RogerBW's Blog

I’ve been doing the Weekly Challenges. The latest involved a date calculation and run-length encoding. (Note that this ends today.)

Task 1: Day of the Year

You are given a date in the format YYYY-MM-DD.

Write a script to find day number of the year that the given date represent.

I always like date problems, because there's such a variety of date support across the languages I'm using (including "none at all" in Lua and PostScript). In this case, some langauges support a day of year calculation natively (usually calling it "ordinal"), but otherwise I just construct a date based on the first of January for the given year and subtract,

So for example this is Ruby which has a day-of-year built in:

def dayoftheyear(a)
  Date.parse(a).yday
end

And this is Lua using my date-to-Julian-day converter:

function dayoftheyear(a)
   local _a, _b, y, m, d = string.find(a, "([0-9][0-9][0-9][0-9])-([0-9][0-9])-([0-9][0-9])")
   local d1 = ymd2jd(y, m, d)
   local d0 = ymd2jd(y, 1, 1)
   return d1 - d0 + 1
end

Of course, a modern strftime can do this directly with the %j format specification, or in a low enough level language you could pull tm_yday directly.

Task 2: Decompressed List

You are given an array of positive integers having even elements.

Write a script to to return the decompress list. To decompress, pick adjacent pair (i, j) and replace it with j, i times.

Another thing that varies across languages: how to take an array by non-overlapping chunks, from the variously named built-ins and libraries (here for Perl):

use List::Util qw(pairs);

sub decompressedlist($a) {
  my @out;
  foreach my $pair ( pairs @{$a} ) {
    my ($k, $v) = @$pair;
    push @out, ($v) x $k;
  }
  \@out;
}

to the mini state machine I built for Lua.

function decompressedlist(a)
   local out = {}
   local n = 0
   for _, v in ipairs(a) do
      if n == 0 then
         n = v
      else
         for i = 1, n do
            table.insert(out, v)
         end
         n = 0
      end
   end
   return out
end

(And some languages have a way of appending multiple values to an array at once, as in the Perl above, and others don't.)

Full code on github.