fixed Beatmap::bpm

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## Upcoming
 
-Nothing as of now
+- __Fixes:__
+  - The `Beatmap::bpm` method now works properly by considering the most common beat length instead of just the first one
 
 # v0.9.3 (2023-01-28)
 

src/beatmap/mod.rs

@@ -1,4 +1,4 @@
-use std::{borrow::Cow, cmp::Ordering};
+use std::{borrow::Cow, cmp::Ordering, collections::HashMap};
 
 use crate::{parse::HitObject, util::SortedVec};
 
@@ -78,10 +78,69 @@ impl Beatmap {
     /// The beats per minute of the map.
     #[inline]
     pub fn bpm(&self) -> f64 {
-        match self.timing_points.first() {
-            Some(point) => point.beat_len.recip() * 1000.0 * 60.0,
-            None => 0.0,
+        // This is incorrect if the last object is a slider since there
+        // is no reasonable way to get the slider end time at this point.
+        let last_time = self
+            .hit_objects
+            .last()
+            .map(HitObject::end_time)
+            .or_else(|| self.timing_points.last().map(|t| t.time))
+            .unwrap_or(0.0);
+
+        /// Maps beat_len to a cumulative duration
+        #[derive(Debug)]
+        struct BeatLenDuration {
+            last_time: f64,
+            map: HashMap<u64, f64>,
         }
+
+        impl BeatLenDuration {
+            fn new(last_time: f64) -> Self {
+                Self {
+                    last_time,
+                    map: HashMap::default(),
+                }
+            }
+
+            fn add(&mut self, beat_len: f64, curr_time: f64, next_time: f64) {
+                let beat_len = (1000.0 * beat_len).round() / 1000.0;
+                let entry = self.map.entry(beat_len.to_bits()).or_default();
+
+                if curr_time <= self.last_time {
+                    *entry += next_time - curr_time;
+                }
+            }
+        }
+
+        let mut bpm_points = BeatLenDuration::new(last_time);
+
+        // * osu-stable forced the first control point to start at 0.
+        // * This is reproduced here to maintain compatibility around
+        // * osu!mania scroll speed and song select display.
+        match &self.timing_points[..] {
+            [curr] => bpm_points.add(curr.beat_len, 0.0, last_time),
+            [curr, next, ..] => bpm_points.add(curr.beat_len, 0.0, next.time),
+            [] => {}
+        }
+
+        self.timing_points
+            .iter()
+            .skip(1)
+            .zip(self.timing_points.iter().skip(2).map(|t| t.time))
+            .for_each(|(curr, next_time)| bpm_points.add(curr.beat_len, curr.time, next_time));
+
+        if let [.., _, curr] = &self.timing_points[..] {
+            bpm_points.add(curr.beat_len, curr.time, last_time);
+        }
+
+        let most_common_beat_len = bpm_points
+            .map
+            .into_iter()
+            // * Get the most common one, or 0 as a suitable default
+            .max_by(|(_, a), (_, b)| a.total_cmp(b))
+            .map_or(0.0, |(beat_len, _)| f64::from_bits(beat_len));
+
+        60_000.0 / most_common_beat_len
     }
 
     /// Sum up the duration of all breaks (in milliseconds).