#!/usr/bin/perl -w

use strict;
use Time::Local;

# Number of horizontal calendars to draw.
use constant CALENDAR_COUNT => 2;

# Color bitmasks.
use constant CURRENT_MONTH => 1;
use constant CURRENT_DAY => 2;
use constant FULL_MOON => 4;

# All variables.
my ($x, $y, $z, $mday, $month, $year, $wday,
    @current, @t, @colors, $i, $j, $k, $l, @c, @age);

# Color palette, indexed by bitmask.
my @palette =
(
   "\e[90;40m",
   "\e[37;40m",   # CURRENT_MONTH
   "\e[30;47m",   # CURRENT_DAY
   "\e[30;47m",   # CURRENT_MONTH | CURRENT_DAY

   "\e[90;40m",   # FULL_MOON (not current month)
   "\e[96;40m",   # FULL_MOON | CURRENT_MONTH
   "\e[30;106m",  # FULL_MOON | CURRENT_DAY
   # CURRENT_MONTH | CURRENT_DAY | FULL_MOON is handled specially.
);


# Split timestamp into components.
sub LocalTime()
{
   return localtime $l;
}

# Align timestamp to midnight.
sub Align()
{
   ($x, $y, $z, $mday, $month, $year) = LocalTime;
   $l = timelocal 0, 0, 0, $k ? $mday : 1, $month, $year;
}

# Conditionally output a string.
sub Output($$)
{
   print $_[0] if $_[1];
}


# Get dates for current month.
@t = (time);
if( !$#ARGV )
{
   if( $ARGV[0] !~ /^(\d+)-(\d+)-(\d+)$/ ||
       !defined($t[0] = eval { timelocal(0, 0, 0, $3, $2 - 1, $1 - 1900) }) )
   {
      die "Unable to parse $ARGV[0] as YYYY-MM-DD\n";
   }
}
for($i = 0; $i < CALENDAR_COUNT; $i++)
{
   $l = $t[$i];
   ($x, $y, $z, $mday, $month, $year) = LocalTime;
   push @current, [$year, $month, $i ? -1 : $mday];

   # Go back to first day of the month, then go back to first Sunday.
   $k = 0;
   for(Align;; Align)
   {
      ($x, $y, $z, $mday, $month, $k, $wday) = LocalTime;
      last if !$wday;
      $l -= 43200;
   }
   $t[$i] = $l;

   @c = @age = ();
   for($j = 0; $j < 6 * 7; $j++)
   {
      ($x, $y, $z, $mday, $month, $year, $wday, $k) = LocalTime;

      # Record moon age for each date.  This is calculated based on
      # number of days since 1900-01-01, which happens to be a new moon.
      # An easier way to compute this is to just take the number of
      # unix seconds and do a modulus, but that relies on the unix
      # epoch being fixed, which isn't true for all platforms.
      $k += $year * 365
            + int(~-$year / 4)
            - int(~-$year / 100)
            + int(($year + 299) / 400);

      # Length of synodic month:
      # https://en.wikipedia.org/wiki/Lunar_phase#Calculating_phase
      use constant PERIOD => 29.530588853;
      push @age, $k - int($k / PERIOD) * PERIOD;

      # Set color based on whether displayed date matches current date.
      push @c,
           ($month - $current[$i][1]
              ? 0
              : $mday - $current[$i][2]
                 ? CURRENT_MONTH
                 : CURRENT_DAY);
      $l += 86400 + 43200;
      Align;
   }

   # Now update colors based on whether moon age between midnights contains
   # 15.5, and also add padding for bottommost row.
   for($j = 0; $j < 6 * 7 - 1; $j++)
   {
      $c[$j] |= FULL_MOON if $age[$j] <= 15.5 && $age[$j + 1] > 15.5;
      push @c, 0;
   }
   push @colors, [@c];

   # Draw headings.
   Output "   ", $i;
   printf "\e[97;40m\e[4m %04d-%02d" . (" " x 26) . "\e[0m",
          $current[$i][0] + 1900, $current[$i][1] + 1;

   # Step forward 6 weeks from first Sunday of current month.  This is
   # guaranteed to put us somewhere in the next month, and definitely not
   # the month after that.
   push @t, $t[$i] + 86400 * 6 * 7;
}
Output "\n", 1;

# Draw calendars.
for($j = 0; $j < 7; $j++)
{
   # Draw odd rows.
   for($i = 0; $i < CALENDAR_COUNT; $i++)
   {
      Output "  ", $i;
      for($k = 0; $k < 7; $k++)
      {
         $x = $colors[$i][$j * 7 + $k];
         $y = $j * 7 + $k > 6 ? $colors[$i][$j * 7 + $k - 7] : 0;
         $z = $x | $y;

         # Special handling for when row above or below is the current date,
         # and the other row is a full moon.  We can't simply use a bitwise
         # OR for this because we need to distinguish the two cases:
         #  (CURRENT_MONTH|FULL_MOON) | CURRENT_DAY -> CURRENT_DAY
         #  (CURRENT_DAY|FULL_MOON) | CURRENT_MONTH -> CURRENT_MONTH|FULL_MOON
         $z = $z - (CURRENT_MONTH | CURRENT_DAY | FULL_MOON)
              ? $z
              : CURRENT_DAY |
                ($x - CURRENT_DAY && $y - CURRENT_DAY
                   ? FULL_MOON : 0);
         Output "$palette[$z]    \e[0m ", 1;
      }
   }
   Output "\n", 1;

   # Draw even rows.
   if( $j < 6 )
   {
      for($i = 0; $i < CALENDAR_COUNT; $i++)
      {
         $l = $t[$i];
         Output "  ", $i;
         for($wday = 0; $wday < 7; $wday++)
         {
            ($x, $y, $z, $mday) = LocalTime;
            printf '%s %2d '."\e[0m ",
                   $palette[$colors[$i][$j * 7 + $wday]], $mday;
            $l += 86400 + 43200;
            Align;
         }
         $t[$i] = $l;
      }
      Output "\n", 1;
   }
}