partition_before.clj

(defn partition-before
  "Partition the input before (pred this-partition) returns true"
  [pred items]
  (if (seq items)
    (reduce (fn [lists item]
              (if (pred (concat (last lists) [item]))
                (concat lists [[item]])
                (concat (butlast lists) [(concat (last lists) [item])])))
            [[(first items)]]
            (rest items))
    []))

(deftest partition-before-test
  (let [sum-gt-10 #(< 10 (apply + %))]
    (is (not (sum-gt-10 [])))
    (is (not (sum-gt-10 [5])))
    (is (not (sum-gt-10 [5 5])))
    (is (sum-gt-10 [5 5 5]))
    (is (= [[0 0 0 6 1] [6 3] [9 0] [2]] (partition-before sum-gt-10 [0 0 0 6 1 6 3 9 0 2])))
    (is (= [[0 7 3] [5] [9] [4 4 2] [7 2]] (partition-before sum-gt-10 [0 7 3 5 9 4 4 2 7 2])))
    (is (= [[5]] (partition-before sum-gt-10 [5])))
    (is (= [] (partition-before sum-gt-10 [])))))