https://en.bitcoin.it/wiki/Test_Cases
http://www.righto.com/2014/02/bitcoin-mining-hard-way-algorithms.html
https://github.com/ckolivas/cgminer/blob/master/sha2.c#L93
can be compiled with the following command
gcc -o bitcoin bitcoinIterative.c -O3 -lssl -lcrypto
mpicc -o bitcoin bitCoinMpi.c -O3 -lssl -lcrypto