exercises

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diff --git a/rosetta/luhn.neb b/rosetta/luhn.neb
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+(func odd-elems (lst)
+    (odd-elems lst (list)))
+
+(func odd-elems (lst acc)
+    (if (eq? 0 (length lst))
+        acc
+        (odd-elems (rest (rest lst)) (append acc (first lst)))))
+
+(func even-elems (lst)
+    (even-elems lst (list)))
+
+(func even-elems (lst acc)
+    (if (>= 1 (length lst))
+        acc
+        (even-elems (rest (rest lst)) (append acc (first (rest lst))))))
+
+(func luhn? (num)
+    ;; algo:
+    ;; with digits reversed, sum odd digits
+    ;; for all even digits:
+    ;;   - double them
+    ;;   - sum the digits (i.e. 16 -> 1 + 6 -> 7)
+    ;;   - sum everything
+    ;; sum both values, and if the result ends in 0, it's valid 
+    (def reverse-digits (reverse (->string num)))
+    ; sum the odd digits
+    (def s1
+        (reduce 
+            (lambda (x y) (+ x (string->int y)))
+            (odd-elems reverse-digits) 0))
+    (def s2
+        ; sum everything
+        (reduce (lambda (x y) (+ x y))
+            ; sum each of the digits
+            (map (lambda (x) (apply + x))
+                ; split the resulting numbers into digits
+                (map (lambda (x) (map string->int (split (->string x))))
+                    ; double the even digits
+                    (map (lambda (x) (* 2 (string->int x))) 
+                        (even-elems reverse-digits)))) 0))
+    (int? (/ (+ s1 s2) 10)))
+
+(func base36-to-base10 (value :string)
+    (base36-to-base10 (ord value)))
+
+(func base36-to-base10 (value :int)
+    (if (< value 32)
+        (+ 9 value)
+        (base36-to-base10 (- value 32))))
+
+(func isin? (value)
+    ;; algo:
+    ;;  - must be 12 digits
+    ;;  - first 2 must be letters
+    ;;  - last must be a number
+    ;;  - interpret all values as base 32
+    ;;  - replace them as if replacing characters in a string
+    ;;  - run luhn
+    (def digits (split "0123456789"))
+    (and
+        (eq? 12 (length value))
+        (not (in? (first value) digits))
+        (not (in? (first (rest value)) digits))
+        (in? (last value) digits)
+        (luhn? (string->int
+            (apply concat
+                (reduce
+                    (lambda (acc val)
+                        (append acc
+                            (->string (try
+                                (string->int val)
+                                (base36-to-base10 val)))))
+                    (split value) (list)))))))