Unchunked range in subsets

Engelberg · Engelberg · commit 4b5312218344 · 2013-11-13T13:35:38.000-08:00
diff --git a/README.md b/README.md
@@ -9,7 +9,7 @@ sequences for common combinatorial functions.
 Releases and Dependency Information
 ========================================
 
-Latest stable release: 0.0.6
+Latest stable release: 0.0.7
 
 * [All Released Versions](http://search.maven.org/#search%7Cgav%7C1%7Cg%3A%22org.clojure%22%20AND%20a%3A%22math.combinatorics%22)
 
@@ -18,7 +18,7 @@ Latest stable release: 0.0.6
 [Leiningen](https://github.com/technomancy/leiningen) dependency information:
 
 ```clojure
-[org.clojure/math.combinatorics "0.0.6"]
+[org.clojure/math.combinatorics "0.0.7"]
 ```
 
 [Maven](http://maven.apache.org/) dependency information:
@@ -27,7 +27,7 @@ Latest stable release: 0.0.6
 <dependency>
   <groupId>org.clojure</groupId>
   <artifactId>math.combinatorics</artifactId>
-  <version>0.0.6</version>
+  <version>0.0.7</version>
 </dependency>
 ```
 
@@ -102,6 +102,8 @@ Developer Information
 
 Changelog
 ========================================
+* Release 0.0.7 on 2013-11-13
+  * Unchunk range in `subsets` to minimize memory usage.
 * Release 0.0.6 on 2013-10-31
   * Removed use of mapv for backwards compat with older versions of Clojure.
 
diff --git a/src/main/clojure/clojure/math/combinatorics.clj b/src/main/clojure/clojure/math/combinatorics.clj
@@ -2,7 +2,7 @@
 ;;; sequences for common combinatorial functions.
 
 ;; by Mark Engelberg (mark.engelberg@gmail.com)
-;; Last updated - October 31, 2013
+;; Last updated - November 13, 2013
 
 (ns
   #^{:author "Mark Engelberg",
@@ -95,12 +95,24 @@ Most of these algorithms are derived from algorithms found in Knuth's wonderful
 		(= n cnt) (list (seq items))
 		:else
 		(map #(map v-items %) (index-combinations n cnt)))))))
+
+(defn- unchunk
+  "Given a sequence that may have chunks, return a sequence that is 1-at-a-time
+lazy with no chunks. Chunks are good for efficiency when the data items are
+small, but when being processed via map, for example, a reference is kept to
+every function result in the chunk until the entire chunk has been processed,
+which increases the amount of memory in use that cannot be garbage
+collected."
+  [s]
+  (lazy-seq
+   (when (seq s)
+     (cons (first s) (unchunk (rest s))))))
 
 (defn subsets
   "All the subsets of items"
   [items]
   (mapcat (fn [n] (combinations items n))
-	  (range (inc (count items)))))
+	  (unchunk (range (inc (count items))))))
 
 (defn cartesian-product
   "All the ways to take one item from each sequence"
