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+; P13 Run-length encoding of a list (direct solution).
+; Implement the so-called run-length encoding data compression method directly. 
+; I.e. don't explicitly create the sublists containing the duplicates, as in problem P09, 
+; but only count them. As in problem P11, simplify the result list by replacing the 
+; singleton lists (1 X) by X.
+
+(def a (list "a" "a" "a" "a" "b" "c" "c" "a" "a" "d" "e" "e" "e" "e"))
+
+(func encode-direct (lst)
+    (encode-direct lst "" (list)))
+
+(func encode-direct (lst :nil prev acc)
+    acc)
+
+(func encode-direct (lst :{:any} prev acc)
+    (if (eq? (first lst) prev)
+        (encode-direct
+            (rest lst)
+            prev
+            (append (most acc)
+                (if (list? (last acc))
+                    (list (+ 1 (first (last acc))) prev)
+                    (list 2 prev))))
+        (encode-direct
+            (rest lst)
+            (first lst)
+            (append acc (first lst)))))
+
+(print (->string a))
+(print (->string (encode-direct a)))