some(
+                (1. / current_retrievability - 1.) / (1. / desired_retrievability - 1.),
+            ))
+        },
+    )
+}
+
 /// Fetch schema version from database.
 /// Return (must_create, version)
 fn schema_version(db: &Connection) -> Result<(bool, u8)> {