Annoy

USearch is a lightweight, header-only C++ library for ANN search with HNSW and scalar quantization, optimized for low RAM and high-speed on CPU. It supports binary and custom metrics for edge devices. Compared to Faiss, USearch is simpler, faster on small datasets, and embeddable without deps.

DiskANN is a pure ANN index library implementing Vamana graphs for disk-based billion-scale approximate nearest neighbor search with low memory footprint. Features GPU acceleration, dynamic updates, cached SSD search, C++/Python bindings. Suited for custom vector engines handling large cold datasets in search/recommendations, benchmarks; more disk-efficient than HNSWLib vs full DBs like Qdrant.

HNSWLIB is a pure ANN index library implementing Hierarchical Navigable Small World (HNSW) graphs for high-performance approximate nearest neighbor search. Features L2/cosine metrics, multi-threading, low memory, C++/Python bindings. Ideal for custom vector engines, benchmarks on millions of vectors; core building block for DBs like Qdrant/Chroma vs complete solutions.

nanoflann is a pure ANN index library implementing KD-trees for nearest neighbor search, header-only C++11 optimized for 2D/3D point clouds. Features efficient spatial queries, no quantization or GPU support, easy integration. Suited for custom vector engines in robotics and computer vision, benchmarks; lightweight building block vs full DBs like Qdrant.

NMSLIB (Non-Metric Space Library) is a pure ANN index library for similarity search in metric and non-metric spaces, implementing HNSW, SW-graph, VPTree. Features Python/C++/Java bindings, custom distance metrics, no built-in quantization/GPU. Ideal for custom vector engines, benchmarks across spaces; versatile building block vs full DBs like Qdrant.

ScaNN (Scalable Nearest Neighbors) is a pure ANN index library using anisotropic vector quantization and scorers for high-recall, high-throughput search at billion-scale. Features CPU/GPU support, TensorFlow/Numpy bindings, advanced quantization. For custom vector engines in recommendations, benchmarks; superior recall/throughput vs Faiss, building block unlike full Qdrant.