order.clj

;Defined in terms of pennies to avoid float error
(def xkcd 
     (sorted-map-by 
      (comp - compare)
      215 :fruit
      275 :fries
      335 :salad
      355 :wings
      420 :cheeze
      580 :sampler))

;Assume seq-utils is used
(defn order
  ([amt menu] (distinct (flatten (vector (order amt menu {})))))
  ([amt menu items]
     (let [affordable (keys-pred drop-while #(< amt %) menu)
	   exact-entry? (affordable amt)]
       (cond
	 (empty? affordable) {}
	 exact-entry? (merge-with + items {exact-entry? 1})
	 true
	 (remove empty?
		 (set 
		  (map 
		   #(let [next-amt (- amt (key %))
			  next-items (merge-with + items {(val %) 1})]
		      (order next-amt affordable next-items))
		   affordable)))))))

user=>(order 1505 xkcd)
({:fruit 1, :wings 2, :sampler 1} {:fruit 7})