Add NgramCharModel to text.py (aimacode#413)

* Add NgramCharModel to text.py

* Update text.py

* Update

Author: lucasmoura

text.py

@@ -26,6 +26,7 @@ def samples(self, n):
         return ' '.join(self.sample() for i in range(n))
 
 
+
 class NgramTextModel(CountingProbDist):
 
     """This is a discrete probability distribution over n-tuples of words.
@@ -50,12 +51,16 @@ def add(self, ngram):
             self.cond_prob[ngram[:-1]] = CountingProbDist()
         self.cond_prob[ngram[:-1]].add(ngram[-1])
 
+    def add_empty(self, words, n):
+        return [''] * (n - 1) + words
+
     def add_sequence(self, words):
         """Add each of the tuple words[i:i+n], using a sliding window.
         Prefix some copies of the empty word, '', to make the start work."""
         n = self.n
-        words = ['', ] * (n - 1) + words
-        for i in range(len(words) - n + 1):
+        words = self.add_empty(words, n)
+
+        for i in range(len(words) - n):
             self.add(tuple(words[i:i + n]))
 
     def samples(self, nwords):
@@ -72,6 +77,15 @@ def samples(self, nwords):
             nminus1gram = nminus1gram[1:] + (wn,)
         return ' '.join(output)
 
+
+class NgramCharModel(NgramTextModel):
+    def add_empty(self, words, n):
+        return  ' ' * (n - 1) + words
+
+    def add_sequence(self, words):
+        for word in words:
+            super().add_sequence(word)
+
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