Added the playlist generator

shanty-playlist/src/ordering.rs

@@ -0,0 +1,57 @@
+use std::collections::BTreeMap;
+
+use rand::prelude::*;
+
+use crate::types::Candidate;
+
+/// Reorder tracks so that artists are evenly spread out.
+/// Greedily picks from the artist with the most remaining tracks,
+/// avoiding back-to-back repeats when possible.
+/// Ported faithfully from drift's interleave_artists().
+pub fn interleave_artists(tracks: Vec<Candidate>) -> Vec<Candidate> {
+    let mut rng = rand::rng();
+
+    let mut by_artist: BTreeMap<String, Vec<Candidate>> = BTreeMap::new();
+    for track in tracks {
+        by_artist
+            .entry(track.artist.clone())
+            .or_default()
+            .push(track);
+    }
+    for group in by_artist.values_mut() {
+        group.shuffle(&mut rng);
+    }
+
+    let mut result = Vec::new();
+    let mut last_artist: Option<String> = None;
+
+    while !by_artist.is_empty() {
+        let mut artists: Vec<String> = by_artist.keys().cloned().collect();
+        artists.sort_by(|a, b| by_artist[b].len().cmp(&by_artist[a].len()));
+
+        let pick = artists
+            .iter()
+            .find(|a| last_artist.as_ref() != Some(a))
+            .or(artists.first())
+            .cloned()
+            .unwrap();
+
+        let group = by_artist.get_mut(&pick).unwrap();
+        let track = group.pop().unwrap();
+        if group.is_empty() {
+            by_artist.remove(&pick);
+        }
+
+        last_artist = Some(pick);
+        result.push(track);
+    }
+
+    result
+}
+
+/// Full random shuffle.
+pub fn shuffle(mut tracks: Vec<Candidate>) -> Vec<Candidate> {
+    let mut rng = rand::rng();
+    tracks.shuffle(&mut rng);
+    tracks
+}