Rewrote the merge sort implementation

Author: codingEzio

chap04_sequences/sort_merge.py

@@ -0,0 +1,66 @@
+def merge(seq, start, mid, stop):
+    """
+    merge lists (sub-lists) that have been sorted
+    """
+
+    lst = []
+    _start = start
+    _mid = mid
+
+    while _start < mid and _mid < stop:
+        if seq[_start] < seq[_mid]:
+            lst.append(seq[_start])
+            _start += 1
+        else:
+            lst.append(seq[_mid])
+            _mid += 1
+
+    # leftover
+    while _start < mid:
+        lst.append(seq[_start])
+        _start += 1
+
+    # copy the items back to the original sequence
+    for idx in range(len(lst)):
+        seq[start + idx] = lst[idx]
+
+
+def merge_sort_recursive(seq, start, stop):
+    """
+    a helper function to get things (split & sort) started
+    """
+
+    if start >= stop - 1:
+        return None
+
+    mid = (start + stop) // 2
+
+    merge_sort_recursive(seq, start, mid)
+    merge_sort_recursive(seq, mid, stop)
+
+    # half, half-of-half, half-of-half-of-half -> merge
+    # half, half-of-half, half-of-half-of-half -> merge another
+    # merge both                               -> the end
+    merge(seq, start, mid, stop)
+
+
+def merge_sort(seq):
+    """
+    O(N logN)
+        O(logN) for dividing the list into lists of size 1
+        O(N)    for merging the sorted sublists
+    """
+    merge_sort_recursive(seq, 0, len(seq))
+
+
+def main():
+    """
+    Not 100% percent get this algorithm, but nearly done :)
+    """
+    d, d_sorted = [3, 7, 9, 40, 1, 13, 10], [1, 3, 7, 9, 10, 13, 40]
+    merge_sort(d)
+    assert d == d_sorted
+
+
+if "__main__" == __name__:
+    main()