fix negative real number encoding in abi

Author: Jan Xie

ethereum/abi.py

@@ -200,7 +200,8 @@ def encode_single(typ, arg):
         high, low = [int(x) for x in sub.split('x')]
         if not -2**(high - 1) <= arg < 2**(high - 1):
             raise ValueOutOfBounds(repr(arg))
-        return zpad(encode_int(int((arg % 2**high) * 2**low)), 32)
+        i = int(arg * 2**low)
+        return zpad(encode_int(i % 2**(high+low)), 32)
     # Strings
     elif base == 'string' or base == 'bytes':
         if not is_string(arg):

ethereum/tests/test_abi.py

@@ -30,6 +30,7 @@ def test_abi_encode_single_int():
 
 def test_abi_encode_single_real():
     assert abi.encode_single(['real', '128x128', []], 1.125) == (b'\x00'*15 + b'\x01' + b'\x20' + b'\x00'*15)
+    assert abi.encode_single(['real', '128x128', []], -1.125) == (b'\xff'*15 + b'\xfe' + b'\xe0' + b'\x00'*15)
 
 def test_abi_encode_single_hash():
     assert abi.encode_single(['hash', '8', []], b'\x00'*8) == b'\x00'*32