Salsa20 Implementation: Sum of 2 Words with Carries Suppressed
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I'm working on the 2nd part of the Salsa20 spec, and I want to implement a closure for the exclusive-or of two words(u32). The author defines the operation as the sum of two words with carries suppressed, but what does "carries suppressed" mean. Then I come across some summation notation:
$u=sum_i2^iu_i$
What does the variable "i" mean in this context exactly? I understand summation notation and the index variable, but the context of the spec doesn't elaborate enough for me to know what "i" indexes.
I realize this all may be very rudimentary, but I am a programmer first.
modular-arithmetic xor salsa20
edited Feb 27 at 6:46
DannyNiu
1,2711628
asked Feb 27 at 4:55
whole_lotta_coinswhole_lotta_coins
232
$endgroup$
add a comment |
$begingroup$
I'm working on the 2nd part of the Salsa20 spec, and I want to implement a closure for the exclusive-or of two words(u32). The author defines the operation as the sum of two words with carries suppressed, but what does "carries suppressed" mean. Then I come across some summation notation:
$u=sum_i2^iu_i$
What does the variable "i" mean in this context exactly? I understand summation notation and the index variable, but the context of the spec doesn't elaborate enough for me to know what "i" indexes.
I realize this all may be very rudimentary, but I am a programmer first.
modular-arithmetic xor salsa20
edited Feb 27 at 6:46
DannyNiu
1,2711628
asked Feb 27 at 4:55
whole_lotta_coinswhole_lotta_coins
232
$endgroup$
1
$begingroup$
$$bigoplus{}$$
$endgroup$
– kelalaka
Feb 27 at 16:43
add a comment |
$begingroup$
I'm working on the 2nd part of the Salsa20 spec, and I want to implement a closure for the exclusive-or of two words(u32). The author defines the operation as the sum of two words with carries suppressed, but what does "carries suppressed" mean. Then I come across some summation notation:
$u=sum_i2^iu_i$
What does the variable "i" mean in this context exactly? I understand summation notation and the index variable, but the context of the spec doesn't elaborate enough for me to know what "i" indexes.
I realize this all may be very rudimentary, but I am a programmer first.
modular-arithmetic xor salsa20
edited Feb 27 at 6:46
DannyNiu
1,2711628
asked Feb 27 at 4:55
whole_lotta_coinswhole_lotta_coins
232
$endgroup$
I'm working on the 2nd part of the Salsa20 spec, and I want to implement a closure for the exclusive-or of two words(u32). The author defines the operation as the sum of two words with carries suppressed, but what does "carries suppressed" mean. Then I come across some summation notation:
$u=sum_i2^iu_i$
What does the variable "i" mean in this context exactly? I understand summation notation and the index variable, but the context of the spec doesn't elaborate enough for me to know what "i" indexes.
I realize this all may be very rudimentary, but I am a programmer first.
modular-arithmetic xor salsa20
modular-arithmetic xor salsa20
edited Feb 27 at 6:46
DannyNiu
1,2711628
asked Feb 27 at 4:55
whole_lotta_coinswhole_lotta_coins
232
edited Feb 27 at 6:46
DannyNiu
1,2711628
asked Feb 27 at 4:55
whole_lotta_coinswhole_lotta_coins
232
edited Feb 27 at 6:46
DannyNiu
1,2711628
edited Feb 27 at 6:46
DannyNiu
1,2711628
edited Feb 27 at 6:46
DannyNiu
1,2711628
1,2711628
asked Feb 27 at 4:55
whole_lotta_coinswhole_lotta_coins
232
asked Feb 27 at 4:55
whole_lotta_coinswhole_lotta_coins
232
asked Feb 27 at 4:55
whole_lotta_coinswhole_lotta_coins
232
232
1
$begingroup$
$$bigoplus{}$$
$endgroup$
– kelalaka
Feb 27 at 16:43
add a comment |
1
$begingroup$
$$bigoplus{}$$
$endgroup$
– kelalaka
Feb 27 at 16:43
1
1
$begingroup$
$$bigoplus{}$$
$endgroup$
– kelalaka
Feb 27 at 16:43
$begingroup$
$$bigoplus{}$$
$endgroup$
– kelalaka
Feb 27 at 16:43
add a comment |
1 Answer
1
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$begingroup$
The sum of two words with carries suppressed is just a convoluted way of saying XOR. You don't need to implement any kind of complicated summation operation. Just perform a bitwise-exclusive OR.
See also https://en.wikipedia.org/wiki/Exclusive_or#Computer_science
answered Feb 27 at 5:03
forestforest
4,3251641
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add a comment |
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$begingroup$
The sum of two words with carries suppressed is just a convoluted way of saying XOR. You don't need to implement any kind of complicated summation operation. Just perform a bitwise-exclusive OR.
See also https://en.wikipedia.org/wiki/Exclusive_or#Computer_science
answered Feb 27 at 5:03
forestforest
4,3251641
$endgroup$
add a comment |
$begingroup$
The sum of two words with carries suppressed is just a convoluted way of saying XOR. You don't need to implement any kind of complicated summation operation. Just perform a bitwise-exclusive OR.
See also https://en.wikipedia.org/wiki/Exclusive_or#Computer_science
answered Feb 27 at 5:03
forestforest
4,3251641
$endgroup$
add a comment |
$begingroup$
The sum of two words with carries suppressed is just a convoluted way of saying XOR. You don't need to implement any kind of complicated summation operation. Just perform a bitwise-exclusive OR.
See also https://en.wikipedia.org/wiki/Exclusive_or#Computer_science
answered Feb 27 at 5:03
forestforest
4,3251641
$endgroup$
The sum of two words with carries suppressed is just a convoluted way of saying XOR. You don't need to implement any kind of complicated summation operation. Just perform a bitwise-exclusive OR.
See also https://en.wikipedia.org/wiki/Exclusive_or#Computer_science
answered Feb 27 at 5:03
forestforest
4,3251641
edited Feb 27 at 8:19
answered Feb 27 at 5:03
forestforest
4,3251641
answered Feb 27 at 5:03
forestforest
4,3251641
answered Feb 27 at 5:03
forestforest
4,3251641
4,3251641
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$begingroup$
$$bigoplus{}$$
$endgroup$
– kelalaka
Feb 27 at 16:43