Solver SUBTRACTING small integers by counting APPLES

Source code of 'SUBTRACTING small integers by counting APPLES'

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==section input
Subtract *[input b=3] from *[input a=7]. ONLY SMALL INTEGERS! The number that you subtract should be LESS than the number you subtract FROM!

==section solution
perl

sub repeat {
  my ($text, $times) = @_;
  return "" unless $times;
  my $result = $text;
  $result .= ",$text" x ($times-1);
  return $result;
}

$diff = $a-$b;
if( $a > 0 && $a <= 10 && $b > 0 && $b <= 10 && $b <= $a ) {
  my $pic = "sprite_apple(0)";

my $p1 = "$a=expr( $pic " . (",$pic" x ($a-1)) . ")";
  print "We start with $a apples: {{{$p1}}}\n\n";

my $p2 = "expr( " . repeat( $pic, $diff );
  $p2 .= ',' if( $diff );
  $p2 .= "highlight( expr( " . repeat( $pic, $b ) . ") ) )";
  print "Subtract $b apples from $a (highlight $b): {{{$p2}}}\n\n";

my $p3 = "expr( " . repeat( $pic, $diff ) . ") = $diff";
  $p3 = "nothing = 0" if $diff == 0;
  print "Here's what we have left: {{{$p3}}}\n\n";

print "<H4>Cartoon</H4>";

print "{{{cartoon( $p1, $p2, $p3 )}}}\n\n";

#print "{{{expr( $pic " . (",$pic" x ($b-1)) . ")=$b}}}\n";
  #print "{{{$a+$b = expr( $pic " . (",$pic" x ($a+$b-1)) . ") = $sum}}}\n";
} else {
  print "Sorry, only numbers between 0 and 10 are allowed. Subtract smaller numbers from greater numbers.\n";
}

==section output
diff

==section check
  a=3 b=2 diff=1