Advent of Code

--- Day 4: The Ideal Stocking Stuffer ---

   Santa needs help mining some AdventCoins (very similar to bitcoins) to use
   as gifts for all the economically forward-thinking little girls and boys.

   To do this, he needs to find MD5 hashes which, in hexadecimal, start with at
   least five zeroes. The input to the MD5 hash is some secret key (your puzzle
   input, given below) followed by a number in decimal. To mine AdventCoins,
   you must find Santa the lowest positive number (no leading zeroes: 1, 2, 3,
   ...) that produces such a hash.

   For example:

     • If your secret key is abcdef, the answer is 609043, because the MD5 hash
       of abcdef609043 starts with five zeroes (000001dbbfa...), and it is the
       lowest such number to do so.

     • If your secret key is pqrstuv, the lowest number it combines with to
       make an MD5 hash starting with five zeroes is 1048970; that is, the MD5
       hash of pqrstuv1048970 looks like 000006136ef....

   Your puzzle answer was 282749.

--- Part Two ---

   Now find one that starts with six zeroes.

   Your puzzle answer was 9962624.

   Both parts of this puzzle are complete! They provide two gold stars: **

   At this point, you should return to your advent calendar and try another
   puzzle.

   Your puzzle input was yzbqklnj.

References

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   . https://en.wikipedia.org/wiki/Bitcoin#Mining
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