Fix potential context search edge case (#3374)

* constrain loss to [0, -1)

* fmt

* update 2-d model link

lib/segment/src/vector_storage/query/context_query.rs

@@ -1,5 +1,6 @@
 use std::iter;
 
+use common::math::fast_sigmoid;
 use common::types::ScoreType;
 use itertools::Itertools;
 
@@ -44,6 +45,8 @@ impl<T> ContextPair<T> {
     ///  -0.4        -0.1 │   +0
     ///                   │
     ///
+    /// Simple 2D model:
+    /// https://www.desmos.com/calculator/lbxycyh2hs
     pub fn loss_by(&self, similarity: impl Fn(&T) -> ScoreType) -> ScoreType {
         const MARGIN: ScoreType = ScoreType::EPSILON;
 
@@ -52,7 +55,7 @@ impl<T> ContextPair<T> {
 
         let difference = positive - negative - MARGIN;
 
-        ScoreType::min(difference, 0.0)
+        fast_sigmoid(ScoreType::min(difference, 0.0))
     }
 }
 
@@ -118,40 +121,31 @@ impl From<ContextQuery<Vector>> for QueryVector {
 
 #[cfg(test)]
 mod test {
-
     use common::types::ScoreType;
-    use rstest::rstest;
+    use proptest::prelude::*;
 
     use super::*;
 
-    fn dummy_similarity(x: &i32) -> ScoreType {
+    fn dummy_similarity(x: &f32) -> ScoreType {
         *x as ScoreType
     }
 
-    /// Test that the score is calculated correctly
-    ///
-    /// for reference:
-    #[rstest]
-    #[case::no_pairs(vec![], 0.0)] // having no input always scores 0
-    #[case::on_negative(vec![(0, 1)], -1.0)]
-    #[case::on_positive(vec![(1, 0)], 0.0)]
-    #[case::on_both(vec![(1, 0), (0, 1)], -1.0)]
-    #[case::positive_positive_negative(vec![(1,0),(1,0),(0,1)], -1.0)]
-    #[case::positive_negative_negative(vec![(1,0),(0,1),(0,1)], -2.0)]
-    #[case::only_positives(vec![(2,-1),(-1,-3),(4,0)], 0.0)]
-    #[case::only_negatives(vec![(-5,-4),(-1,3),(0,2)], -7.0)]
-    fn scoring(#[case] pairs: Vec<(i32, i32)>, #[case] expected: f32) {
-        let pairs = pairs.into_iter().map(ContextPair::from).collect();
+    /// Possible similarities
+    fn sim() -> impl Strategy<Value = f32> {
+        (-100.0..=100.0).prop_map(|x| x as f32)
+    }
 
-        let query = ContextQuery::new(pairs);
+    proptest! {
+        #![proptest_config(ProptestConfig::with_cases(1000))]
 
-        let score = query.score_by(dummy_similarity);
+        /// Checks that the loss is between 0 and -1
+        #[test]
+        fn loss_is_not_more_than_1_per_pair((p, n) in (sim(), sim())) {
+            let query = ContextQuery::new(vec![ContextPair::from((p, n))]);
 
-        assert!(
-            score > expected - 0.00001 && score < expected + 0.00001,
-            "score: {}, expected: {}",
-            score,
-            expected
-        );
+            let score = query.score_by(dummy_similarity);
+            assert!(score <= 0.0, "similarity: {}", score);
+            assert!(score > -1.0, "similarity: {}", score);
+        }
     }
 }