Added solution to 1.19.

diff --git a/chapter1/ch1_2.clj b/chapter1/ch1_2.clj
index 0cf3c556e8e2dc15024fe9eeec1caaa12bf815e8..13b99d1f881433a0e844eb8d88cf03bd6fbff1a1 100644 (file)
--- a/chapter1/ch1_2.clj
+++ b/chapter1/ch1_2.clj
@@ -536,3 +536,22 @@ TRACE t2916: |    => 6
 TRACE t2915: => 6
 6
 )
+
+;; exercise 1.19: fast fibonacci
+;; see the pdf of the notebook scan for the derivation of p' and q'
+(defn ffib [n]
+  (ffib-iter 1 0 0 1 n))
+
+(defn ffib-iter [a b p q count]
+  (cond (= count 0) b
+       (even? count)
+       (ffib-iter a
+                  b
+                  (+ (* p p) (* q q))
+                  (+ (* 2 p q) (* q q))
+                  (/ count 2))
+       :else (ffib-iter (+ (* b q) (* a q) (* a p))
+                        (+ (* b p) (* a q))
+                        p
+                        q
+                        (- count 1))))