Wrap legacy --fbin vector index instead of panicking past the end of the file

src/generators/legacy.rs

@@ -45,7 +45,10 @@ impl PointGenerator for LegacyGenerator {
         };
 
         let vectors: Vectors = if let Some(reader) = &self.reader {
-            reader.read_vector(idx as usize).to_vec().into()
+            // Cycle through the file when more points are requested than it holds,
+            // like `source: {type: file}` does in YAML configs.
+            let n = reader.num_vectors.max(1) as usize;
+            reader.read_vector(idx as usize % n).to_vec().into()
         } else if self.args.vectors_per_point != 1 {
             let vectors_map: HashMap<_, _> = (0..self.args.vectors_per_point)
                 .map(|i| (format!("{i}"), random_vector(&mut rng, &self.args)))
@@ -96,3 +99,52 @@ impl PointGenerator for LegacyGenerator {
         random_payload(&mut rng, &self.args, self.zipf.as_ref())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use std::io::Write;
+
+    use clap::Parser;
+    use qdrant_client::qdrant::vector::Vector as VectorVariant;
+
+    use super::*;
+
+    /// Write `vectors` to a temporary file in fbin layout.
+    fn fbin_file(vectors: &[Vec<f32>]) -> tempfile::NamedTempFile {
+        let mut file = tempfile::NamedTempFile::new().unwrap();
+        file.write_all(&(vectors.len() as i32).to_le_bytes())
+            .unwrap();
+        file.write_all(&(vectors[0].len() as i32).to_le_bytes())
+            .unwrap();
+        for value in vectors.iter().flatten() {
+            file.write_all(&value.to_le_bytes()).unwrap();
+        }
+        file.flush().unwrap();
+        file
+    }
+
+    fn dense(point: PointStruct) -> Vec<f32> {
+        match point.vectors.unwrap().vectors_options.unwrap() {
+            VectorsOptions::Vector(vector) => match vector.vector.unwrap() {
+                VectorVariant::Dense(dense) => dense.data,
+                _ => panic!("expected a dense vector"),
+            },
+            _ => panic!("expected the unnamed default vector"),
+        }
+    }
+
+    /// `--fbin` with more points (or a larger `--offset`) than the file holds
+    /// must cycle through the file instead of panicking on an out-of-range read.
+    #[test]
+    fn fbin_vectors_wrap_past_the_end_of_the_file() {
+        let file = fbin_file(&[vec![1.0, 2.0], vec![3.0, 4.0]]);
+        let reader = FBinReader::new(file.path()).unwrap();
+        let args = Args::parse_from(["bfb", "--dim", "2"]);
+        let generator = LegacyGenerator::new(args, Some(reader));
+
+        assert_eq!(dense(generator.make_point(0)), [1.0, 2.0]);
+        assert_eq!(dense(generator.make_point(1)), [3.0, 4.0]);
+        assert_eq!(dense(generator.make_point(2)), [1.0, 2.0]);
+        assert_eq!(dense(generator.make_point(5)), [3.0, 4.0]);
+    }
+}