s. Otherwise, it returns the
		 * decoded JSON or null for the string `null`.
		 */
		$decoded_json = json_decode( $directive_value, true );
		if ( null !== $decoded_json || 'null' === $directive_value ) {
			$directive_value = $decoded_json;
		}

		return array( $default_namespace, $directive_value );
	}

	/**
	 * Transforms a kebab-case string to camelCase.
	 *
	 * @param string $str The kebab-case string to transform to camelCase.
	 * @return string The transformed camelCase string.
	 */
	private function kebab_to_camel_case( string $str ): string {
		return lcfirst(
			preg_replace_callback(
				'/(-)([a-z])/',
				function ( $matches ) {
					return strtoupper( $matches[2] );
				},
				strtolower( rtrim( $str, '-' ) )
			)
		);
	}

	/**
	 * Processes the `data-wp-interactive` directive.
	 *
	 * It adds the default store namespace defined in the directive value to the
	 * stack so that it's available for the nested interactivity elements.
	 *
	 * @since 6.5.0
	 *
	 * @param WP_Interactivity_API_Directives_Processor $p               The directives processor instance.
	 * @param string                                    $mode            Whether the processing is entering or exiting the tag.
	 * @param array                                     $context_stack   The reference to the context stack.
	 * @param array                                     $namespace_stack The reference to the store namespace stack.
	 */
	private function data_wp_interactive_processor( WP_Interactivity_API_Directives_Processor $p, string $mode, array &$context_stack, array &$namespace_stack ) {
		// When exiting tags, it removes the last namespace from the stack.
		if ( 'exit' === $mode ) {
			array_pop( $namespace_stack );
			return;
		}

		// Tries to decode the `data-wp-interactive` attribute value.
		$attribute_value = $p->get_attribute( 'data-wp-interactive' );

		/*
		 * Pushes the newly defined namespace or the current one if the
		 * `data-wp-interactive` definition was invalid or does not contain a
		 * namespace. It does so because the function pops out the current namespace
		 * from the stack whenever it finds a `data-wp-interactive`'s closing tag,
		 * independently of whether the previous `data-wp-interactive` definition
		 * contained a valid namespace.
		 */
		$new_namespace = null;
		if ( is_string( $attribute_value ) && ! empty( $attribute_value ) ) {
			$decoded_json = json_decode( $attribute_value, true );
			if ( is_array( $decoded_json ) ) {
				$new_namespace = $decoded_json['namespace'] ?? null;
			} else {
				$new_namespace = $attribute_value;
			}
		}
		$namespace_stack[] = ( $new_namespace && 1 === preg_match( '/^([\w\-_\/]+)/', $new_namespace ) )
			? $new_namespace
			: end( $namespace_stack );
	}

	/**
	 * Processes the `data-wp-context` directive.
	 *
	 * It adds the context defined in the directive value to the stack so that
	 * it's available for the nested interactivity elements.
	 *
	 * @since 6.5.0
	 *
	 * @param WP_Interactivity_API_Directives_Processor $p               The directives processor instance.
	 * @param string                                    $mode            Whether the processing is entering or exiting the tag.
	 * @param array                                     $context_stack   The reference to the context stack.
	 * @param array                                     $namespace_stack The reference to the store namespace stack.
	 */
	private function data_wp_context_processor( WP_Interactivity_API_Directives_Processor $p, string $mode, array &$context_stack, array &$namespace_stack ) {
		// When exiting tags, it removes the last context from the stack.
		if ( 'exit' === $mode ) {
			array_pop( $context_stack );
			return;
		}

		$attribute_value = $p->get_attribute( 'data-wp-context' );
		$namespace_value = end( $namespace_stack );

		// Separates the namespace from the context JSON object.
		list( $namespace_value, $decoded_json ) = is_string( $attribute_value ) && ! empty( $attribute_value )
			? $this->extract_directive_value( $attribute_value, $namespace_value )
			: array( $namespace_value, null );

		/*
		 * If there is a namespace, it adds a new context to the stack merging the
		 * previous context with the new one.
		 */
		if ( is_string( $namespace_value ) ) {
			$context_stack[] = array_replace_recursive(
				end( $context_stack ) !== false ? end( $context_stack ) : array(),
				array( $namespace_value => is_array( $decoded_json ) ? $decoded_json : array() )
			);
		} else {
			/*
			 * If there is no namespace, it pushes the current context to the stack.
			 * It needs to do so because the function pops out the current context
			 * from the stack whenever it finds a `data-wp-context`'s closing tag.
			 */
			$context_stack[] = end( $context_stack );
		}
	}

	/**
	 * Processes the `data-wp-bind` directive.
	 *
	 * It updates or removes the bound attributes based on the evaluation of its
	 * associated reference.
	 *
	 * @since 6.5.0
	 *
	 * @param WP_Interactivity_API_Directives_Processor $p               The directives processor instance.
	 * @param string                                    $mode            Whether the processing is entering or exiting the tag.
	 * @param array                                     $context_stack   The reference to the context stack.
	 * @param array                                     $namespace_stack The reference to the store namespace stack.
	 */
	private function data_wp_bind_processor( WP_Interactivity_API_Directives_Processor $p, string $mode, array &$context_stack, array &$namespace_stack ) {
		if ( 'enter' === $mode ) {
			$all_bind_directives = $p->get_attribute_names_with_prefix( 'data-wp-bind--' );

			foreach ( $all_bind_directives as $attribute_name ) {
				list( , $bound_attribute ) = $this->extract_prefix_and_suffix( $attribute_name );
				if ( empty( $bound_attribute ) ) {
					return;
				}

				$attribute_value = $p->get_attribute( $attribute_name );
				$result          = $this->evaluate( $attribute_value, end( $namespace_stack ), end( $context_stack ) );

				if (
					null !== $result &&
					(
						false !== $result ||
						( strlen( $bound_attribute ) > 5 && '-' === $bound_attribute[4] )
					)
				) {
					/*
					 * If the result of the evaluation is a boolean and the attribute is
					 * `aria-` or `data-, convert it to a string "true" or "false". It
					 * follows the exact same logic as Preact because it needs to
					 * replicate what Preact will later do in the client:
					 * https://github.com/preactjs/preact/blob/ea49f7a0f9d1ff2c98c0bdd66aa0cbc583055246/src/diff/props.js#L131C24-L136
					 */
					if (
						is_bool( $result ) &&
						( strlen( $bound_attribute ) > 5 && '-' === $bound_attribute[4] )
					) {
						$result = $result ? 'true' : 'false';
					}
					$p->set_attribute( $bound_attribute, $result );
				} else {
					$p->remove_attribute( $bound_attribute );
				}
			}
		}
	}

	/**
	 * Processes the `data-wp-class` directive.
	 *
	 * It adds or removes CSS classes in the current HTML element based on the
	 * evaluation of its associated references.
	 *
	 * @since 6.5.0
	 *
	 * @param WP_Interactivity_API_Directives_Processor $p               The directives processor instance.
	 * @param string                                    $mode            Whether the processing is entering or exiting the tag.
	 * @param array                                     $context_stack   The reference to the context stack.
	 * @param array                                     $namespace_stack The reference to the store namespace stack.
	 */
	private function data_wp_class_processor( WP_Interactivity_API_Directives_Processor $p, string $mode, array &$context_stack, array &$namespace_stack ) {
		if ( 'enter' === $mode ) {
			$all_class_directives = $p->get_attribute_names_with_prefix( 'data-wp-class--' );

			foreach ( $all_class_directives as $attribute_name ) {
				list( , $class_name ) = $this->extract_prefix_and_suffix( $attribute_name );
				if ( empty( $class_name ) ) {
					return;
				}

				$attribute_value = $p->get_attribute( $attribute_name );
				$result          = $this->evaluate( $attribute_value, end( $namespace_stack ), end( $context_stack ) );

				if ( $result ) {
					$p->add_class( $class_name );
				} else {
					$p->remove_class( $class_name );
				}
			}
		}
	}

	/**
	 * Processes the `data-wp-style` directive.
	 *
	 * It updates the style attribute value of the current HTML element based on
	 * the evaluation of its associated references.
	 *
	 * @since 6.5.0
	 *
	 * @param WP_Interactivity_API_Directives_Processor $p               The directives processor instance.
	 * @param string                                    $mode            Whether the processing is entering or exiting the tag.
	 * @param array                                     $context_stack   The reference to the context stack.
	 * @param array                                     $namespace_stack The reference to the store namespace stack.
	 */
	private function data_wp_style_processor( WP_Interactivity_API_Directives_Processor $p, string $mode, array &$context_stack, array &$namespace_stack ) {
		if ( 'enter' === $mode ) {
			$all_style_attributes = $p->get_attribute_names_with_prefix( 'data-wp-style--' );

			foreach ( $all_style_attributes as $attribute_name ) {
				list( , $style_property ) = $this->extract_prefix_and_suffix( $attribute_name );
				if ( empty( $style_property ) ) {
					continue;
				}

				$directive_attribute_value = $p->get_attribute( $attribute_name );
				$style_property_value      = $this->evaluate( $directive_attribute_value, end( $namespace_stack ), end( $context_stack ) );
				$style_attribute_value     = $p->get_attribute( 'style' );
				$style_attribute_value     = ( $style_attribute_value && ! is_bool( $style_attribute_value ) ) ? $style_attribute_value : '';

				/*
				 * Checks first if the style property is not falsy and the style
				 * attribute value is not empty because if it is, it doesn't need to
				 * update the attribute value.
				 */
				if ( $style_property_value || $style_attribute_value ) {
					$style_attribute_value = $this->merge_style_property( $style_attribute_value, $style_property, $style_property_value );
					/*
					 * If the style attribute value is not empty, it sets it. Otherwise,
					 * it removes it.
					 */
					if ( ! empty( $style_attribute_value ) ) {
						$p->set_attribute( 'style', $style_attribute_value );
					} else {
						$p->remove_attribute( 'style' );
					}
				}
			}
		}
	}

	/**
	 * Merges an individual style property in the `style` attribute of an HTML
	 * element, updating or removing the property when necessary.
	 *
	 * If a property is modified, the old one is removed and the new one is added
	 * at the end of the list.
	 *
	 * @since 6.5.0
	 *
	 * Example:
	 *
	 *     merge_style_property( 'color:green;', 'color', 'red' )      => 'color:red;'
	 *     merge_style_property( 'background:green;', 'color', 'red' ) => 'background:green;color:red;'
	 *     merge_style_property( 'color:green;', 'color', null )       => ''
	 *
	 * @param string            $style_attribute_value The current style attribute value.
	 * @param string            $style_property_name   The style property name to set.
	 * @param string|false|null $style_property_value  The value to set for the style property. With false, null or an
	 *                                                 empty string, it removes the style property.
	 * @return string The new style attribute value after the specified property has been added, updated or removed.
	 */
	private function merge_style_property( string $style_attribute_value, string $style_property_name, $style_property_value ): string {
		$style_assignments    = explode( ';', $style_attribute_value );
		$result               = array();
		$style_property_value = ! empty( $style_property_value ) ? rtrim( trim( $style_property_value ), ';' ) : null;
		$new_style_property   = $style_property_value ? $style_property_name . ':' . $style_property_value . ';' : '';

		// Generates an array with all the properties but the modified one.
		foreach ( $style_assignments as $style_assignment ) {
			if ( empty( trim( $style_assignment ) ) ) {
				continue;
			}
			list( $name, $value ) = explode( ':', $style_assignment );
			if ( trim( $name ) !== $style_property_name ) {
				$result[] = trim( $name ) . ':' . trim( $value ) . ';';
			}
		}

		// Adds the new/modified property at the end of the list.
		$result[] = $new_style_property;

		return implode( '', $result );
	}

	/**
	 * Processes the `data-wp-text` directive.
	 *
	 * It updates the inner content of the current HTML element based on the
	 * evaluation of its associated reference.
	 *
	 * @since 6.5.0
	 *
	 * @param WP_Interactivity_API_Directives_Processor $p               The directives processor instance.
	 * @param string                                    $mode            Whether the processing is entering or exiting the tag.
	 * @param array                                     $context_stack   The reference to the context stack.
	 * @param array                                     $namespace_stack The reference to the store namespace stack.
	 */
	private function data_wp_text_processor( WP_Interactivity_API_Directives_Processor $p, string $mode, array &$context_stack, array &$namespace_stack ) {
		if ( 'enter' === $mode ) {
			$attribute_value = $p->get_attribute( 'data-wp-text' );
			$result          = $this->evaluate( $attribute_value, end( $namespace_stack ), end( $context_stack ) );

			/*
			 * Follows the same logic as Preact in the client and only changes the
			 * content if the value is a string or a number. Otherwise, it removes the
			 * content.
			 */
			if ( is_string( $result ) || is_numeric( $result ) ) {
				$p->set_content_between_balanced_tags( esc_html( $result ) );
			} else {
				$p->set_content_between_balanced_tags( '' );
			}
		}
	}

	/**
	 * Returns the CSS styles for animating the top loading bar in the router.
	 *
	 * @since 6.5.0
	 *
	 * @return string The CSS styles for the router's top loading bar animation.
	 */
	private function get_router_animation_styles(): string {
		return <<<CSS
			.wp-interactivity-router-loading-bar {
				position: fixed;
				top: 0;
				left: 0;
				margin: 0;
				padding: 0;
				width: 100vw;
				max-width: 100vw !important;
				height: 4px;
				background-color: #000;
				opacity: 0
			}
			.wp-interactivity-router-loading-bar.start-animation {
				animation: wp-interactivity-router-loading-bar-start-animation 30s cubic-bezier(0.03, 0.5, 0, 1) forwards
			}
			.wp-interactivity-router-loading-bar.finish-animation {
				animation: wp-interactivity-router-loading-bar-finish-animation 300ms ease-in
			}
			@keyframes wp-interactivity-router-loading-bar-start-animation {
				0% { transform: scaleX(0); transform-origin: 0 0; opacity: 1 }
				100% { transform: scaleX(1); transform-origin: 0 0; opacity: 1 }
			}
			@keyframes wp-interactivity-router-loading-bar-finish-animation {
				0% { opacity: 1 }
				50% { opacity: 1 }
				100% { opacity: 0 }
			}
CSS;
	}

	/**
	 * Outputs the markup for the top loading indicator and the screen reader
	 * notifications during client-side navigations.
	 *
	 * This method prints a div element representing a loading bar visible during
	 * navigation, as well as an aria-live region that can be read by screen
	 * readers to announce navigation status.
	 *
	 * @since 6.5.0
	 */
	public function print_router_loading_and_screen_reader_markup() {
		echo <<<HTML
			<div
				class="wp-interactivity-router-loading-bar"
				data-wp-interactive="core/router"
				data-wp-class--start-animation="state.navigation.hasStarted"
				data-wp-class--finish-animation="state.navigation.hasFinished"
			></div>
			<div
				class="screen-reader-text"
				aria-live="polite"
				data-wp-interactive="core/router"
				data-wp-text="state.navigation.message"
			></div>
HTML;
	}

	/**
	 * Processes the `data-wp-router-region` directive.
	 *
	 * It renders in the footer a set of HTML elements to notify users about
	 * client-side navigations. More concretely, the elements added are 1) a
	 * top loading bar to visually inform that a navigation is in progress
	 * and 2) an `aria-live` region for accessible navigation announcements.
	 *
	 * @since 6.5.0
	 *
	 * @param WP_Interactivity_API_Directives_Processor $p               The directives processor instance.
	 * @param string                                    $mode            Whether the processing is entering or exiting the tag.
	 */
	private function data_wp_router_region_processor( WP_Interactivity_API_Directives_Processor $p, string $mode ) {
		if ( 'enter' === $mode && ! $this->has_processed_router_region ) {
			$this->has_processed_router_region = true;

			// Initialize the `core/router` store.
			$this->state(
				'core/router',
				array(
					'navigation' => array(
						'texts' => array(
							'loading' => __( 'Loading page, please wait.' ),
							'loaded'  => __( 'Page Loaded.' ),
						),
					),
				)
			);

			// Enqueues as an inline style.
			wp_register_style( 'wp-interactivity-router-animations', false );
			wp_add_inline_style( 'wp-interactivity-router-animations', $this->get_router_animation_styles() );
			wp_enqueue_style( 'wp-interactivity-router-animations' );

			// Adds the necessary markup to the footer.
			add_action( 'wp_footer', array( $this, 'print_router_loading_and_screen_reader_markup' ) );
		}
	}

	/**
	 * Processes the `data-wp-each` directive.
	 *
	 * This directive gets an array passed as reference and iterates over it
	 * generating new content for each item based on the inner markup of the
	 * `template` tag.
	 *
	 * @since 6.5.0
	 *
	 * @param WP_Interactivity_API_Directives_Processor $p               The directives processor instance.
	 * @param string                                    $mode            Whether the processing is entering or exiting the tag.
	 * @param array                                     $context_stack   The reference to the context stack.
	 * @param array                                     $namespace_stack The reference to the store namespace stack.
	 * @param array                                     $tag_stack       The reference to the tag stack.
	 */
	private function data_wp_each_processor( WP_Interactivity_API_Directives_Processor $p, string $mode, array &$context_stack, array &$namespace_stack, array &$tag_stack ) {
		if ( 'enter' === $mode && 'TEMPLATE' === $p->get_tag() ) {
			$attribute_name   = $p->get_attribute_names_with_prefix( 'data-wp-each' )[0];
			$extracted_suffix = $this->extract_prefix_and_suffix( $attribute_name );
			$item_name        = isset( $extracted_suffix[1] ) ? $this->kebab_to_camel_case( $extracted_suffix[1] ) : 'item';
			$attribute_value  = $p->get_attribute( $attribute_name );
			$result           = $this->evaluate( $attribute_value, end( $namespace_stack ), end( $context_stack ) );

			// Gets the content between the template tags and leaves the cursor in the closer tag.
			$inner_content = $p->get_content_between_balanced_template_tags();

			// Checks if there is a manual server-side directive processing.
			$template_end = 'data-wp-each: template end';
			$p->set_bookmark( $template_end );
			$p->next_tag();
			$manual_sdp = $p->get_attribute( 'data-wp-each-child' );
			$p->seek( $template_end ); // Rewinds to the template closer tag.
			$p->release_bookmark( $template_end );

			/*
			 * It doesn't process in these situations:
			 * - Manual server-side directive processing.
			 * - Empty or non-array values.
			 * - Associative arrays because those are deserialized as objects in JS.
			 * - Templates that contain top-level texts because those texts can't be
			 *   identified and removed in the client.
			 */
			if (
				$manual_sdp ||
				empty( $result ) ||
				! is_array( $result ) ||
				! array_is_list( $result ) ||
				! str_starts_with( trim( $inner_content ), '<' ) ||
				! str_ends_with( trim( $inner_content ), '>' )
			) {
				array_pop( $tag_stack );
				return;
			}

			// Extracts the namespace from the directive attribute value.
			$namespace_value         = end( $namespace_stack );
			list( $namespace_value ) = is_string( $attribute_value ) && ! empty( $attribute_value )
				? $this->extract_directive_value( $attribute_value, $namespace_value )
				: array( $namespace_value, null );

			// Processes the inner content for each item of the array.
			$processed_content = '';
			foreach ( $result as $item ) {
				// Creates a new context that includes the current item of the array.
				$context_stack[] = array_replace_recursive(
					end( $context_stack ) !== false ? end( $context_stack ) : array(),
					array( $namespace_value => array( $item_name => $item ) )
				);

				// Processes the inner content with the new context.
				$processed_item = $this->process_directives_args( $inner_content, $context_stack, $namespace_stack );

				if ( null === $processed_item ) {
					// If the HTML is unbalanced, stop processing it.
					array_pop( $context_stack );
					return;
				}

				// Adds the `data-wp-each-child` to each top-level tag.
				$i = new WP_Interactivity_API_Directives_Processor( $processed_item );
				while ( $i->next_tag() ) {
					$i->set_attribute( 'data-wp-each-child', true );
					$i->next_balanced_tag_closer_tag();
				}
				$processed_content .= $i->get_updated_html();

				// Removes the current context from the stack.
				array_pop( $context_stack );
			}

			// Appends the processed content after the tag closer of the template.
			$p->append_content_after_template_tag_closer( $processed_content );

			// Pops the last tag because it skipped the closing tag of the template tag.
			array_pop( $tag_stack );
		}
	}
}