From efb0329fda9f16b84e4ba75f825f08ee1cd22b49 Mon Sep 17 00:00:00 2001
From: Mike Cifelli <michael.cifelli@gmail.com>
Date: Sun, 12 Nov 2017 16:40:49 -0500
Subject: [PATCH] Implement TCO for cons cell toString

---
 lisp/tail/length-recursion.lisp               | 11 ++++++++++
 .../to-string-recursion.lisp}                 |  2 +-
 src/sexpression/Cons.java                     | 21 ++++++++++++-------
 3 files changed, 26 insertions(+), 8 deletions(-)
 create mode 100644 lisp/tail/length-recursion.lisp
 rename lisp/{random/problem.lisp => tail/to-string-recursion.lisp} (73%)

diff --git a/lisp/tail/length-recursion.lisp b/lisp/tail/length-recursion.lisp
new file mode 100644
index 0000000..fe6adeb
--- /dev/null
+++ b/lisp/tail/length-recursion.lisp
@@ -0,0 +1,11 @@
+(load "../lang/functions.lisp")
+
+(defun build (n lst)
+  (if (= n 0) lst
+    (build (- n 1) (cons (car lst) lst))))
+
+(defun build2 (n lst)
+  (if (= n 0) lst
+    (build2 (- n 1) (build 200 lst))))
+
+(length (build2 200 (build2 200 (build2 200 (build2 200 '(1 1 1 1 0 0 0 0)))))) 
diff --git a/lisp/random/problem.lisp b/lisp/tail/to-string-recursion.lisp
similarity index 73%
rename from lisp/random/problem.lisp
rename to lisp/tail/to-string-recursion.lisp
index ad4b69a..f2d9c69 100644
--- a/lisp/random/problem.lisp
+++ b/lisp/tail/to-string-recursion.lisp
@@ -8,4 +8,4 @@
   (if (= n 0) lst
     (build2 (- n 1) (build 200 lst))))
 
-(length (build2 200 (build2 200 '(1 1 1 1 0 0 0 0))))
+(build2 200 (build2 200 (build2 200 (build2 200 '(1 1 1 1 0 0 0 0)))))
diff --git a/src/sexpression/Cons.java b/src/sexpression/Cons.java
index 3bdbc4b..da6a243 100644
--- a/src/sexpression/Cons.java
+++ b/src/sexpression/Cons.java
@@ -1,5 +1,10 @@
 package sexpression;
 
+import static recursion.tail.TailCalls.done;
+import static recursion.tail.TailCalls.tailCall;
+
+import recursion.tail.TailCall;
+
 @DisplayName("list")
 public class Cons extends SExpression {
 
@@ -34,16 +39,18 @@ public class Cons extends SExpression {
 
     @Override
     public String toString() {
-        return ("(" + toStringAux());
+        return toStringTailRecursive("(").invoke();
     }
 
-    private String toStringAux() {
-        if (rest.isNull())
-            return (first.toString() + ")");
-        else if (rest.isCons())
-            return (first.toString() + " " + ((Cons) rest).toStringAux());
+    private TailCall<String> toStringTailRecursive(String precedingString) {
+        String leadingString = precedingString + first.toString();
 
-        return (first.toString() + " . " + rest.toString() + ")");
+        if (rest.isNull())
+            return done(leadingString + ")");
+        else if (rest.isCons())
+            return tailCall(() -> ((Cons) rest).toStringTailRecursive(leadingString + " "));
+
+        return done(leadingString + " . " + rest.toString() + ")");
     }
 
 }