From a7f7b48a777a2e8d9fbefa83344cfd05c81c1806 Mon Sep 17 00:00:00 2001
From: Dan Buch <daniel.buch@gmail.com>
Date: Mon, 15 Aug 2011 22:34:46 -0400
Subject: [PATCH] working through merge sort

---
 sorting.py      | 38 ++++++++++++++++++++++++++++++++++++++
 test_sorting.py | 41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 79 insertions(+)

diff --git a/sorting.py b/sorting.py
index 32cdaea..cbddb26 100644
--- a/sorting.py
+++ b/sorting.py
@@ -1,4 +1,6 @@
 # vim:fileencoding=utf-8
+import math
+
 
 def insertion_sort(a):
     """ Θ(n^2) """
@@ -9,3 +11,39 @@ def insertion_sort(a):
             a[i + 1] = a[i]
             i = i - 1
         a[i + 1] = key
+
+
+def merge_sort(m):
+    n = len(m)
+    if n <= 1:
+        return m
+
+    middle = math.ceil(n / 2)
+
+    left = m[:middle]
+    right = m[middle:]
+
+    left = merge_sort(left)
+    right = merge_sort(right)
+
+    return merge(left, right)
+
+
+def merge(left, right):
+    result = []
+
+    while len(left) > 0 or len(right) > 0:
+        if len(left) > 0 and len(right) > 0:
+            if left[0] <= right[0]:
+                result.append(left.pop(0))
+            else:
+                result.append(right.pop(0))
+
+        elif len(left) > 0:
+            result.append(left.pop(0))
+
+        elif len(right) > 0:
+            result.append(right.pop(0))
+
+    return result
+
diff --git a/test_sorting.py b/test_sorting.py
index a34694b..55bcbf4 100644
--- a/test_sorting.py
+++ b/test_sorting.py
@@ -45,5 +45,46 @@ class TestInsertionSort(unittest.TestCase):
         self.assertEqual(expected, a)
 
 
+class TestMergeSort(unittest.TestCase):
+
+    def test_example(self):
+        tmpl = [8, 2, 4, 9, 3, 6]
+        a = tmpl[:]
+        expected = [2, 3, 4, 6, 8, 9]
+
+        actual = sorting.merge_sort(a)
+
+        self.assertEqual(expected, actual)
+
+    def test_100_example(self):
+        tmpl = list(range(0, 1000))
+        a = tmpl[:]
+        random.shuffle(a)
+        expected = tmpl[:]
+
+        actual = sorting.merge_sort(a)
+
+        self.assertEqual(expected, actual)
+
+    def test_reversed_100_example(self):
+        tmpl = list(range(0, 1000))
+        a = list(reversed(tmpl[:]))
+        expected = tmpl[:]
+
+        actual = sorting.merge_sort(a)
+
+        self.assertEqual(expected, actual)
+
+    def test_1000_example(self):
+        tmpl = list(range(0, 1000))
+        a = tmpl[:]
+        random.shuffle(a)
+        expected = tmpl[:]
+
+        actual = sorting.merge_sort(a)
+
+        self.assertEqual(expected, actual)
+
+
 if __name__ == '__main__':
     unittest.main()