xor swap

The operation xor has dual functions,
as opposite checker or as conditional flipper.

a b | xor
---------
0 0 | 0
0 1 | 1
1 0 | 1
1 1 | 0

So, a fast swap method can be done by

# produce a flag indicating whether
# the two are opposite (1) or equal (0)
a' = a xor b

# re-create a based on flag and orig b, and store it in b
b = a' xor b

# re-create b based on flag and orig a, and store it in a
a = a' xor b

After statement 1, the orig a is no longer in use.
So, in practice, to be more space-saving, use a for a'

a = a xor b
b = a xor b
a = a xor b

何以名之？姑名之曰：三儍對調法.
xor swap 雖然工巧, 但對於當代的 CPU 而言,
標準的 temp swap 不涉及運算, 比它更為迅速, .

tmp = a
a = b
b = tmp

編碼用意也更為明確.