add transducer step to combine abutting ranges

ranges have previously been consolidated by having overlapping ranges be
combined, resuting in a (possibly) consolidated set of ranges, which
are now ordered by start and end dates.

Add a transducer step to walk those ranges and find any pairs of ranges
where the pair of ranges abutt each other, and combine them into
a single range.

src/challenge/discrete_value_range.clj

@@ -28,7 +28,8 @@
   (value-after [this])
   (start [this])
   (end [this])
-  (range-type [this]))
+  (range-type [this])
+  (union [range1 range2]))
 
 (defn- ordered-range-values
   "Builds an ordered list or 'fenceposts' for the start and end
@@ -56,7 +57,7 @@
 (defmulti ->discrete-value-range (fn [range-type _start _end]
                                    range-type))
 
-(defn combine-overlapping-ranges
+(defn- combine-overlapping-ranges
   "transducer to find and combine overlapping ranges by looking at
   ordered range value markers.
   
@@ -91,9 +92,41 @@
                       result
                       (xf result (->discrete-value-range (:type input) @start (:value input))))))))))))
 
+(defn- combine-abutting-ranges
+  "transducer to join ranges where the start and end of two ranges are consective
+  discrete values.
+
+  Assumes the ranges are already in order and any overlapping ranges have already been
+  combined."
+  []
+  (fn [xf]
+    (let [prev (volatile! nil)]
+      (fn
+        ([] (xf))
+        ([result] (if @prev
+                    (xf (xf result @prev))
+                    (xf result)))
+        ([result input]
+         (cond
+           (reduced? input) result
+
+           (nil? @prev) (do
+                          (vreset! prev input)
+                          result)
+
+           (abuts @prev input)
+           (let [item (union @prev input)]
+             (vreset! prev item)
+             result)
+
+           :else  (let [item @prev]
+                    (vreset! prev input)
+                    (xf result item))))))))
+
 (def consolidate-ranges-xf
   (comp
-   (combine-overlapping-ranges)))
+   (combine-overlapping-ranges)
+   (combine-abutting-ranges)))
 
 (defn consolidate [ranges]
   (eduction consolidate-ranges-xf (ordered-range-values ranges)))