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SICP/exercise_1_11.janet

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+
+(defn f1 [n]
+  (cond
+    (< n 3) n
+    (+
+     (f1 (- n 1))
+     (* 2 (f1 (- n 2)))
+     (* 3 (f1 (- n 3))))))
+
+(defn f2-s [ a b c ]
+  (+ a (* 2 b) (* 3 c)))
+
+(defn f2-iter [ a b c i n ]
+  (cond
+    (= i 0) (cond
+              (= n 0 ) a
+              (f2-iter a b (f2-s a b c) (+ i 1) (- n 1)))
+    (= i 1) (cond
+              (= n 0 ) c
+              (f2-iter a (f2-s c a b) c (+ i 1) (- n 1)))
+    (= i 2) (cond
+              (= n 0 ) b
+              (f2-iter (f2-s b c a) b c 0 (- n 1)))))
+
+(defn f2 [n]
+  (cond
+    (< n 3 ) n
+    (f2-iter 2 1 0 0 (- n 2))))
+
+(defn f3-iter [a b c n]
+  (def new_c (f2-s a b c))
+  (def new_b (f2-s new_c a b))
+  (def new_a (f2-s new_b new_c a))
+  (cond
+    (= n 1) new_c
+    (= n 2) new_b
+    (= n 3) new_a
+    (f3-iter new_a new_b new_c (- n 3))))
+
+(defn f3 [n]
+  (cond
+    (< n 3) n
+    (f3-iter 2 1 0 (- n 2))))
+
+(print (f3 30))
+(print (f2 30))
+(print (f1 30))