Explain 1-liner party of a US President from IOCCC 2013 - c

Question

I found this code on ioccc and I have trouble even beginning to understand how it works!
void main(int riguing, char** acters) {
puts(1[acters-~!(*(int*)1[acters]%4796%275%riguing)]);
}
An explanation of how this is valid code and how it actually works would be fantastic!

Answer 1

First, in C (and C++) k[pointer] and pointer[k] mean exactly the same thing, which is *(k + pointer) and *(pointer + k), respectively. Code obfuscators often seem to like to use the first version because many people find it unusual. But it's reasonably obvious that pointer + k and k + pointer are the same computation.
The other little diversion in the snippet is the use of
pointer-~!(something)
which is exactly the same as
pointer + (something == 0 ? 2 : 1)
How this works:
The ! operator turns any true (non-zero) value into 0 and the false (0) value into a boolean true (1):
!something: something == 0 ? 1 : 0
The ~ operator is bitwise inverse, so it turns 0 into the number consisting of all 1 bits, which is -1 and 1 into the number consisting of all 1 bits except the last bit, which is -2. See Wikipedia article on two's complement.
~!something: something == 0 ? -2 : -1
Subtracting that from something is the same as adding the negative (a - -b == a + b)
a-~!something: something == 0 ? a + 2 : a + 1
Finally
1[a-~!something]: something == 0 ? a[3] : a[2]
So it selects either the second or third command line argument based on whether some computation is zero or not.
So now we need to decipher "some computation". We start with the type-punning operator *(T *)(pointer), in this case *(int*)(char*), reads out whatever the pointer points to as though it were a T. So in this case, it reads the first sizeof(int) characters from 1[acters] -- that is, from the first command-line argument (argv[1]) -- as though they were the internal representation of an integer. That will code every president as an integer based on the first four characters of their surname.
While there have been several repeated presidential surnames in US history, it's not a problem as long as the two presidents with the same name shared a political party.
One such pair, the father and son John Adams, Jr. (a Federalist), and John Quincy Adams (elected to the senate as a Federalist and to the presidency as a Democratic-Republican), are eliminated as being prior to the first valid president (Franklin Pierce), as is the elder Harrison (William Henry, a Whig) whose grandson Benjamin was elected as a Republican. The father and son George H.W. and George W. Bush are both Republicans. And the two Johnsons, Andrew and Lyndon Baines (as far as I know, not related to each other), were both Democrats.
So that leaves only the two Roosevelts, Theodore (Republican) and Franklin Delano (Democrat). The great-great-great-great-grandfathers of the two Roosevelt presidents were the brothers Johannes and Jacobus, sons of Nicholas Roosevelt (or Nicholas van Rosenvelt) (1658-1742) and grandsons of the Dutch immigrant Claes Maartenszen Van Rosenvelt, making them fifth cousins. However, the presidents were more closely related to each other through Eleanor Roosevelt, Theodore's niece and FDR's wife. In order for the IOCCC entry to work, it's necessary to represent the younger Roosevelt as "fdr", as he was commonly known.
So that only leaves (integer)%4796%275%riguing, or (integer)%4796%275%4, since riguing (aka argc) is 4. That's a simple hash function, which I imagine was discovered by trial and error using the list of presidential surnames and their affiliations.