Add modulo functionality

Add an if to the pre_check that catches cases where there are less items
to choose from than sides on the die. If this is the case, we can still
pick items, but we have to use a modulo.
Pick the `num_rolls` so that the result generated after rolling is larger
 than `100*len(sequence)`.
The `pre_check()` function now has to return the modified `num_rolls` as
well as a boolean to indicate whether modulo needs to be applied on the
result.

diceware/random_sources.py

@@ -131,9 +131,15 @@ class RealDiceRandomSource(object):
         We make sure that `num_rolls`, the number of rolls, is in an
         acceptable range and issue an hint about the procedure.
         """
+        use_modulo=False
+        if num_rolls == 0:
+           raise(ValueError)
         if num_rolls < 1:
-            raise ValueError(
-                "Must provide at least %s items" % self.dice_sides)
+           # If this happens, there are less values in the sequence to choose from than there are dice sides.
+           # We will generate a large integer (larger than 100*len(sequence)),
+           # and use modulo to select from it
+           num_rolls = int(math.ceil(math.log(100*len(sequence), self.dice_sides)))
+           use_modulo=True
         if (self.dice_sides ** num_rolls) < len(sequence):
             print(
                 "Warning: entropy is reduced! Using only first %s of %s "
@@ -144,13 +150,13 @@ class RealDiceRandomSource(object):
         print(
             "Please roll %s dice (or a single dice %s times)." % (
                 num_rolls, num_rolls))
-        return
+        return num_rolls, use_modulo
 
     def choice(self, sequence):
         """Pick one item out of `sequence`.
         """
         num_rolls = int(math.log(len(sequence), self.dice_sides))
-        self.pre_check(num_rolls, sequence)
+        num_rolls, use_modulo = self.pre_check(num_rolls, sequence)
         result = 0
         for i in range(num_rolls, 0, -1):
             rolled = None
@@ -159,4 +165,6 @@ class RealDiceRandomSource(object):
                 rolled = input_func(
                     "What number shows dice number %s? " % (num_rolls - i + 1))
             result += ((self.dice_sides ** (i - 1)) * (int(rolled) - 1))
+            if use_modulo:
+                result = result % len(sequence)
         return sequence[result]