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protobuf: improve description of sint_to_uint

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matejcik 2018-05-07 14:09:56 +02:00
parent df8c3da1a2
commit 28264a4fe0
1 changed files with 7 additions and 9 deletions
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16
trezorlib/protobuf.py
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@ -77,19 +77,17 @@ def dump_uvarint(writer, n):
#
# To achieve this with a twos-complement number:
# 1. shift left by 1, leaving LSbit free
# 2. XOR with "all sign bits" - 0s for positive, 1s for negative
# 2. if the number is negative, do bitwise negation.
# This keeps positive number the same, and converts negative from twos-complement
# to the appropriate value, while setting the sign bit. Cute and efficient.
# to the appropriate value, while setting the sign bit.
#
# The original algorithm makes use of the fact that arithmetic (signed) shift
# keeps the sign bits, so for a n-bit number, (x >> n+1) gets us the "all sign bits".
# keeps the sign bits, so for a n-bit number, (x >> n) gets us "all sign bits".
# Then you can take "number XOR all-sign-bits", which is XOR 0 (identity) for positive
# and XOR 1 (bitwise negation) for negative. Cute and efficient.
#
# But this is harder in Python because we don't know the bit size of the number.
# We could simply shift by 65, relying on the fact that the biggest type for other
# languages is sint64. Or we could shift by 1000 to be extra sure.
#
# But instead, we'll do it less elegantly, with an if branch:
# if the number is negative, do bitwise negation (which is the same as "xor all ones").
# But this is harder in Python because we don't natively know the bit size of the number.
# So we have to branch on whether the number is negative.
def sint_to_uint(sint):
res = sint << 1