sub readbackwards_utf8 { # returns an iterator
	my ($fn, $window) = @_;
	die "Bad window $window" unless $window>=4;
	open my $fh, '<:raw', $fn or die "open $fn: $!";
	my $curpos = -s $fh;
	return sub {
		if ( $curpos<1 ) { close $fh if $fh; $fh=undef; return }
		my $bytes = $curpos < $window ? $curpos : $window;
		seek($fh, $curpos-=$bytes, 0) or die "seek $curpos $fn: $!";
		read($fh, my $buf, $bytes) == $bytes
			or die "read $bytes bytes at $curpos from $fn: $!";
		while ( (ord(substr $buf, 0, 1) & 0b11000000)==0b10000000 )
			{ $buf = substr $buf, 1; $curpos++ }
		utf8::decode($buf);
		return $buf;
	}
}

##</code><code>##

use open qw/:std :utf8/;
use Test::More;
use File::Temp qw/tempfile/;

my ($tempfh, $filename) = tempfile( UNLINK => 1 );
binmode $tempfh, ':encoding(UTF-8)';
print $tempfh "H\N{U+20AC}ll\N{U+00F6}, \N{U+1F5FA}!\n";
close $tempfh;
#system('hexdump','-C',$filename);

my $four = readbackwards_utf8($filename, 4);
is $four->(), "!\n";
is $four->(), "\N{U+1F5FA}";
is $four->(), "\N{U+00F6}, ";
is $four->(), "ll";
is $four->(), "H\N{U+20AC}";
is $four->(), undef;
is $four->(), undef;

my $five = readbackwards_utf8($filename, 5);
is $five->(), "!\n";
is $five->(), " \N{U+1F5FA}";
is $five->(), "ll\N{U+00F6},";
is $five->(), "H\N{U+20AC}";
is $five->(), undef;

my $six = readbackwards_utf8($filename, 6);
is $six->(), "\N{U+1F5FA}!\n";
is $six->(), "ll\N{U+00F6}, ";
is $six->(), "H\N{U+20AC}";
is $six->(), undef;

my $seven = readbackwards_utf8($filename, 7);
is $seven->(), " \N{U+1F5FA}!\n";
is $seven->(), "ll\N{U+00F6},";
is $seven->(), "H\N{U+20AC}";
is $seven->(), undef;

for my $n (8..9) {
	my $eight = readbackwards_utf8($filename, $n);
	is $eight->(), ", \N{U+1F5FA}!\n";
	is $eight->(), "H\N{U+20AC}ll\N{U+00F6}";
	is $eight->(), undef;
}

my $ten = readbackwards_utf8($filename, 10);
is $ten->(), "\N{U+00F6}, \N{U+1F5FA}!\n";
is $ten->(), "H\N{U+20AC}ll";
is $ten->(), undef;

my $eleven = readbackwards_utf8($filename, 11);
is $eleven->(), "l\N{U+00F6}, \N{U+1F5FA}!\n";
is $eleven->(), "H\N{U+20AC}l";
is $eleven->(), undef;

for my $n (12..14) {
	my $twelve = readbackwards_utf8($filename, $n);
	is $twelve->(), "ll\N{U+00F6}, \N{U+1F5FA}!\n";
	is $twelve->(), "H\N{U+20AC}";
	is $twelve->(), undef;
}

my $fifteen = readbackwards_utf8($filename, 15);
is $fifteen->(), "\N{U+20AC}ll\N{U+00F6}, \N{U+1F5FA}!\n";
is $fifteen->(), "H";
is $fifteen->(), undef;

for my $n (16..17) {
	my $sixteen = readbackwards_utf8($filename, 16);
	is $sixteen->(), "H\N{U+20AC}ll\N{U+00F6}, \N{U+1F5FA}!\n";
	is $sixteen->(), undef;
}

done_testing;