# subroutine name: process_text
# Input: template text, reference to hash {tag_name=>tag_value}
# Processes string using an input template text and <!--$ANYTHING--> tag values hash.
# Supports also <!--%% "perl commands" %%--> tags.
# Usage example: process_text('I <!--$WORDTWO--> free scripts, do <!--$WORDONE-->?', {WORDONE => "you", WORDTWO => "like"});
#        Result: "I like free scripts, do you?"
sub process_text {
	local ($templatetext,$namespace) = @_;
	$templatetext =~ s/<\!--\%\%(.+?)\%\%-->/&evaluate($1)/egs;
	my @blocks = split(/(<!--if .*?-->|<!--else-->|<!--endif-->)/, $templatetext);
	my @stack=(); # format: ["true|false", "expression", "true string", "false (else) string"]
	my $result="";
	foreach my $block (@blocks) {
		if ($block =~ m/^<!--if (.*)-->$/s) {
			push (@stack, ["true", $1, "", ""]);
		} elsif ($block eq "<!--else-->") {
			if (scalar @stack > 0) {
				$stack[-1]->[0] = "false";
			} else {
				die "Nesting error: too many <!--else--> tags.";
			}
		} elsif ($block eq "<!--endif-->") {
			if (scalar @stack > 0) {
				my $expr = $stack[-1]->[1];
				my $r="";
				if ( &exec_expression($expr, $namespace) ) {
					$r = $stack[-1]->[2];
				} else {
					$r = $stack[-1]->[3];
				};
				pop(@stack);
				if (scalar @stack > 0) {
					if ($stack[-1]->[0] eq "true") {
						# Append result to "true" part of parent expression
						$stack[-1]->[2] .= $r;
					} else {
						# Append result to "false" part of parent expression
						$stack[-1]->[3] .= $r;
					}
				} else {
					$result .= $r;
				}
			} else {
				die "Nesting error: too many <!--endif--> tags.";
			}
		} else {
			# Text
			my $r = $block;
			if (scalar @stack > 0) {
				if ($stack[-1]->[0] eq "true") {
					# Append result to "true" part of upper-level expression
					$stack[-1]->[2] .= $r;
				} else {
					# Append result to "false" part of upper-level expression
					$stack[-1]->[3] .= $r;
				}
			} else {
				$result .= $r;
			}
		}
	}
	if (scalar @stack > 0) {
		die "Nesting error: too many <!--if ...--> tags.";
	} else {
		$templatetext = $result;
	}
	undef $result;
	
	# $templatetext =~ s/<!--if (.*?)-->(.*?)<!--endif-->/my ($expr,$yes)=($1,$2); my $no=""; if ($yes =~ m\/^(.*?)<!--else-->(.*)$\/s) {($yes,$no)=($1,$2)}; &exec_expression($expr, $namespace) ? $yes : $no /sge;
	$templatetext =~ s/<!--\$([A-Za-z\-_\.\d]+)-->/$namespace->{$1}/sg;
	$templatetext;
}

sub exec_expression {
	my ($expr, $namespace) = @_;
	my $original = $expr;
	my $perl_expr;
	while ($expr ne "") {
		$expr = " $expr";
		if ($expr =~ s/^\s*\$([A-Za-z\-_\.\d]+)//s) {
			$perl_expr.= "\$namespace->{'$1'}";
		} elsif ($expr =~ s/^\s*(==|!=|>=|>|<=|<>|<|=|\+|\-|\/|\*)//s) {
			if ($1 eq "<>") {
				$perl_expr.= "!=";
			} elsif ($1 eq "=") {
				$perl_expr.= "==";
			} else {
				$perl_expr.= $1;
			};
		} elsif ($expr =~ s/^\s+(eq|ne|le|lt|ge|gt)\s//si) {
			$perl_expr.= lc(" $1 ");
		} elsif ($expr =~ s/^\s+(and|\&\&)\s//si) {
			$perl_expr.= " && ";
		} elsif ($expr =~ s/^\s+(or|\|\|)\s//si) {
			$perl_expr.= " || ";
		} elsif ($expr =~ s/^\s+(not|!)\s//si) {
			$perl_expr.= " ! ";
		} elsif ($expr =~ s/^\s*"((?:\\"|\\\\|[^"])*)"//s) {
			my $str=$1;
			# unquote \ and "
			$str=~s/\\("|\\)/$1/gs; 
			# quote ",\,@,$
			$str=~s/([\"\\\@\$])/\\$1/gs;
			$perl_expr.= " \"$str\" ";
		} elsif ($expr =~ s/^\s*(\d*\.?\d+)//s) {
			$perl_expr.= " $1 ";
		} elsif ($expr =~ s/^\s+$//s) {
			# end of expression
		} else {
			die "Can't parse expression: $original. Parser stopped at position: $expr";
		};
	};
	return evaluate($perl_expr, "ignore");
}