Improve command suggestions (new algorithm)

Signed-off-by: Chris Warrick <kwpolska@gmail.com>

nikola/__main__.py

@@ -317,10 +317,15 @@ def run(self, cmd_args):
         if args[0] not in sub_cmds.keys():
             LOGGER.error("Unknown command {0}".format(args[0]))
             sugg = defaultdict(list)
-            for c in sub_cmds.keys():
-                d = lev(c, args[0])
+            sub_filtered = (i for i in sub_cmds.keys() if i != 'run')
+            for c in sub_filtered:
+                d = levenshtein(c, args[0])
                 sugg[d].append(c)
-            LOGGER.info('Did you mean "{}"?', '" or "'.join(sugg[min(sugg.keys())]))
+            best_sugg = sugg[min(sugg.keys())]
+            if len(best_sugg) == 1:
+                LOGGER.info('Did you mean "{}"?'.format(best_sugg[0]))
+            else:
+                LOGGER.info('Did you mean "{}" or "{}"?'.format('", "'.join(best_sugg[:-1]), best_sugg[-1]))
             return 3
         if sub_cmds[args[0]] is not Help and not isinstance(sub_cmds[args[0]], Command):  # Is a doit command
             if not self.nikola.configured:
@@ -334,12 +339,32 @@ def print_version():
         print("Nikola v" + __version__)
 
 
-# Stolen from http://stackoverflow.com/questions/4173579/implementing-levenshtein-distance-in-python
-def lev(a, b):
-    if not a or not b:
-        return max(len(a), len(b))
-    return min(lev(a[1:], b[1:]) + (a[0] != b[0]), lev(a[1:], b) + 1, lev(a, b[1:]) + 1)
-
+def levenshtein(s1, s2):
+    """Calculate the Levenshtein distance of two strings.
+
+    Implementation from Wikibooks:
+    https://en.wikibooks.org/w/index.php?title=Algorithm_Implementation/Strings/Levenshtein_distance&oldid=2974448#Python
+    Copyright © The Wikibooks contributors (CC BY-SA/fair use citation); edited to match coding style and add an exception.
+    """
+    if len(s1) < len(s2):
+        return levenshtein(s2, s1)
+
+    # len(s1) >= len(s2)
+    if len(s2) == 0:
+        return len(s1)
+
+    previous_row = range(len(s2) + 1)
+    for i, c1 in enumerate(s1):
+        current_row = [i + 1]
+        for j, c2 in enumerate(s2):
+            # j+1 instead of j since previous_row and current_row are one character longer than s2
+            insertions = previous_row[j + 1] + 1
+            deletions = current_row[j] + 1
+            substitutions = previous_row[j] + (c1 != c2)
+            current_row.append(min(insertions, deletions, substitutions))
+        previous_row = current_row
+
+    return previous_row[-1]
 
 if __name__ == "__main__":
     sys.exit(main(sys.argv[1:]))