hnsw_knn: Find Nearest Neighbors and Distances

In RcppHNSW: 'Rcpp' Bindings for 'hnswlib', a Library for Approximate Nearest Neighbors

Find Nearest Neighbors and Distances

Description

A k-nearest neighbor algorithm using the hnswlib library (https://github.com/nmslib/hnswlib).

Usage

hnsw_knn(
  X,
  k = 10,
  distance = "euclidean",
  M = 16,
  ef_construction = 200,
  ef = 10,
  verbose = FALSE,
  progress = "bar",
  n_threads = 0,
  grain_size = 1,
  byrow = TRUE
)

Arguments

X	A numeric matrix of n items to search for neighbors. If byrow = TRUE (the default) then each row of X stores an item to be searched. Otherwise, each item should be stored in the columns of X.
k	Number of neighbors to return.
distance	Type of distance to calculate. One of: "l2" Squared L2, i.e. squared Euclidean. "euclidean" Euclidean. "cosine" Cosine. "ip" Inner product: 1 - sum(ai * bi), i.e. the cosine distance where the vectors are not normalized. This can lead to negative distances and other non-metric behavior.
M	Controls the number of bi-directional links created for each element during index construction. Higher values lead to better results at the expense of memory consumption. Typical values are 2 - 100, but for most datasets a range of 12 - 48 is suitable. Can't be smaller than 2.
ef_construction	Size of the dynamic list used during construction. A larger value means a better quality index, but increases build time. Should be an integer value between 1 and the size of the dataset.
ef	Size of the dynamic list used during search. Higher values lead to improved recall at the expense of longer search time. Can take values between k and the size of the dataset and may be greater or smaller than ef_construction. Typical values are 100 - 2000.
verbose	If TRUE, log messages to the console.
progress	defunct and has no effect.
n_threads	Maximum number of threads to use. The exact number is determined by grain_size.
grain_size	Minimum amount of work to do (rows in X to add or search for) per thread. If the number of rows in X isn't sufficient, then fewer than n_threads will be used. This is useful in cases where the overhead of context switching with too many threads outweighs the gains due to parallelism.
byrow	if TRUE (the default), this indicates that the items to be processed in X are stored in each row of X. Otherwise, the items are stored in the columns of X. Storing items in each column reduces the overhead of copying data to a form that can be used by the hnsw library. Note that if byrow = FALSE, any matrices returned from this function will also store the items by column.

Value

a list containing:

• idx a matrix containing the nearest neighbor indices.

• dist a matrix containing the nearest neighbor distances.

The dimensions of the matrices respect the storage (row or column-based) of X as indicated by the byrow parameter. If byrow = TRUE (the default) each row of idx and dist contain the neighbor information for the item passed in the equivalent row of X, i.e. the dimensions are ⁠n x k⁠ where n is the number of items in X. If byrow = FALSE, then each column of idx and dist contain the neighbor information for the item passed in the equivalent column of X, i.e. the dimensions are ⁠k x n⁠.

Every item in the dataset is considered to be a neighbor of itself, so the first neighbor of item i should always be i itself. If that isn't the case, then any of M, ef_construction or ef may need increasing.

Hnswlib Parameters

Some details on the parameters used for index construction and search, based on https://github.com/nmslib/hnswlib/blob/master/ALGO_PARAMS.md:

• M Controls the number of bi-directional links created for each element during index construction. Higher values lead to better results at the expense of memory consumption, which is around M * 8-10 bytes per bytes per stored element. High intrinsic dimensionalities will require higher values of M. A range of 2 - 100 is typical, but 12 - 48 is ok for most use cases.

• ef_construction Size of the dynamic list used during construction. A larger value means a better quality index, but increases build time. Should be an integer value between 1 and the size of the dataset. A typical range is 100 - 2000. Beyond a certain point, increasing ef_construction has no effect. A sufficient value of ef_construction can be determined by searching with ef = ef_construction, and ensuring that the recall is at least 0.9.

• ef Size of the dynamic list used during index search. Can differ from ef_construction and be any value between k (the number of neighbors sought) and the number of elements in the index being searched.

References

Malkov, Y. A., & Yashunin, D. A. (2016). Efficient and robust approximate nearest neighbor search using Hierarchical Navigable Small World graphs. arXiv preprint arXiv:1603.09320.

Examples

iris_nn_data <- hnsw_knn(as.matrix(iris[, -5]), k = 10)

RcppHNSW documentation built on Sept. 19, 2023, 9:06 a.m.