added tests, made correct.

palindrome.rb

@@ -1,30 +1,50 @@
-def expand_from_center(str, top, bottom)
-  # return the max length you can get to given a top/bottom
-  while(bottom >= 0 && top < str.length) do
-    break if str[bottom] != str[top]
-    bottom -= 1
-    top += 1
+require "rubygems"
+require "test/unit"
+class PalindromeTest < Test::Unit::TestCase
+  def expand_from_center(str, top, bottom)
+    # return the max length you can get to given a top/bottom
+    while(bottom >= 0 && top < str.length) do
+      break if str[bottom] != str[top]
+      bottom -= 1
+      top += 1
+    end
+    # top and bottom are one char too far away from center, and if they are ==,
+    # then we want length 1
+    return top - bottom - 1
   end
-  return top - bottom - 1 # explain this.
-end
 
-def find_longest_palindrome(str)
-  max_length = 0
-  longest_pal = nil
+  def find_longest_palindrome(str)
+    max_length = 0
+    longest_pal = ""
 
-  (0...str.length).each do |i|
-    odd  = expand_from_center str, i, i
-    if odd > max_length
-      max_length = odd
-      start = i - odd / 2 # ruby truncates
-      longest_pal = str[start, odd ]
-    end
-    even = expand_from_center str, i, i + 1
-    if even > max_length
-      max_length = even
-      start = i - even / 2
-      longest_pal = str[start, even ]
+    (0...str.length).each do |i|
+      odd  = expand_from_center str, i, i
+      if odd > max_length
+        max_length = odd
+        start = i - odd / 2 # ruby truncates
+        longest_pal = str[start, odd ]
+      end
+      even = expand_from_center str, i, i + 1
+      if even > max_length
+        max_length = even
+        start = i - even / 2 + 1
+        longest_pal = str[start, even ]
+      end
     end
+    longest_pal
+  end
+
+  def test_palindrome
+    assert_equal "", find_longest_palindrome("")
+    assert_equal "a", find_longest_palindrome("abc")
+    assert_equal "c", find_longest_palindrome("cba")
+    assert_equal "aa", find_longest_palindrome("aa")
+    assert_equal "aa", find_longest_palindrome("aabb")
+    assert_equal "aa", find_longest_palindrome("aabbcc")
+    assert_equal "racecar", find_longest_palindrome("racecar")
+    assert_equal "racecar", find_longest_palindrome("I have a racecar.")
+    assert_equal "ooooooooooo", find_longest_palindrome("I have a racecar.bxoooooooooooaxb")
+    assert_equal "bxoooooooooooxb", find_longest_palindrome("I have a racecar.bxoooooooooooxb")
+    # assert_equal "", find_longest_palindrome()
   end
-  longest_pal
 end