Wednesday, September 1, 2010

Back to Basics: Bubble Sorted Date Strings in C.

Earlier today I ran across a job posting that had a small test, I wasn't interested in the job but I liked the test.  One problem was to write a date sorter without using a date parsing library.  The format to be parsed is: "Sep 1 2010".  Normally I would write something like this in a few lines of Python or Perl, but I decided to tackle the problem in C, just to make it harder.  Since regular expressions were allowed, I thought it was okay to use sscanf.

One of the first thing's you'll see is that I took a page from Perl's spaceship operator, and defined a macro that will compare two numbers and return -1 if it is less, 0 if equal and 1 if greater.  Normally for a sorting algorithm you would only need to know if it is less or not, but I wanted to chain comparisons together by falling down to the next comparison when two values are equal.

Another trick I used is a bit of a hack to find the the number corresponding to the month.  You'll notice that I cast char *a to uint32_t as a way to avoid strcmp.  I'm not sure if I would do this in production code, but its a fun trick.

Full code is below.


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