/// Cosine similarity between two embedding vectors.
/// Returns value in [-1, 1] range; higher = more similar.
pub fn cosine_similarity(a: &[f32], b: &[f32]) -> f32 {
    debug_assert_eq!(a.len(), b.len(), "embedding dimensions must match");
    let dot: f32 = a.iter().zip(b).map(|(x, y)| x * y).sum();
    let norm_a: f32 = a.iter().map(|x| x * x).sum::<f32>().sqrt();
    let norm_b: f32 = b.iter().map(|x| x * x).sum::<f32>().sqrt();
    if norm_a == 0.0 || norm_b == 0.0 {
        return 0.0;
    }
    dot / (norm_a * norm_b)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_cosine_similarity_identical() {
        let v = [1.0, 2.0, 3.0];
        let sim = cosine_similarity(&v, &v);
        assert!((sim - 1.0).abs() < 1e-6);
    }

    #[test]
    fn test_cosine_similarity_orthogonal() {
        let a = [1.0, 0.0, 0.0];
        let b = [0.0, 1.0, 0.0];
        let sim = cosine_similarity(&a, &b);
        assert!(sim.abs() < 1e-6);
    }

    #[test]
    fn test_cosine_similarity_zero_vector() {
        let a = [1.0, 2.0, 3.0];
        let b = [0.0, 0.0, 0.0];
        let sim = cosine_similarity(&a, &b);
        assert!((sim - 0.0).abs() < 1e-6);
    }

    #[test]
    fn test_cosine_similarity_opposite() {
        let a = [1.0, 2.0, 3.0];
        let b = [-1.0, -2.0, -3.0];
        let sim = cosine_similarity(&a, &b);
        assert!((sim - (-1.0)).abs() < 1e-6);
    }
}