Area Under Curve [AUC] reduce function

auc-iter.clj

(defn iter
  "Area Under Curve [AUC] reduce function.
  The element is a vector of three elements: the ID of the model,
  whether the classifier example is positive (pos),
  and the probabilistic value (f).
  The accumulator tracks six values, aggregated area, the last f value (f'),
  negative examples (fp and fp'), and positive examples (tp and tp')."
  [[area f' fp fp' tp tp'] [_ pos f]]
  (if (= f' f)
    [area
     f'
     (if pos fp (inc fp))
     fp'
     (if pos (inc tp) tp)
     tp']
    ;; else
    [(+ area (trap-area fp fp' tp tp'))
     f
     (if pos fp (inc fp))
     fp
     (if pos (inc tp) tp)
     tp]))