Improvements and simplification in corpus' handling randomisation code

- Remove useless casts to bool
- Use something faster than random.randint: this cuts in half the time spent
  in the random.py file, without raising anything else
- use random.choice where possible, instead of `mylist[_rand(len(mylist))]`
- Add some comments explaining the if…if…else structure, to prevent
  future refactoring from transforming them into an if…elif…else

On a local fuzzer, with 10000 iterations, this decrease the amount of time
spent in corpus' mutate function from ~0.2 seconds to ~0.1 seconds, for a total
run time of ~9 seconds, which seems to not be a lot, but since 8.60s is spent
in connection.py's poll function waiting for the fuzzer, it's a worthy
optimization.

pythonfuzz/corpus.py

@@ -41,9 +41,14 @@ def length(self):
 
     @staticmethod
     def _rand(n):
-        if n == 1 or n == 0:
+        if n < 2:
             return 0
-        return random.randint(0, n-1)
+
+        k = (n-1).bit_length()
+        r = random.getrandbits(k)  # 0 <= r < 2**k
+        while r >= n:
+            r = random.getrandbits(k)
+        return r
 
     # Exp2 generates n with probability 1/2^(n+1).
     @staticmethod
@@ -97,7 +102,7 @@ def generate_input(self):
             self.put(zero_test_case)
             return zero_test_case
         else:
-            buf = self._inputs[self._rand(len(self._inputs))]
+            buf = random.choice(self._inputs)
             return self.mutate(buf)
 
     def mutate(self, buf):
@@ -182,7 +187,7 @@ def mutate(self, buf):
                     continue
                 pos = self._rand(len(res))
                 v = self._rand(35) + 1
-                if bool(random.getrandbits(1)):
+                if random.getrandbits(1):
                     res[pos] = numpy.uint8(res[pos]) + numpy.uint8(v)
                 else:
                     res[pos] = numpy.uint8(res[pos]) - numpy.uint8(v)
@@ -193,9 +198,9 @@ def mutate(self, buf):
                     continue
                 pos = self._rand(len(res) - 1)
                 v = numpy.uint16(self._rand(35) + 1)
-                if bool(random.getrandbits(1)):
+                if random.getrandbits(1):
                     v = numpy.uint16(0) - v
-                if bool(random.getrandbits(1)):
+                if random.getrandbits(1):  # pick the endianness
                     v = struct.pack('>H', v)
                 else:
                     v = struct.pack('<H', v)
@@ -208,9 +213,9 @@ def mutate(self, buf):
                     continue
                 pos = self._rand(len(res) - 3)
                 v = numpy.uint32(self._rand(35) + 1)
-                if bool(random.getrandbits(1)):
+                if random.getrandbits(1):
                     v = numpy.uint32(0) - v
-                if bool(random.getrandbits(1)):
+                if random.getrandbits(1):  # pick the endianness
                     v = struct.pack('>I', v)
                 else:
                     v = struct.pack('<I', v)
@@ -223,9 +228,9 @@ def mutate(self, buf):
                     continue
                 pos = self._rand(len(res) - 7)
                 v = numpy.uint64(self._rand(35) + 1)
-                if bool(random.getrandbits(1)):
+                if random.getrandbits(1):
                     v = numpy.uint64(0) - v
-                if bool(random.getrandbits(1)):
+                if random.getrandbits(1):  # pick the endianness
                     v = struct.pack('>Q', v)
                 else:
                     v = struct.pack('<Q', v)
@@ -237,15 +242,15 @@ def mutate(self, buf):
                     i -= 1
                     continue
                 pos = self._rand(len(res))
-                res[pos] = numpy.uint8(INTERESTING8[self._rand(len(INTERESTING8))])
+                res[pos] = numpy.uint8(random.choice(INTERESTING8))
             elif x == 12:
                 # Replace an uint16 with an interesting value.
                 if len(res) < 2:
                     i -= 1
                     continue
                 pos = self._rand(len(res) - 1)
-                v = numpy.uint16(INTERESTING16[self._rand(len(INTERESTING16))])
-                if bool(random.getrandbits(1)):
+                v = numpy.uint16(random.choice(INTERESTING16))
+                if random.getrandbits(1):  # pick the endianness
                     v = struct.pack('>H', v)
                 else:
                     v = struct.pack('<H', v)
@@ -257,8 +262,8 @@ def mutate(self, buf):
                     i -= 1
                     continue
                 pos = self._rand(len(res) - 3)
-                v = numpy.uint32(INTERESTING32[self._rand(len(INTERESTING32))])
-                if bool(random.getrandbits(1)):
+                v = numpy.uint32(random.choice(INTERESTING32))
+                if random.getrandbits(1):  # pick the endianness
                     v = struct.pack('>I', v)
                 else:
                     v = struct.pack('<I', v)
@@ -270,7 +275,7 @@ def mutate(self, buf):
                 for k in range(len(res)):
                     if ord('0') <= res[k] <= ord('9'):
                         digits.append(k)
-                if len(digits) == 0:
+                if not digits:
                     i -= 1
                     continue
                 pos = self._rand(len(digits))