It's working more or less...

src/playlist.cpp

@@ -1,9 +1,61 @@
 #include "playlist.h"
 
+Playlist::Playlist(String id) {
+	rfid_id = id;
+}
+
+String Playlist::get_rfid_id() {
+	return rfid_id;
+}
+
 void Playlist::add_file(String filename) {
 	files.push_back(filename);
 }
 
 void Playlist::sort() {
 	std::sort(files.begin(), files.end());
+}
+
+void Playlist::set_current_position(uint8_t file, uint32_t seconds) {
+	current_file = file;
+	current_time = seconds;
+}
+
+String Playlist::get_current_file_name() {
+	if (current_file >= files.size()) {
+		Serial.printf("Requested a file number %d, which is not available in this playlist. Starting over.\n", current_file);
+		set_current_position(0);
+	}
+	return files[current_file];
+}
+
+bool Playlist::next_track() {
+	if (files.size() <= current_file + 1) {
+		Serial.println("next_track does not exist. Resetting playlist and returning false.");
+		set_current_position(0, 0);
+		return false;
+	}
+	set_current_position(current_file + 1, 0);
+	return true;
+}
+
+bool Playlist::prev_track() {
+	if (current_file == 0) {
+		return false;
+	}
+	
+	set_current_position(current_file - 1, 0);
+	return true;
+}
+
+void Playlist::restart() {
+	current_time = 0;
+}
+
+void Playlist::set_current_time(uint32_t pos) {
+	current_time = pos;
+}
+
+uint32_t Playlist::get_current_time() {
+	return current_time;
 }