Reservoir Sampling in Clojure

Update

I now have a whole fancy library for random sampling in Clojure, including reservoir sampling with lots of extras (with and without replacement, weighting, seeds). It's available here:
https://github.com/bigmlcom/sampling

---

A little snippet of Clojure for reservoir sampling:

	;; For a fully fleshed out library, see:
	;; https://github.com/bigmlcom/sampling
	;; -------------------------------------
	(ns sample.reservoir
	"Functions for sampling without replacement using a reservoir.")
	(defn create
	"Creates a sample reservoir."
	[reservoir-size]
	(with-meta [] {:reservoir-size reservoir-size
	:insert-count 0}))
	(defn insert
	"Inserts a value into the sample reservoir."
	[reservoir val]
	(let [{:keys [reservoir-size insert-count]} (meta reservoir)
	insert-count (inc insert-count)
	index (rand-int insert-count)]
	(with-meta (cond (< insert-count reservoir-size)
	(conj reservoir val)
	(= insert-count reservoir-size)
	(shuffle (conj reservoir val))
	(< index reservoir-size)
	(assoc reservoir index val)
	:else reservoir)
	{:reservoir-size reservoir-size
	:insert-count insert-count})))
	(defn sample
	"Returns a reservoir sample for a collection."
	[coll reservoir-size]
	(reduce insert (create reservoir-size)
	coll))

view raw reservoir.clj hosted with ❤ by GitHub

BigML's Fancy Histograms

One of my recent BigML projects:

http://blog.bigml.com/2012/06/18/bigmls-fancy-histograms/