Started chapter 17.

ch-17-hard/01-add-without-plus.py

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+# Add two numbers without using +.
+
+import ctypes
+
+def add_without_plus(a, b):
+  both_bits = a & b
+  single_bits = a ^ b
+  if both_bits == 0:
+    return single_bits
+  carry_bits = (both_bits << 1) & 0x1FFFFFFFFFFFFFFFF
+  if carry_bits & (1 << 64):
+    return ctypes.c_long(single_bits).value
+  return add_without_plus(single_bits, carry_bits)
+
+import unittest
+
+class Test(unittest.TestCase):
+  def test_add_without_plus(self):
+    self.assertEqual(add_without_plus(1, 1), 2)
+    self.assertEqual(add_without_plus(1, 2), 3)
+    self.assertEqual(add_without_plus(1001, 234), 1235)
+    self.assertEqual(add_without_plus(5, -1), 4)
+    self.assertEqual(add_without_plus(7,-5), 2)
+    self.assertEqual(add_without_plus(7,-29), -22)
+    self.assertEqual(add_without_plus(-2,10), 8)
+
+if __name__ == "__main__":
+  unittest.main()
+