From 345df3d6d08e95309d3a9cce8b6ffe57be84a7a0 Mon Sep 17 00:00:00 2001
From: Colin Jones <trptcolin@gmail.com>
Date: Sun, 7 Nov 2010 10:17:58 -0600
Subject: [PATCH] Add some background knowledge to the recursion section.

---
 src/koans/recursion.clj      | 18 ++++++++++++++++++
 src/path_to_answer_sheet.clj | 10 ++++++----
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/src/koans/recursion.clj b/src/koans/recursion.clj
index 7f3b2ef..6cd34ba 100644
--- a/src/koans/recursion.clj
+++ b/src/koans/recursion.clj
@@ -1,7 +1,25 @@
+(defn is-even? [n]
+  (if (= n 0) __
+      (___ (is-even? (dec n)))))
+
+(defn is-even-bigint? [n]
+  (loop [n n acc true]
+    (if (= n 0) __
+        (recur (dec n) (not acc)))))
+
 (defn factorial [n]
   __)
 
 (meditations
+  "Recursion ends with a base case"
+  (= true (is-even? 0))
+
+  "And starts by moving toward that base case"
+  (= false (is-even? 1))
+
+  "Having too many stack frames requires explicit tail calls with recur"
+  (= false (is-even-bigint? 100003N))
+
   "Simple things may appear simple."
   (= 1 (factorial 1))
 
diff --git a/src/path_to_answer_sheet.clj b/src/path_to_answer_sheet.clj
index ba2f1ff..72a4d53 100644
--- a/src/path_to_answer_sheet.clj
+++ b/src/path_to_answer_sheet.clj
@@ -55,7 +55,7 @@
                       10 5
                       30 2
                       15
-                      20 "*"]
+                      20 '*]
                 "___" ['(fn [f] (f 5))
                        '(fn [f] 25)]}
    "conditionals" {"__" [:a
@@ -63,7 +63,7 @@
                          0
                          :glory
                          4 6 :your_road
-                         "'doom" 0 ]}
+                         ''doom 0 ]}
    "higher_order_functions" {"__" [4 8 12
                                    '(* x x)
                                    [false false true false false]
@@ -90,10 +90,12 @@
                               :a :b :c :d
                               :c :d]
                         "___" ["multiply-by-5"]}
-   "recursion" {"__" ['(loop [n n acc 1]
+   "recursion" {"__" [true 'acc
+                      '(loop [n n acc 1]
                          (if (zero? n)
                              acc
-                             (recur (dec n) (* acc n))))]}
+                             (recur (dec n) (* acc n))))]
+                "___" ['not]}
    "destructuring" {"__" ["\":bar:foo\""]}
    })