Add Countdown.clj and update the wordgen.clj because assoc can take a

default value.

countdown.clj

@@ -0,0 +1,39 @@
+;;; Solving Countdown by Bruteforce
+;;; jeff.foster@acm.org
+
+(ns countdown
+  (:use clojure.contrib.combinatorics)
+  (:use clojure.contrib.seq-utils))
+
+(def *operators* [+ - / *])
+
+(defn is-valid [op a b]
+  (cond 
+   (= '+ op) true
+   (= '- op) (> a b)
+   (= '* op) true
+   (= '/ op) (= 0 (mod a b))))
+
+(defn expr 
+  "A list of expressions for a and b"
+  [a b]
+  (map (fn [x] (x a b)) *operators*))
+
+(defn expressions-helper 
+  "Given a lst, build up all valid Countdown expressions"
+  [x]
+  (cond
+   (< (count x) 2) x
+   (= 2 (count x)) (apply expr x)
+   :else
+     (let [exps (apply expr (take 2 x))
+	   remd (drop 2 x)]
+       (mapcat expressions-helper (map (fn [x] (cons x remd)) exps)))))
+
+(defn expressions [lst]
+  (if (nil? lst)
+    nil
+    (lazy-cat
+     (mapcat expressions-helper (permutations lst))
+     (expressions (rest lst)))))
+

wordgen.clj

@@ -10,10 +10,8 @@
 	      (let [w1 (first v) w2 (second v) val (get accum w1)]
 		(if (nil? val)
 		  (assoc accum w1 {w2 1})
-		  (let [currentVal (get val w2)]
-		    (if (nil? currentVal)
-		      (assoc accum w1 (conj val {w2 1}))
-		      (assoc accum w1 (conj val {w2 (inc currentVal)})))))))
+		  (let [currentVal (get val w2 0)]
+		    (assoc accum w1 (conj val {w2 (inc currentVal)}))))))
 	    {} word-pairs)))
 
 (defn frequency-map-count [m word]