add get_colliding_prefix_bits, docs and tests

lbry/utils.py

@@ -474,3 +474,21 @@ def release(self):
             return super().release()
         finally:
             self._lock_held_time_metric.observe(time.perf_counter() - self._lock_acquired_time)
+
+
+def get_colliding_prefix_bits(first_value: bytes, second_value: bytes, size: int):
+    """
+    Calculates the amount of colliding bits between <first_value> and <second_value> over the <size> first bits.
+    This is given by the amount of bits that are the same until the first different one (via XOR).
+    :param first_value: first value to compare, bigger than size.
+    :param second_value: second value to compare, bigger than size.
+    :param size: prefix size in bits.
+    :return: amount of prefix colliding bits.
+    """
+    assert size % 8 == 0, "size has to be a multiple of 8"
+    size_in_bytes = size // 8
+    assert len(first_value) >= size_in_bytes, "first_value has to be larger than size parameter"
+    first_value = int.from_bytes(first_value[:size_in_bytes], "big")
+    assert len(second_value) >= size_in_bytes, "second_value has to be larger than size parameter"
+    second_value = int.from_bytes(second_value[:size_in_bytes], "big")
+    return size - (first_value ^ second_value).bit_length()

tests/unit/test_utils.py

@@ -0,0 +1,19 @@
+import unittest
+from lbry import utils
+
+
+class UtilsTestCase(unittest.TestCase):
+
+    def test_get_colliding_prefix_bits(self):
+        self.assertEqual(
+            0, utils.get_colliding_prefix_bits(0xffffff.to_bytes(4, "big"), 0x00000000.to_bytes(4, "big"), 32))
+        self.assertEqual(
+            1, utils.get_colliding_prefix_bits(0xefffff.to_bytes(4, "big"), 0x00000000.to_bytes(4, "big"), 32))
+        self.assertEqual(
+            8, utils.get_colliding_prefix_bits(0x00ffff.to_bytes(4, "big"), 0x00000000.to_bytes(4, "big"), 16))
+        self.assertEqual(
+            8, utils.get_colliding_prefix_bits(0x00ffff.to_bytes(4, "big"), 0x00000000.to_bytes(4, "big"), 8))
+        self.assertEqual(
+            1, utils.get_colliding_prefix_bits(0xefffff.to_bytes(4, "big"), 0x00000000.to_bytes(4, "big"), 16))
+        self.assertEqual(
+            1, utils.get_colliding_prefix_bits(0xefffff.to_bytes(4, "big"), 0x00000000.to_bytes(4, "big"), 8))