RogerBW's Blog

The Weekly Challenge 175: Perfect Sunday 26 July 2022

I’ve been doing the Weekly Challenges. The latest involved date calculations and number theory. (Note that this is open until 31 July 2022.)

Task 1: Last Sunday

Write a script to list last sunday of every month in the given year.

Date calculations are one of those things like cryptography with all sorts of traps for the unwary, so I've used system libraries wherever possible. For Perl I used the good library rather than the older built-ins:

use DateTime;

sub lastsunday($year) {
  my @o;
  foreach my $month (1..12) {
    my $dt = DateTime->last_day_of_month(year => $year,
                                         month => $month);
    my $dl = $dt->day_of_week();
    if ($dl != 7) {
      $dt->subtract(days => $dl);
    push @o,$dt->strftime('%Y-%m-%d');
  return \@o;

Raku's built-in Date class also offers a last-date-in-month: it works slightly differently, but the basic idea is the same. For the other languages, which don't have this, I bodged it by going back a day from the first day of the next month, as in this Kotlin (which uses the LocalDate and Period libraries from Java):

fun lastsunday(year0: Int): List<String> {
    var year = year0
    var o = ArrayList<String>()
    val formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd")
    for (month0 in 2..13) {
        var month = month0
        if (month0 == 13) {
            year += 1
            month -= 12
        var dt = LocalDate.of(year,month,1).minusDays(1)
        val dl = dt.dayOfWeek.getValue()
        if (dl < 7) {
            dt = dt.minusDays(dl.toLong())
    return o

Some languages give a Sunday as 0 in their day-of-week function, which lets me remove the test on dl.

Lua can convert date-times back and forth to an internal representation, but can't do much with them: on Unix it uses seconds since the epoch as that representation, but there's no guarantee of the size or base of the unit elsewhere, so I'd have had to write day-of-the-week code from scratch. So I didn't write a Lua version of this.

Of course, PostScript has even less date support, but as it turns out I'd already written a Julian day number conversion library (originally for challenge #132), which does the job nicely here. (I suspect I should write a sprintf / strftime-style formatting function.)

Task 2: Perfect Totient Numbers

Write a script to generate first 20 Perfect Totient Numbers.

Given the speed of modern languages and machines, I didn't bother with much optimisation. It would be faster (though of course it would use more memory) to keep a map of iterated totient values to avoid repeated calculation; but it was hot and I felt little enthusiasm for writing that across multiple languages.

Here's the Python for the Euler totient (I've written gcd functions before for languages that need them):

def eulertotient(n):
  return sum(1 for k in range(1,n+1) if gcd(n,k) == 1)

Then the iterated version (with an optimisation you should remove if you actually want the iterated totient value for some reason - p > n0 implies that p is not equal to n0 and since that's all we care about we needn't calculate it further).

def iteratedtotient(n0):
  p = 0
  n = n0
  while True:
    n = eulertotient(n)
    p += n
    if n == 1:
    if p > n0:
  return p

Finally the list-building top-level function:

def perfecttotient(ct):
  o = []
  n = 1
  while len(o) < ct:
    n += 1
    if iteratedtotient(n) == n:
  return o

The algorithm in other languages is basically the same.

Full code on github.

See also:
Perl Weekly Challenge 132: Hash on the Mirror

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