changes

crates/domain/tests/correlation/spearman_test.rs

@@ -0,0 +1,117 @@
+use domain::correlation::{CorrelationStrategy, Observation};
+use domain::entry::{DayMood, Mood};
+
+fn day(mood: Mood) -> DayMood {
+    DayMood::of(&[mood]).unwrap()
+}
+
+fn observations(pairs: &[(f64, Mood)]) -> Vec<Observation> {
+    pairs
+        .iter()
+        .map(|(value, mood)| Observation::new(*value, day(*mood)))
+        .collect()
+}
+
+fn spearman(pairs: &[(f64, Mood)]) -> Option<f64> {
+    CorrelationStrategy::Spearman
+        .score(&observations(pairs))
+        .map(|coefficient| coefficient.value())
+}
+
+#[test]
+fn a_perfectly_ordered_pair_of_series_scores_one() {
+    let score = spearman(&[
+        (1.0, Mood::Awful),
+        (2.0, Mood::Bad),
+        (3.0, Mood::Meh),
+        (4.0, Mood::Good),
+    ])
+    .unwrap();
+
+    assert!((score - 1.0).abs() < 1e-12);
+}
+
+#[test]
+fn a_perfectly_inverted_pair_of_series_scores_minus_one() {
+    let score = spearman(&[
+        (1.0, Mood::Rad),
+        (2.0, Mood::Good),
+        (3.0, Mood::Meh),
+        (4.0, Mood::Bad),
+    ])
+    .unwrap();
+
+    assert!((score + 1.0).abs() < 1e-12);
+}
+
+#[test]
+fn only_the_order_matters_not_the_distance() {
+    let gentle = spearman(&[
+        (1.0, Mood::Awful),
+        (2.0, Mood::Bad),
+        (3.0, Mood::Meh),
+        (4.0, Mood::Good),
+    ])
+    .unwrap();
+
+    let one_enormous_day = spearman(&[
+        (1.0, Mood::Awful),
+        (2.0, Mood::Bad),
+        (3.0, Mood::Meh),
+        (100_000.0, Mood::Good),
+    ])
+    .unwrap();
+
+    assert!((gentle - one_enormous_day).abs() < f64::EPSILON);
+}
+
+#[test]
+fn tied_moods_share_their_average_rank_rather_than_taking_the_shortcut() {
+    let score = spearman(&[
+        (1_000.0, Mood::Meh),
+        (2_000.0, Mood::Meh),
+        (3_000.0, Mood::Meh),
+        (4_000.0, Mood::Good),
+        (5_000.0, Mood::Good),
+        (6_000.0, Mood::Rad),
+    ])
+    .unwrap();
+
+    let tie_corrected = 0.925_820_099_8;
+    let textbook_shortcut = 0.928_571_428_6;
+
+    assert!(
+        (score - tie_corrected).abs() < 1e-9,
+        "expected the tie-corrected coefficient, got {score}"
+    );
+    assert!(
+        (score - textbook_shortcut).abs() > 1e-9,
+        "this is the 6*d^2 shortcut, which is wrong when ranks are tied"
+    );
+}
+
+#[test]
+fn a_day_count_below_two_scores_nothing() {
+    assert!(spearman(&[(1.0, Mood::Good)]).is_none());
+    assert!(spearman(&[]).is_none());
+}
+
+#[test]
+fn an_unvarying_series_scores_nothing() {
+    let same_mood_every_day = spearman(&[
+        (1.0, Mood::Meh),
+        (2.0, Mood::Meh),
+        (3.0, Mood::Meh),
+        (4.0, Mood::Meh),
+    ]);
+
+    let same_value_every_day = spearman(&[
+        (5_000.0, Mood::Awful),
+        (5_000.0, Mood::Bad),
+        (5_000.0, Mood::Good),
+        (5_000.0, Mood::Rad),
+    ]);
+
+    assert!(same_mood_every_day.is_none());
+    assert!(same_value_every_day.is_none());
+}