Implement new *content functions* feature (readme)

This commit implements a new feature called *content functions* that is
similar but much more powerful than the already existing *content tags*
which you may have already used (`{POST[1]}`, for example).

You now can also add your own *content functions* to do some interesting
things like embedding a YouTube video or other things to prevent typing
repetitive lines of text or code in your entities content.

Read the corresponding wiki page to learn more about this:
https://github.com/Nerdmind/Blog/wiki/Content-functions

3 files changed, 159 insertions, 1 deletions

diff --git a/core/functions.php b/core/functions.php
index 97ff802..cf35aa9 100644
--- a/core/functions.php
+++ b/core/functions.php
@@ -9,6 +9,7 @@ use Template\Template as Template;
 use Template\Factory as TemplateFactory;
 
 use Parsers\ArgumentParser;
+use Parsers\FunctionParser;
 use Parsers\EmoticonParser;
 use Parsers\MarkdownParser;
 
@@ -190,7 +191,8 @@ function parseContentTags(string $text): string {
 	$text = preg_replace($base_tag, \Application::getURL('$1'), $text);
 	$text = preg_replace($file_tag, \Application::getFileURL('$1'), $text);
 
-	return $text;
+	$FunctionParser = new FunctionParser;
+	return $FunctionParser->transform($text);
 }
 
 #===============================================================================
@@ -318,6 +320,23 @@ function generateSlug($string, $separator = '-') {
 	return trim($string, $separator);
 }
 
+#===========================================================================
+# Callback for (CATEGORY|PAGE|POST|USER) content function
+#===========================================================================
+function getEntityMarkdownLink($ns, $id, $text = NULL, $info = NULL): string {
+	if(!$Entity = Application::getRepository($ns)->find($id)) {
+		return sprintf('`{%s: *Reference error*}`', strtoupper($ns));
+	}
+
+	$title = htmlspecialchars($Entity->get('name') ?? $Entity->get('fullname'));
+	$href = Application::getEntityURL($Entity);
+	$text = $text ?: "»{$title}«";
+	$info = $info ?: sprintf('%s »%s«',
+		Application::getLanguage()->text(strtolower($ns)), $title);
+
+	return sprintf('[%s](%s "%s")',	$text, $href, $info);
+}
+
 #===============================================================================
 # Function for use in templates to get data of a category
 #===============================================================================
@@ -369,3 +388,45 @@ function USER(int $id): array {
 
 	return [];
 }
+
+#===========================================================================
+# Get base URL (optionally extended by $extend)
+#===========================================================================
+FunctionParser::register('BASE_URL', function($extend = '') {
+	return Application::getURL($extend);
+});
+
+#===========================================================================
+# Get file URL (optionally extended by $extend)
+#===========================================================================
+FunctionParser::register('FILE_URL', function($extend = '') {
+	return Application::getFileURL($extend);
+});
+
+#===========================================================================
+# Get Markdown formatted *category* link
+#===========================================================================
+FunctionParser::register('CATEGORY', function($id, $text = NULL, $title = NULL) {
+	return getEntityMarkdownLink('Category', $id, $text, $title);
+});
+
+#===========================================================================
+# Get Markdown formatted *page* link
+#===========================================================================
+FunctionParser::register('PAGE', function($id, $text = NULL, $title = NULL) {
+	return getEntityMarkdownLink('Page', $id, $text, $title);
+});
+
+#===========================================================================
+# Get Markdown formatted *post* link
+#===========================================================================
+FunctionParser::register('POST', function($id, $text = NULL, $title = NULL) {
+	return getEntityMarkdownLink('Post', $id, $text, $title);
+});
+
+#===========================================================================
+# Get Markdown formatted *user* link
+#===========================================================================
+FunctionParser::register('USER', function($id, $text = NULL, $title = NULL) {
+	return getEntityMarkdownLink('User', $id, $text, $title);
+});
diff --git a/core/namespace/Application.php b/core/namespace/Application.php
index 5690841..140baac 100644
--- a/core/namespace/Application.php
+++ b/core/namespace/Application.php
@@ -1,5 +1,6 @@
 <?php
 use ORM\EntityInterface;
+use Parsers\FunctionParser;
 
 class Application {
 
@@ -214,6 +215,18 @@ class Application {
 	}
 
 	#===============================================================================
+	# Add a custom content function
+	#===============================================================================
+	public static function addContentFunction(string $name, callable $callback): void {
+		if(!preg_match('#^([0-9A-Z_]+)$#', $name)) {
+			throw new Exception('The name for adding a content function must
+				contain only numbers, uppercase letters and underscores!');
+		}
+
+		FunctionParser::register($name, $callback);
+	}
+
+	#===============================================================================
 	# Exit application with a custom message and status code
 	#===============================================================================
 	public static function exit($message = '', $code = 503): void {
diff --git a/core/namespace/Parsers/FunctionParser.php b/core/namespace/Parsers/FunctionParser.php
new file mode 100644
index 0000000..9f2f72e
--- /dev/null
+++ b/core/namespace/Parsers/FunctionParser.php
@@ -0,0 +1,84 @@
+<?php
+namespace Parsers;
+use ReflectionFunction;
+
+class FunctionParser implements ParserInterface {
+	private static $functions = [];
+
+	#===============================================================================
+	# Regular expressions
+	#===============================================================================
+	private const FUNCTION_SHELL_REGEX = '#\{\s?(%s)%s\s?\}#';
+	private const ARGUMENT_PARTS_REGEX = '(?:\:( (?:(?:"[^"]*"|[0-9]+)(?:,[\s]*)?)+))?';
+	private const ARGUMENT_SPLIT_REGEX = '#("[^"]*"|[0-9]+)(?:,[\s]*)?#';
+
+	#===============================================================================
+	# Register function
+	#===============================================================================
+	public static function register(string $name, callable $callback): void {
+		$Function = new ReflectionFunction($callback);
+		self::$functions[$name] = [
+			'callback' => $callback,
+			'required' => $Function->getNumberOfRequiredParameters()
+		];
+	}
+
+	#===============================================================================
+	# Parse functions
+	#===============================================================================
+	public function parse(string $text): array {
+		$functionNames = array_keys(self::$functions);
+		$functionNames = implode('|', $functionNames);
+
+		$pattern = self::FUNCTION_SHELL_REGEX;
+		$options = self::ARGUMENT_PARTS_REGEX;
+
+		preg_match_all(sprintf($pattern, $functionNames, $options), $text, $matches);
+
+		foreach(array_map(function($name, $parameters) {
+			return [$name , $this->parseParameterString($parameters)];
+		}, $matches[1], $matches[2]) as $match) {
+			$functions[$match[0]][] = $match[1];
+		}
+
+		return $functions ?? [];
+	}
+
+	#===============================================================================
+	# Transform functions
+	#===============================================================================
+	public function transform(string $text): string {
+		$functionData = self::$functions;
+		$functionNames = array_keys($functionData);
+		$functionNames = implode('|', $functionNames);
+
+		$pattern = self::FUNCTION_SHELL_REGEX;
+		$options = self::ARGUMENT_PARTS_REGEX;
+
+		return preg_replace_callback(sprintf($pattern, $functionNames, $options),
+		function($matches) use($functionData) {
+			$function = $matches[1];
+			$callback = $functionData[$function]['callback'];
+			$required = $functionData[$function]['required'];
+
+			$arguments = $this->parseParameterString($matches[2] ?? '');
+
+			if(count($arguments) < $required) {
+				return sprintf('`{%s: *Missing arguments*}`', $function);
+			}
+
+			return $callback(...$arguments);
+		}, $text);
+	}
+
+	#===============================================================================
+	# Parse the parameter string found within the function shell
+	#===============================================================================
+	private function parseParameterString(string $parameters): array {
+		preg_match_all(self::ARGUMENT_SPLIT_REGEX, $parameters, $matches);
+
+		return array_map(function($argument) {
+			return trim($argument, '"');
+		}, $matches[1]);
+	}
+}