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+;;; seq_utils.clj -- Sequence utilities for Clojure
+
+;; by Stuart Sierra, http://stuartsierra.com/
+;; last updated August 12, 2008
+
+;; Copyright (c) 2008 Stuart Sierra. All rights reserved.  The use and
+;; distribution terms for this software are covered by the Common
+;; Public License 1.0 (http://www.opensource.org/licenses/cpl1.0.php)
+;; which can be found in the file CPL.TXT at the root of this
+;; distribution.  By using this software in any fashion, you are
+;; agreeing to be bound by the terms of this license.  You must not
+;; remove this notice, or any other, from this software.
+
+
+(ns clojure.contrib.seq-utils)
+
+
+;; 'flatten' written by Rich Hickey,
+;; see http://groups.google.com/group/clojure/msg/385098fabfcaad9b
+(defn flatten
+  "Takes any nested combination of sequential things (lists, vectors,
+  etc.) and returns their contents as a single, flat sequence."
+  [x]
+  (let [s? #(instance? clojure.lang.Sequential %)]
+    (filter (complement s?) (tree-seq s? seq x))))
+
+(defn separate
+  "Returns a vector:
+   [ (filter f s), (filter (complement f) s) ]"
+  [f s]
+  [(filter f s) (filter (complement f) s)])
+
+(defn includes?
+  "Returns true if s contains something equal (with =) to x."
+  [x s]
+  (if (some (fn [y] (= y x)) s)
+    true false))
+
+(defn indexed
+  "Returns a lazy sequence of [index, item] pairs, where items come
+  from 's' and indexes count up from zero.
+
+  (indexed '(a b c d))  =>  ([0 a] [1 b] [2 c] [3 d])"
+  [s]
+  (map vector (iterate inc 0) s))
+
+;; group-by written by Rich Hickey;
+;; see http://paste.lisp.org/display/64190
+(defn group-by [f coll]
+  "Returns a sorted map of the elements of coll keyed by the result of
+  f on each element. The value at each key will be a vector of the
+  corresponding elements, in the order they appeared in coll."
+  (reduce
+   (fn [ret x]
+     (let [k (f x)]
+       (assoc ret k (conj (get ret k []) x))))
+   (sorted-map) coll))
+
+;; partition-by written by Rich Hickey;
+;; see http://paste.lisp.org/display/64190
+(defn partition-by [f coll]
+  "Applies f to each value in coll, splitting it each time f returns
+   a new value.  Returns a lazy seq of lazy seqs."
+  (when-let [s (seq coll)]
+    (let [fv (f (first s))
+          ends (drop-while #(= fv (f %)) (rest s))
+          tw (fn this [s]
+               (when-not (identical? s ends)
+                 (lazy-cons (first s) (this (rest s)))))]
+      (lazy-cons (tw s) (partition-by f ends)))))