Exercise 1.18 (#13)

src/sicp/chapter_1/ex_1_18.clj

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+(ns sicp.chapter-1.ex-1-18
+  (:require [sicp.chapter-1.ex-1-16 :refer [expt]]))
+
+; Exercise 1.18
+; Using the results of Exercise 1.16 and Exercise 1.17, devise a procedure that generates an iterative
+; process for multiplying two integers in terms of adding, doubling, and halving and uses a logarithmic number of steps.
+
+(defn fast-mult-v2
+  [a b product counter]
+  (cond (= a 0) product
+        (= (mod a 2) 1) (fast-mult-v2 (/ a 2)
+                                      b
+                                      (+ product (* (expt 2 counter) b))
+                                      (+ counter 1))
+        :else (fast-mult-v2 (/ a 2)
+                            b
+                            product
+                            (+ counter 1))))
+
+(defn mult
+  [a b]
+  (fast-mult-v2 a b 0 0))

test/sicp/chapter_1/ex_1_18_test.clj

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+(ns sicp.chapter-1.ex-1-18-test
+  (:require [clojure.test :refer :all])
+  (:require [sicp.chapter-1.ex-1-18 :refer [mult]]))
+
+(deftest mult-test
+  (is (= 0 (mult 0 19))))