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+'''
+Simple Diff for Python version 1.0
+
+Annotate two versions of a list with the values that have been
+changed between the versions, similar to unix's `diff` but with
+a dead-simple Python interface.
+
+(C) Paul Butler 2008-2012 <http://www.paulbutler.org/>
+May be used and distributed under the zlib/libpng license
+<http://www.opensource.org/licenses/zlib-license.php>
+'''
+
+__all__ = ['diff', 'string_diff', 'html_diff']
+__version__ = '1.0'
+
+
+def diff(old, new):
+    '''
+    Find the differences between two lists. Returns a list of pairs, where the
+    first value is in ['+','-','='] and represents an insertion, deletion, or
+    no change for that list. The second value of the pair is the list
+    of elements.
+
+    Params:
+        old     the old list of immutable, comparable values (ie. a list
+                of strings)
+        new     the new list of immutable, comparable values
+   
+    Returns:
+        A list of pairs, with the first part of the pair being one of three
+        strings ('-', '+', '=') and the second part being a list of values from
+        the original old and/or new lists. The first part of the pair
+        corresponds to whether the list of values is a deletion, insertion, or
+        unchanged, respectively.
+
+    Examples:
+        >>> diff([1,2,3,4],[1,3,4])
+        [('=', [1]), ('-', [2]), ('=', [3, 4])]
+
+        >>> diff([1,2,3,4],[2,3,4,1])
+        [('-', [1]), ('=', [2, 3, 4]), ('+', [1])]
+
+        >>> diff('The quick brown fox jumps over the lazy dog'.split(),
+        ...      'The slow blue cheese drips over the lazy carrot'.split())
+        ... # doctest: +NORMALIZE_WHITESPACE
+        [('=', ['The']),
+         ('-', ['quick', 'brown', 'fox', 'jumps']),
+         ('+', ['slow', 'blue', 'cheese', 'drips']),
+         ('=', ['over', 'the', 'lazy']),
+         ('-', ['dog']),
+         ('+', ['carrot'])]
+
+    '''
+
+    # Create a map from old values to their indices
+    old_index_map = dict()
+    for i, val in enumerate(old):
+        old_index_map.setdefault(val,list()).append(i)
+
+    # Find the largest substring common to old and new.
+    # We use a dynamic programming approach here.
+    # 
+    # We iterate over each value in the `new` list, calling the
+    # index `inew`. At each iteration, `overlap[i]` is the
+    # length of the largest suffix of `old[:i]` equal to a suffix
+    # of `new[:inew]` (or unset when `old[i]` != `new[inew]`).
+    #
+    # At each stage of iteration, the new `overlap` (called
+    # `_overlap` until the original `overlap` is no longer needed)
+    # is built from the old one.
+    #
+    # If the length of overlap exceeds the largest substring
+    # seen so far (`sub_length`), we update the largest substring
+    # to the overlapping strings.
+
+    overlap = dict()
+    # `sub_start_old` is the index of the beginning of the largest overlapping
+    # substring in the old list. `sub_start_new` is the index of the beginning
+    # of the same substring in the new list. `sub_length` is the length that
+    # overlaps in both.
+    # These track the largest overlapping substring seen so far, so naturally
+    # we start with a 0-length substring.
+    sub_start_old = 0
+    sub_start_new = 0
+    sub_length = 0
+
+    for inew, val in enumerate(new):
+        _overlap = dict()
+        for iold in old_index_map.get(val,list()):
+            # now we are considering all values of iold such that
+            # `old[iold] == new[inew]`.
+            _overlap[iold] = (iold and overlap.get(iold - 1, 0)) + 1
+            if(_overlap[iold] > sub_length):
+                # this is the largest substring seen so far, so store its
+                # indices
+                sub_length = _overlap[iold]
+                sub_start_old = iold - sub_length + 1
+                sub_start_new = inew - sub_length + 1
+        overlap = _overlap
+
+    if sub_length == 0:
+        # If no common substring is found, we return an insert and delete...
+        return (old and [('-', old)] or []) + (new and [('+', new)] or [])
+    else:
+        # ...otherwise, the common substring is unchanged and we recursively
+        # diff the text before and after that substring
+        return diff(old[ : sub_start_old], new[ : sub_start_new]) + \
+               [('=', new[sub_start_new : sub_start_new + sub_length])] + \
+               diff(old[sub_start_old + sub_length : ],
+                       new[sub_start_new + sub_length : ])
+
+
+def string_diff(old, new):
+    '''
+    Returns the difference between the old and new strings when split on
+    whitespace. Considers punctuation a part of the word
+
+    This function is intended as an example; you'll probably want
+    a more sophisticated wrapper in practice.
+
+    Params:
+        old     the old string
+        new     the new string
+
+    Returns:
+        the output of `diff` on the two strings after splitting them
+        on whitespace (a list of change instructions; see the docstring
+        of `diff`)
+
+    Examples:
+        >>> string_diff('The quick brown fox', 'The fast blue fox')
+        ... # doctest: +NORMALIZE_WHITESPACE
+        [('=', ['The']),
+         ('-', ['quick', 'brown']),
+         ('+', ['fast', 'blue']),
+         ('=', ['fox'])]
+
+    '''
+    return diff(old.split(), new.split())
+
+
+def html_diff(old, new):
+    '''
+    Returns the difference between two strings (as in stringDiff) in
+    HTML format. HTML code in the strings is NOT escaped, so you
+    will get weird results if the strings contain HTML.
+
+    This function is intended as an example; you'll probably want
+    a more sophisticated wrapper in practice.
+
+    Params:
+        old     the old string
+        new     the new string
+
+    Returns:
+        the output of the diff expressed with HTML <ins> and <del>
+        tags.
+
+    Examples:
+        >>> html_diff('The quick brown fox', 'The fast blue fox')
+        'The <del>quick brown</del> <ins>fast blue</ins> fox'
+    '''
+    con = {'=': (lambda x: x),
+           '+': (lambda x: "<ins>" + x + "</ins>"),
+           '-': (lambda x: "<del>" + x + "</del>")}
+    return " ".join([(con[a])(" ".join(b)) for a, b in string_diff(old, new)])
+
+
+def check_diff(old, new):
+    '''
+    This tests that diffs returned by `diff` are valid. You probably won't
+    want to use this function, but it's provided for documentation and
+    testing.
+
+    A diff should satisfy the property that the old input is equal to the
+    elements of the result annotated with '-' or '=' concatenated together.
+    Likewise, the new input is equal to the elements of the result annotated
+    with '+' or '=' concatenated together. This function compares `old`,
+    `new`, and the results of `diff(old, new)` to ensure this is true.
+
+    Tests:
+        >>> check_diff('ABCBA', 'CBABA')
+        >>> check_diff('Foobarbaz', 'Foobarbaz')
+        >>> check_diff('Foobarbaz', 'Boobazbam')
+        >>> check_diff('The quick brown fox', 'Some quick brown car')
+        >>> check_diff('A thick red book', 'A quick blue book')
+        >>> check_diff('dafhjkdashfkhasfjsdafdasfsda', 'asdfaskjfhksahkfjsdha')
+        >>> check_diff('88288822828828288282828', '88288882882828282882828')
+        >>> check_diff('1234567890', '24689')
+    '''
+    old = list(old)
+    new = list(new)
+    result = diff(old, new)
+    _old = [val for (a, vals) in result if (a in '=-') for val in vals]
+    assert old == _old, 'Expected %s, got %s' % (old, _old)
+    _new = [val for (a, vals) in result if (a in '=+') for val in vals]
+    assert new == _new, 'Expected %s, got %s' % (new, _new)
+