More accurate sorting by R (#3747)

* Match calc of relative overdueness in SM2 and FSRS

* Fix calculation of FSRS relative overdueness

* Improve readability by avoiding double negative

* Move comment line

rslib/src/storage/card/mod.rs

@@ -775,7 +775,8 @@ impl fmt::Display for ReviewOrderSubclause {
             ReviewOrderSubclause::DifficultyDescending => "extract_fsrs_variable(data, 'd') desc",
             ReviewOrderSubclause::RetrievabilitySm2 { today, order } => {
                 temp_string = format!(
-                    "ivl / cast({today}-due+0.001 as real) {order}",
+                    // - (elapsed days+0.001)/(scheduled interval)
+                    "-(1 + cast({today}-due+0.001 as real)/ivl) {order}",
                     today = today
                 );
                 &temp_string

rslib/src/storage/sqlite.rs

@@ -388,7 +388,9 @@ fn add_extract_fsrs_relative_retrievability(db: &Connection) -> rusqlite::Result
                 .max(0.0001);
 
             Ok(Some(
-                -(1. / current_retrievability - 1.) / (1. / desired_retrievability - 1.),
+                // power should be the reciprocal of the value of DECAY in FSRS-rs, which is
+                // currently -0.5
+                -(current_retrievability.powi(-2) - 1.) / (desired_retrievability.powi(-2) - 1.),
             ))
         },
     )