Weekly Challenge - #079 Task #2 :: Perl

It has been about 10 weeks since I joined perl weekly challenge.
I'm not a professional programmer but I have experience with perl and some C lang. this is basically for perl/raku language but you can solve the many interesting challenges in "Other Language" - which is called "Guest" in their site - and have look into other's code and enjoy the fantastic reviews as well.

I have learnt many things from others, some of them are still in mystery but it was such a great chance~!!

I will be probably too busy to code from next week and probably I cannot participate as I did in past weeks. I'd like to thank you them all and decided to post a blog about it.

so..
This is task #2 solution and there must be a wide range of solution method.
I'm starting from the left hand side data and have a look only right hand side and if there is any bigger (or equal) wall than left one.
I calculated the water capacity at every chance a possible reservoir can hold.

...

for ( my $x = 1; $x < @T; ) {
    if ( $left <= $T[$x] ) { # increasing only: no useful data
        ( $start, $left ) = ( $x, $T[$x] );
        ++$x;
    }
    elsif ( (my $ri = firstidx { $_ >= $left } @T[$x..$#T]) >= 0 ) {
        my $abs_ri = $x+$ri;
        my $water_level = min( $left, $T[$abs_ri] );
        for (($start+1) .. ($abs_ri-1)) { # water area only
            $W[$_] = $water_level - $T[$_]
        }

        ( $start, $left ) = ( $x, $T[$abs_ri] );
        $x += $ri;
    }

...

but this approach have a problem at last section. I had to amend it
by finding second tallest wall(which is smaller than left boundary though) and calculate the capacity if I had to.

   else { # generally decreasing ...
           # find a tallest one as right boundary
        my $tallest = max @T[$x .. $#T];
        if ( (my $ri = firstidx { $_ == $tallest } @T[$x..$#T]) >= 0 ) {
            my $abs_ri = $x+$ri;
            my $water_level = min( $left, $T[$abs_ri] );

            for (($start+1) .. ($abs_ri-1)) {
                $W[$_] = $water_level - $T[$_];
            }
            ( $start, $left ) = ( $x, $T[$x] );
            ++$x;
        }
    }
...

so if we sum @W array here we can get the total amount of capacity
but this is not only thing we have to do.

we have to print Histogram!! I have some experience with printing on #75 so it wasn't very hard. but the code itself is a bit long
because I'd like to add some unicode character and color.

so this is how I look at if it is on my rxvt-unicode

and this is code if you want to look at

sub ssprintf ($$) { sprintf "%#$_[0]s", $_[1] }
sub map_ssprintf  { map { sprintf "%#$_[0]s", $_ } @_[1..$#_] }

sub u_($) { # unicode
    return $_[0] unless $::utf8;
    my $a = shift;
    state %u = ( qw{` └
                    - ─
                    | │
                    ~ ≈}, '#' => '■' );
    $u{$a} // $a
}

sub ch($$;$) { # unicode with colour
    my $a  = shift;
    my $ww = shift // 2;
    my $colour_str = shift;

    if ( $::colour and defined $colour_str ) {
        $a eq '#' and $a = ' '; # distinguish by color
        return colored( [ $colour_str ], ssprintf $ww, u_$a );
    }
    return ssprintf $ww, u_$a;
}

sub printTrappedInWater {
    my @T = @{$_[0]}; # territory heights
    my @W = @{$_[1]}; # water capacty

    my $maxh = max @T;                  # max height
    my $ww   = 1 + length $maxh;        # word width

    for my $y ( reverse 1 .. $maxh ) {
        my $line = ssprintf $ww, $y;
        $line .= u_"|";
        for my $x ( 0.. $#T ) {
            my $ch;
            if ( $T[$x] >= $y ) {
                $ch = ch("#", $ww, 'black on_yellow' );
            }
            elsif ( $W[$x] > 0 and $T[$x]+$W[$x] >= $y ) {
                $ch = ch("~", $ww, 'black on_cyan');
            }
            else {
                $ch = ch(" ", $ww);
            }
            $line .= $ch;
        }
        say $line;
    }

    say ssprintf( $ww, " " ), u_"`",
      map_ssprintf( $ww, ( (u_("-") x $ww ) x scalar @T ) );
    say ssprintf($ww, " "), " ",map_ssprintf( $ww, @T );
}

so.. how do you think? why don't you join PWC??